anti dive setup [Archive] - FSAE.com Forums

gautham

08-01-2013, 05:41 AM

hello karam thanks for ur response
my svsa length - 767.23
wheel base 1600
CG 960mm from front at height 310mm
i have done all my design and calculation
i came up with this results not just with 2d sketches but with dedicated softwares
i just have one ques for an 50% antidive setup do u think this is valid

Originally posted by Karam Atteia:
gautham,

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">, its like my lower wishbone is parallel to ground and upper wishbone is making 16deg in side view
now my question is that for 50% anti dive is this values of declination perfect or have i gone wrong somewere

There is something missing here to answer your question. You are talking about the side view geometry, then whats your car's wheel base, Side view swing arm length and CG height and distance from the front axle ?? In-board/Out-board Brakes??

But I want to take this discussion to another route where we can all learn something valuable.

1- Do we really need anti-dive setup for our car?? why??
2- know the values of the max. brake force at the front axle and the stiffness of the front suspension springs, If all these forces go to compress the springs and nothing through the wishbones. Then, does the chassis will hit the ground??
3- whats the effect of this anti-dive % on the position of the pitch center??
4- whats the effect of this anti-dive % on the longitudinal load transfer when braking??
5- whats the effect of this % on the rate of castor change?? Is this important for driver feedback??

I don't want you to answer these questions, but i'm presenting my thoughts about this subject.

Please all comment and share your thoughts. Thank you. </div></BLOCKQUOTE>

Karam Atteia

08-01-2013, 12:45 PM

gautham,

i just have one ques for an 50% antidive setup do u think this is valid

Excuse me, You are the one that should answer this question. So, please share your thoughts about why did you choose 50 % anti-dive for your car.

gautham

08-01-2013, 06:07 PM

we have caster of 8.8 deg, now having a parallel side view setup will lead to an anti dive value of -40% so now why anti dive is that our front ground clearance is close to 2 inches, considerin safety factor i can allow it to move upto 1-1.25 inch but the dive value i got from brakes guy at max force is even more high..........

and one more thing what i am really tryin is to get sugesstion from u guys whether my design is valid cause this the first time i am designin stuffs so dont get me in a wrong way

Originally posted by Karam Atteia:
gautham,

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">i just have one ques for an 50% antidive setup do u think this is valid

Excuse me, You are the one that should answer this question. So, please share your thoughts about why did you choose 50 % anti-dive for your car. </div></BLOCKQUOTE>

08-01-2013, 07:15 PM

Originally posted by gautham:
we have caster of 8.8 deg, now having a parallel side view setup will lead to an anti dive value of -40%
Gautham,

1. Caster in not a major factor in anti-dive.

2. What is "a parallel side view setup" parallel to, and why will it give -40% anti-dive?

3. Explanations for anti-dive can be found in lots of places (ask your teachers... or, at worst, try the interweb...).

4. Asking the same question on multiple different threads is counterproductive (it annoys people).

5. As Karam asked, why do you want 50% anti-dive? Could you not make up your mind between 0% and 100%, so you just thought you would split the difference? What is wrong with 0%? What is wrong with 100%? What is wrong with using the springs?

6. Don't expect anyone to answer above question until you have tried a lot harder to answer it ...

Z

Jay Lawrence

08-01-2013, 07:31 PM

Gautham,

Is anti-dive the only way to stop your car from pitching too much? It sounds like you are the 'suspension guy,' so why are you getting suspension travel information from your 'brakes guy'? How did you arrive at 50% (or -40% for that matter)?

Also, who are you and where are you from?

Edit: Z, beat me to it!

mams

10-12-2013, 04:56 PM

Gautham,

Though these forums can be helpful, the one thing I dislike about them is how condescending some of the people on here can be. Its difficult to start out on a new design as a first timer (I'm in the same boat as you are) so I understand where you are coming from. The best thing we can do for you is give you where to start in terms of research. The book Suspension Geometry and Computation by John C Dixon is a really good place to build from. Your school library probably has access to it online.

A little tip, you design in any kind of anti because you predict a certain amount weight transfer and a certain reaction from your springs. You cant just decide 50% anti dive without first knowing if you need that much.

Also how much static caster is set may not be important when considering your anti dive setup but how much caster variation results from your setup is.

dynatune

10-13-2013, 12:34 PM

Gautham,

as many people mentioned you should first question yourself about the question why do I want anti-dive and what are the pro and con's. Do not mind the sometimes rude comments, I do get them too. But as Z said, go read literature, without that it is impossible to learn. Now let me try to help you out with some of the learnings I made on anti-dive over the last 25 years (I might have forgotten one or two..)

The pro's of anti-dive are:

1) that you will have less pitch under braking which in many cases will help you to run a weaker front spring. Typically rear engined cars would need to get the best front end grip a soft front end spring which would hurt brake pitching. Anti-dive certainly helps to resolve that compromise.
2) Anti-dive helps to control the roll-center position better. Typically on the front the car would dive, lowering the roll center significantly causing the vehicle balance to become more "oversteer" - and that on top on the effect of load transfer. Same thing rear lift ... increasing anti-dive on the front can solely due to this effect improve corner entry stability.
3) If your car suffers from pitch sensitive aerodynamics a certain percentage anti-dive can help to keep your aerodynamics under control. Nowadays some fancy hydraulic push-rods try to do the same thing....
4) An anti-dive geometry will increase caster angle in jounce motion, this will help braking stability (causing extra understeer in braking) and will also in cornering increase the caster angle on the outside wheel causing better camber gains with steering angle towards less understeer.
5) Since anti-dive is actually describing the "link load transfer" the response on the chassis is immediate with as soon as the "acceleration" applies. A damper reacts with "velocity" and is thus an "integrational loop" lagging being the acceleration and a spring is lagging another "integrational loop" behind being driven by displacement. So in short Anti-Dive is about 3 times quicker than spring reaction ......

Now the con's of anti-dive:

a) Typically anti-dive tends to make the car less dive, which said differently increases the wheelrate. A car with 100% anti-dive will not move a mm under braking which is "technically" the same as an infinitely rigid suspension. We know that these kind of "rigid" suspension setups are bad for grip. So if you are running your car on a pool billiard table no problem ... elsewise reconsider....
b) as a bonus to a) anti-dive makes the contact patch move forward in impact. This makes the suspensions typically more harsh and as said under a) does not help grip under braking.
c) Anti-Dive being a link load transfer does unfortunately "interpret" the longitudinal component of the front tire lateral forces (which are perpendicular to the wheel but not to the car/suspension links) as a braking force and act thus in a similar way as the roll center causing jacking forces.

Practically it is very difficult to decide what is the optimum value and most of the times it depends on what particular kind of car you are designing. Fundamental however is to have some kind of tool to start doing your trade-off studies. Otherwise it is very difficult to get to grips with all of this.

Cheers,
dynatune, www.dynatune-xl.com

10-14-2013, 06:26 AM

Do not mind the sometimes rude comments...

In the spirit of above quote, I have to point out that...

4) An anti-dive geometry will increase caster angle in jounce motion, this will help braking stability (causing extra understeer in braking) and will also in cornering increase the caster angle on the outside wheel causing better camber gains with steering angle towards less understeer.
... is a "school-boy error". Unfortunately, it is all too often repeated by all too many experts!

For the record, "anti-dive" is NOT dependent on "Side-View-Swing-Arm length". SVSA length has a direct effect on (instantaneous) castor change, but NOT on anti-dive. Put another way, you pick the anti-dive percentage you want, and I guarantee I can achieve same with a short SVSA, or a long one, or with the SVSA IC behind the front wheels, or with the IC in front of the front wheels, or with the IC way off at either front or rear infinity...

Anti-dive is directly dependent on the front wheelprint's longitudinal n-line slope (sometimes referred to as "side-view force-line slope", eg. by Mark Ortiz and others). The single number describing said slope is all you need (well, together with CG position, braking split, etc.). The front suspension's SVSA length, or SVSA IC position along the wheelprint's n-line, are NOT needed.

Other than that, I agree with most of Dynatune's above comments.

Furthermore, (and IMO), 0% anti-dive is a reasonable number. 10% is also ok. 50% is getting on the high side, but might be workable in FSAE (because very smooth tracks...). Good luck if you try 100%!!!

Z

dynatune

10-14-2013, 03:13 PM

Z,

I should have added "in general" to the comment. There are many roads to ancient Rome and everybody will have to find his own.

Claude Rouelle

10-14-2013, 03:24 PM

A few additional perspectives to previous valuable comments;

1. Different antidive = different bumpsteer. If you change your antidive, you change the caster variation. As the upright rotates around its own hub (while the hub goes up or down Vs the chassis) the pick up point of the toe link on the upright will also follow a different trajectory, That will change your toe and your toe variation (bumpsteer). A lot? Well it depends of your caster, your antidive, your wheel Vs chassis movement. Just check it. It will also slightly change your Ackermann. In any case worth to look at it in holistic, global way. And in 3D.

2. The more antidive the less dive under braking so the less spring movement. Yeah but the energy has to go somewhere. And where does it go? In your suspensions elements. I have seen racing team redesigning their anti (dive or squat) from 0 to 50 % and suddendly the rod ends are too weak and/or they break suspension linkages.

3. The more anti (dive or squat) the less the spring and damper will move under longitudinal acceleration. So...another way to look at anti: the more anti the less each click of damper tuning will do something for you.

4. The issue I have with no antidive and no antisquat is the lack of control of your kinematics pitch center. It is the same issue than front view with top and bottom horizontal wishbone; the kinematic roll center is on the ground but its lateral position movement is undefined and uncontrolled. I see the usual can of worm about kinematic and force based roll and pitch center being opened again so let's cut is short: NO the suspended mass do NOT move around the kinematic roll axis or the kinematic pitch axis. But that does not make the kinematic irrelevant. The best prove is that if you change one pick up point on your car while testing it on a K&C you will find different roll and pitch center, motion ratio etc....

5. I keep wondering why some people look at the front roll center when they look at the car from the front and antidive, antilift and antisquat when they look at the car view from the side. When people speak about antidive they look at the front of the car as if the rear was not existing. Why in that case don't they look at the front view "anti" or "pro" of the left side of the suspension ignoring the right side. The notion of kinematic pitch center (pitch axis in 3D) speak to me better as it look at the whole car with integrated front and rear suspensions.

Food for thoughts.

Claude Rouelle

10-14-2013, 03:42 PM

Ahhh and one more comment

6. On some quite stiff race car (often aerodynamic cars very sensitive to ride height), the wheel rate could be about the same value as the tire vertical stiffness. In simple terms, the rule of thumb is that 1/2 of the ride height variation is due to the suspension movement and the other 1/2 to the tire deflection. (That is if you ignore the compliance) So, on that kind of car, if you go from 0 to 50 % antidive in simple steady state approximation you only change your ride height variation by 25%. The tire still deflects the same amount. For a given lateral and/or longitudinal acceleration, in a simplified steady state calculation, the "anti" value do change the distribution between geometric (through wishbones) and elastic (through spring, ARB and dampers) weight transfer but it does not change the tire deflection.

Tim.Wright

10-15-2013, 03:05 AM

On point 5 Claude, I imagine this is because in front view you have a largely symmetrical (at least in the linear range) arrangement in terms of the applied forces.

From the side view you typically have all the traction forces coming from one side and most of the braking only from the other.

10-15-2013, 06:14 AM

A few additional perspectives ...

1. ... If you change your antidive, you change the caster variation.

Originally posted by Z:
^^^ ... is a "school-boy error". Unfortunately, it is all too often repeated by all too many experts!

For the record, "anti-dive" is NOT dependent on "Side-View-Swing-Arm length". SVSA length has a direct effect on (instantaneous) castor change, but NOT on anti-dive.

I will try one more time.

If you change your antidive, then YOU DO NOT NECESSARILY CHANGE THE CASTER VARIATION!!!!!!

Z

Claude Rouelle

10-15-2013, 09:08 AM

To Tim,

I have worked in the US on asymmetrical cars where LF and RF kinematics are different. Ditto for LR and RR. I ended up acquiring a front view asymmetry perspective that I mentally used on the side view. If we want to speak about antidive and antilift and antisquat, ignoring the front while we look at the rear or vice versa then you also need to look at the LF and RF separately.

PS I sent you a private message did you see it?

Claude Rouelle

10-15-2013, 11:05 AM

It is all about making sure we have the same definition.

Some people define the side virtual swing axle as the longitudinal distance (if you want measured on the ground or parallel to the ground) between the tire contact patch (or the wheel center) and the side view instant center. In that definition the side view instant center altitude is irrelevant. Some other (I am one of them) look at the side view virtual swing axle as the direct distance between the contact patch and the side view instant center. In that case the instant center altitude is important. For a given side view instant center X coordinate if the instance center Z coordinate is 0 (side IC on the round) or 1 meter above the ground you will not have the same wheelbase or caster variation.

Similarly.... some people define the anti-dive taking into account

First definition

A. If the brakes are inboard the ratio between 2 angles
a) the first angle is defined by the ground and the line between the contact patch (contact patch is assimilated to a point that is kinematics not K&C) and the kinematics side view instant center
b) the second angle is defined by the ground and the line going from the tire contact patch and the SUSPENDED mass CG

B. If the brakes are outboard
a) the first angle is defined by a parallel to the ground passing through the wheel center and the line between the wheel center and the side view kinematic instant center
b) the second angle is defined by a parallel to the ground passing through the wheel center and the line going from the wheel center and the SUSPENDED mass CG.

That definition does not take into account the tire longitudinal grip or the brake balance. I am not comfortable with that approach because I know for a fact (simulation and experimentation) that for a given kinematics if you change the front tire grip and / or the brake balance for a given longitudinal deceleration the suspended mass will not dive (pitch) the same way nor the same amount.

Second definition

A. If the brakes are inboard the ratio between 2 angles
a) the first angle is defined by the ground and the line between the contact patch and the kinematics side view instant center
b) the second angle is defined the tire longitudinal force and the VARIATION of the tire vertical load

B. If the brakes are outboard
a) the first angle is defined by a parallel to the ground passing through the wheel center and the line between the wheel center and the side view kinematic instant center
b) the second angle is defined the tire longitudinal force and the VARIATION of the tire vertical load

Again ... we can all agree about numbers but we need to make sure we know what method / definition each other is using to calculate these numbers.

10-15-2013, 09:39 PM

Claude,

In both your first and second "definitions" above it seems you have the explanations for Inboard and Outboard brakes back-to-front.

That is, 1st-A and 2nd-A should refer to Outboard brakes, and 1st-B and 2nd-B should refer to Inboard brakes.
~~~o0o~~~

[2nd defn.]
...the second angle is defined the tire longitudinal force and the VARIATION of the tire vertical load.

More importantly, this above quote just doesn't make sense to me,

What is the "VARIATION of the tire vertical load" due to?

Is this "second angle" = arctan(vertical-jacking-force/longitudinal-tyre-(braking)force)?
If so, then it is the same angle as the "first angle" (ie. wrong!)!

Or is the "second angle" = arctan(longitudinal-(vertical)load-transfer/longitudinal-tyre-(braking)force)?
In which case the "kinematic %-anti-dive" varies according to how hard you push the brake pedal (ie. wrong!)!
~~~o0o~~~

Anyway, none of the anti-dive "definitions" above suggest any sort of connection between %-anti-dive and castor-variation.

There is NO direct dependence between %-anti-dive and castor-change. To suggest that there is a dependence is MISLEADING (albeit all too common these days... :().
~~~o0o~~~

4. The issue I have with no antidive and no antisquat is the lack of control of your kinematics pitch center. It is the same issue than front view with top and bottom horizontal wishbone; the kinematic roll center is on the ground but its lateral position movement is undefined and uncontrolled. I see the usual can of worm about kinematic and force based roll and pitch center being opened again so let's cut is short: NO the suspended mass do NOT move around the kinematic roll axis or the kinematic pitch axis. But that does not make the kinematic irrelevant. The best prove is that if you change one pick up point on your car while testing it on a K&C you will find different roll and pitch center, motion ratio etc....

MUCH MORE IMPORTANTLY...

In the above, are you suggesting that a horizontally "migrating" pitch centre is a bad thing?

Do you also believe that a horizontally "migrating" roll centre is a bad thing??

If so, then can you give a RATIONAL MECHANICAL EXPLANATION for WHY these horizontally "uncontrolled" pitch or roll centres are bad things???

(Please, no meaningless anecdotes about "... we changed one pick up point and the car was 2 seconds slower...". That is NOT good reasoning.)

Z

Claude Rouelle

10-15-2013, 11:30 PM

Yep Z is correct (Z is going to print this and pin it on his bedroom wall! :) ) I did mess up inboard and outboard on my last post. Sorry to everybody.

For the rest I have chosen long time ago to avoid entering in unfruitful controversy, even more as most answer to Z questions / comments have already been posted on other areas of this forum.

mams

10-17-2013, 07:08 AM

Z,
I also have a question regarding how much anti-anything is good. The way I see it my goal should be to design in as much anti as I can without passing a certain threshold (the threshold being the point where jacking forces are created with steering angle increase thus undesirable handling). Is there any way to quantify that point? Also I understand that anti basically takes a percentage of the longitudinal load transfer away from the springs and into the a-arms, do control of the pitch center movements have any kind of correlation, direct or indirect, with longitudinal acceleration? Obviously I don't want my centers to be skyrocketing every time I brake but compared to roll center movement, how important is pitch center variation under braking and acceleration.

Honestly any guidance would be much appreciated. Anymore all nighters for suspension and I'll be taking too much time away from other aspects of the car. Our suspension team has designed a setup that handles the front view fairly well ( minimizes roll center variation and theoretically will achieve the desired camber angle in a corner) but I would like to go to competition knowing exactly why I chose 6% anti-dive or anti-squat over 10% or 15%. Thanks in advance!

Also I'm from Temple University and this is my first time as suspension lead

edit: Hah, I should pay attention to who is posting. Claude your more than welcome to answer. Also I posted without realizing there was a second page, looks like I have some reading to do.

10-17-2013, 09:01 PM

The way I see it my goal should be to design in as much anti as I can without passing a certain threshold...
...
Honestly any guidance would be much appreciated...
Mams,

My genuine, non-condescending, advice is, YOUR GOAL IS TO BUILD A CAR THAT CAN TRAVEL 20 MILES, AT AN AVERAGE SPEED OF 30 MPH, WITHOUT BREAKING DOWN.

No amount of "optimising" anti-dive will help you achieve this. Rather, time spent optimising will most likely harm your efforts. More below, but in short, any anti-dive between about -10% and +30% could be workable on a winning car.
~~~o0o~~~

Part of the reason for the above wideish range of acceptable numbers is that a secondary goal in FSAE (IMO) is to USE THE BRAKES AS LITTLE AS POSSIBLE. Instead of using the brakes, and then also having to stomp on the accelerator, just go fast around the corners, and thus be the fastest car WITH the least fuel burnt.

Dynatune covered most of the pros and cons of different levels of anti-dive back on page 1. On a smooth FSAE-typical track you can have quite high anti-dive without adverse consequences. On a bumpy road-racing circuit I would stay under ~20% (better under 10%?, else the front-wheels will start getting air over the bumps on corner entry).

Off-road racers typically have large amounts of pro-dive, mainly to give lots of "wheel recession" for better ride over the bumps (wheels move backward in bounce). But off-road racers also have front brakes the size of milk bottle tops (the 2WD cars are rear heavy, and racing is about going fast, not slow!).

There was a period in Lotus's history when they were trying to get as much pro-dive as possible. This was early in the aero era, and Chapman figured that lowering the front wing, via pro-dive, on corner entry would give the car better downforce through the rest of the corner. They also had pro-roll (RC below ground level) to jack the car DOWN in corners, because the straights were quite bumpy and the car had to have a highish static ride height.
~~~o0o~~~

... compared to roll center movement, how important is pitch center variation under braking and acceleration.

For some understanding of antis (pitch or roll) have a look at the "Jacking Force" thread. (http://www.fsae.com/forums/showthread.php?4063-Jacking-force&p=17492&viewfull=1#post17492)

The key point to take from that thread is that you only have to know the longitudinal (for pitch) and lateral (for roll) n-line slopes (aka "force line slopes") to be able to calculate the anti-pitch, anti-roll, and jacking behaviour of the car. (Well, you should also know how to calculate the slopes correctly, realise that a single wheel can have different longitudinal n-line slopes for accelerating and braking (because, say, inboard drive and outboard brake), and you must know the magnitude of Fx and Fy road-to-wheelprint forces at each wheel.)

But, VERY IMPORTANTLY, the location of the intersection point of the pair of close to horizontal n-lines in side-view (= the "PC"), or in end-view (= the "RC"), DOES NOT MATTER AT ALL!!!!!

It should be apparent that n-lines that are always close to horizontal wrt the car floor-plane, have an intersection point ("PC" or "RC") that zooms off to infinity ("parallel lines meet at infinity"), or at least out past Pluto, with any small amount of body pitch or roll. But this make NO DIFFERENCE to the amount of anti-pitch, anti-roll, or jacking forces, which, again, depend only on the slopes of the n-lines.

Of course, Claude feels that the PCs/RCs have some sort of magical powers, the details of which he doesn't want to reveal, so you might get marked down in the Design tent if you give him an old-fashioned "Mechanical" explanation for how these things work.

Based on the longitudinal and lateral n-line slopes of all the cars I have ever driven, I reckon the closer to horizontal (wrt car floor) that the n-lines are (and remain), then the more benign the handling, and the better the ride and grip over bumps.
~~~o0o~~~

Bottom line, pick the antis that give you the simplest, quickest-to-build, most reliable car you can manage, providing the antis are roughly in the range given above (ie. n-lines closer to horizontal are better). If a particular concept has anti-dive = 13.7819...%, but it also gives you an exceptionally neat and tidy (and strong and stiff and light and easy-to-build+++) car, then go for it!

Z

dynatune

10-20-2013, 02:25 PM

Mams,

I think you have gotten a lot of useful hints. Remember that there is no magic to physics (it is not quantum physics) and many people have their own opinions on what is important and what is less important, but the bottom line is understand your system. If you do need a practical number to start with go for a value between 25% to 35%, I have designed quite a lot of car's and that is a "walk in the park" number on which you cannot do much wrong. If I may recommend, do create for yourself a simple excel sheet with some basic formulas that are important for you and do your trade off studies,excel is the perfect tool for that. Feel free to look around on my site too for inspiration.

Cheers,
dynatune, www.dynatune-xl.com

10-20-2013, 09:01 PM

Mams,

Some further comments regarding "How to design an FSAE car?". :)
~~~o0o~~~

This is the WRONG WAY.
====================
Scene - FSAE Team Weekly Product Development Design Review Committee Meeting.

Team-Leader "Geez guys, we're already a cupla weeks behind, and we gotta get this design stuff sorted soon! Suspension-Guy, we're still waiting for your kinematic numbers ..."

Suspension-Guy "Nuh, no problems, I got'em all here. See, I benchmarked all the expert opinions, ... aaaannd ... based on that I did this great Matlab-based Monte-Carlo optimisation routine ... computer had to run all weekend!!! Anyhooos, ... it's given me these super accurate results for all the main parameters. Like here it says we have to run 18.74281% anti-dive, and here ..."

T-L "Hang on ... what's that Monte stuff anyway? How does it work?"

S-G "Err ..., not really sure. But it IS super accurate ..."

Frame-Guy "Whoaaa!!! These wishbone mounting points are gonna make the frame a freakin' nightmare! There's gonna be tubes running everywhere... And count the nodes!!!"

S-G "Yeah, well that's your job... That's why you're on the team..."

Aero-Guy "I don't think I can fit my undertray in that space..."

S-G "Oh, stop wingeing A-G, and there is no "I" in "team". Anyway, aero's a wank! FSAE is all about screamin' acceleration!! Pull-your-face-off braking!!! Stuff like that..."
~~~o0o~~~

This might be a BETTER WAY.
=======================

T-L "So, updates everyone. Keep in mind we're cutting metal in five weeks. What you got S-G?"

S-G "Well, we chopped up that old car and made all the suspension parts super adjustable. Got in a good 12 hours testing on it so far. But here's the funny thing,.. err, sorta funny-strange... All the good drivers said they could FEEL a difference between the settings, but for most of the settings, sorta in the middle of the range, well, ... the drivers couldn't really say which was better or worse??? Bottom line, I think we've got quite a bit of freedom in how we do this..."

F-G "I'll jump in here ..., ummm, 'cos I was doing some of that driving. Anyway, I reckon if we do this sort of layout, ... look, here's a sketch ..., then we can get really good camber behaviour ..., it's that short-FVSA stuff you talk about S-G ..., AND the frame can be super simple! Look, just a cupla straightish tubes running through here, and a minimum of nodes..."

A-G "Yeah, I like that too, because it gives me plenty of room for the main undertray stiffening ribs..."

S-G "Hmmm, well, yeah, maybe ... but it gives kinda low anti-dive ... though I guess the testing showed that didn't matter too much..."

A-G "Well, I want to put a main undertray cross-spar here, to take the aero loads out to the wheels, and you could fit a "third-spring" directly to it... You know, like the aero-springs on "real" racecars ..., but more direct..."

S-G "Yeah, I see... Then I can run softer corner springs, which is always good for grip. Let's have a closer look at that sketch..."
~~~o0o~~~

Of course, when you get real jobs you will probably have to do it the top way. FSAE might be your last and only chance to try it the bottom way... :)

Z

mams

10-23-2013, 02:37 PM

Thanks guys, I really appreciate the input. Bottom line, design something that is manufacturable while keeping the other aspects of the car in mind

jpusb

11-15-2013, 03:19 AM

To mams and gautham:
Claude put a very subtle but important point in his previous post (point number 5). An anti-dive front suspension setup will partly help control the pitch attitude of the car. That, along WITH the rear suspension geometry is what defines the real pitch attitude you will see. At the end, you can do many very different things, and as many have said already, it depends on WHY you want (or need) anti-something, and why do you think it will help (or will it?).
Both of you (gotham and mams) did not put here the reasoning that lead you to think or decide you needed anti dive.
And for anybody else that does not have these concepts clear, I recommend to go ride a bike with suspension, more than 100mm of travel, to amplify effects (hint: it has independent front and rear brakes, and anti-lift geometry at the rear).

DannytheRadomski

11-17-2013, 08:09 PM

http://www.youtube.com/watch?v=bbgjRBT4ltM

Claude Rouelle

11-18-2013, 04:47 AM

jpusb

Good point.

The front AND the rear "anti" geometry will play a roll in your pitch and heave Vs time during longitudinal acceleration. But so will your springs and dampers and suspended mass and non suspended mass inertia. Kinematics is not everything but it is the first thing you should be looking at because you can't patch a bad kinematics with spring and damper adjustments. Archimedes said "give me a leverage and a application point and I will lift the world". A force is defined by an application point and a direction and an intensity. Worry about the application point and the direction before you worry about the intensity.

Some people look at the anti-dive totally ignoring the rear suspension as if there was no rear part of the car. Or the anti-squat as if the front was not existing. If they want to think that way then they need to think about the anti outside wheel going up and the nati inside wheel in droop with the car in roll in front view.

FSAE / FS car have about 1200 mm of front and rear track and about 1.5 to 2.8 (with downforce) G of lateral acceleration. And about 1600 mm of wheelbase and 1.5 to 2.5 and even 3.0 G in braking (with good drivers). So it is not as if longitudinal weight transfer is negligible, correct? Then I wonder why some students spend weeks thing about roll centers (axis) (kinematics and ideally force based) but they do not think about pitch centers (axis). The car is a whole, it has 4 wheels and you need to think 3D. Looking at the car in 2D is a good, simplified process but it is only a beginning.

When students see their car lifting the inside rear wheel they think, spring, damper, roll centers etc.... but what if this inside rear wheel lift started in the braking zone...?

And do not forget your compliances; it can completely screwed up your kinematics target reaching.

11-18-2013, 07:15 PM

Claude,

Since you often promote the idea that Roll and Pitch Centres (or -centers, or -axes) are a good way of thinking about the car as a whole, can you please help me with this following problem?

In a simplified 2-D analysis, consider the end-view, or side-view, of a car (doesn't matter which). The horizontal distance between the two wheelprints (= track or wheelbase) is 2 metres. The left wheelprint has a "force-line" (or "n-line", or "line-from-wheelprint-through-its-IC"...) that slopes up-to-right at 1:100 (= rise:run, or ~0.01 radians up from horizontal). The right wheelprint has its "force-line" sloping up-to-right at 0.010,000,000,1 radians.

By my figuring, the above car has its Roll or Pitch Centre (which is the intersection point of the above two force-lines, or n-lines), at an altitude of 2,000 KILOMETRES (ie. well and truly into space!), and at a lateral position of 200,000 KILOMETRES to the right of the car (ie. half way to the Moon!!!).

So here is my problem.

Q1. Is the above a GOOD place, or a BAD place, to have a Roll or Pitch Centre? Please give reasons WHY???

Q2. If the right wheelprint has its force-line slope up-to-LEFT at 0.01 radians (ie. still VERY close to horizontal), then the intersection point (= R/PC) is at mid-track/wheelbase, and at a height of 1 cm. Is this a better position for the "Centre"? And, most importantly, WHY???

Awaiting your helpful insights... :)

Z

Claude Rouelle

11-19-2013, 12:45 PM

So here is my problem.

Z

I think you have more than one.

Tim.Wright

11-19-2013, 02:31 PM

This just tells me you have either low anti-dive and low pro-lift (side view) or a slightly asymmetrical roll setup (front view).

Whether you choose to define it in coordinates of:
[n-line slope front, n-line slope rear]
[n-line slope left, n-line slope right]
[pitch cen Z, pitch cen X]
[rol cen Z, roll cen Y]

is pretty irrelevant IMO.

There seems to be a growing acceptance that the body doesn't roll about the roll centre or pitch about the pitch centre so I don't really see the problem with using them as a set of "general coordinates" to define a suspension design. As you have demonstrated, sometimes they aren't convenient to use - but often they are. Really not worth agonizing over...

Big Bird

11-19-2013, 04:13 PM

Some entertaining discussion here, I've picked up quite some knowledge about anti-'s from reading through. Thanks all.

From my perspective, (and with my usual " what are the project priorities" cap on), I think the OP might want to answer a few higher level questions first.

1. What is your understanding of the track you will be driving on? e.g. How many corners will you be braking into? Get some old track maps, analyse, get a feel for the percentage of corners entered under significant braking versus those entered at a reasonably constant speed. In my year the previous years track was 20% former, 80% latter

2. What is your vehicle concept? A car with a stonking great motor that goes like stink in a straight line but needs to slow right down to get around the corners, is going to be a lot more sensitive to braking anti's than a smaller, lower horsepower car that might not be so fast in a straight line but carries higher mid corner speed

3. Driving style? Some drivers go like stink in a straight line, and almost come to a stop mid corner (squaring off the corner), others prefer to run the car around as wide an arc as possible and maintain as high a mid corner speed as possible. The latter is less brake dependant

4. What is your understanding of the returns of braking versus cornering? A simulation I did with the above mentioned track indicated that a 1% increase in cornering performance would give 14 times the points return of a 1% increase in braking performance (due to the small number of braking points on the track and our low-power, high mid-corner speed vehicle design objective)

Working through the above process, we put anti-dive on the minor priority list and just made sure we had some adjustment in the ball-joint locations so we could experiment if it became a priority

Hope that is a useful contribution,

Geoff

DannytheRadomski

11-19-2013, 06:25 PM

Triplex suspension keeps you from diving or squatting http://www.youtube.com/watch?v=bbgjRBT4ltM

11-19-2013, 07:34 PM

Claude,

Your reluctance to offer an answer to this problem strongly suggests that you do NOT understand it. Certainly, at your 2002 seminar that I attended you didn't seem to understand it.

Do you really believe that you should continue charging students (and OEM Engineers+++) for your seminars, when you cannot explain this very simple, and very typical, Vehicle Dynamic problem?

Maybe the following will help.
~~~~~o0o~~~~~

Tim,

As you have demonstrated, sometimes they [RC or PC] aren't convenient to use...

Your quote is at the crux of the matter. My example is typical of any car that normally has close-to-horizontal n-lines wrt car-floor, (or close-to-ground-level RC/PC), and has then rolled or pitched about 0.6 degrees in an anticlockwise direction (so both n-lines now slope up-to-right at ~0.6 degrees = 0.01 radians). This is an EXTREMELY COMMON occurance for many real cars...

Now, here is where the problem gets interesting. Any further very small movement of the car can have the R/P "centre" shooting off from its nearby 200,000 km, to rightmost INFINITY. In another blink of the eye the "centre" is at leftmost infinity, and coming at you at googleplex x the speed of light. Very importantly, this crazy R/PC movement makes absolutely no difference to the dynamic behaviour of the car, which generally behaves very well with these sorts of n-line slopes.

BUT!!! Any calculation that a Suspension Engineer tries to do will immediately CRASH!!! "Machine overflow"! "Variable out of range"! "Divide by Zero"! And so on...

Essentially, the choice of using the horizontal-vertical coordinates (eg. RC(x,z)) of these particular "centres" to characterise the suspension is a bad one. This is because right in the middle of the range of well behaved suspensions (ie. those that typically have horizontalish n-lines), the R/P centres dance around far too much. Off to infinity they go in the blink of an eye...

In computer programming terms, the calculation code is poorly structured. There is a part of the code where a middle-of-the-range number is often divided by an extremely small number. Hence the "divide by zero" error, or at least, much loss of accuracy. Specifically, in my earlier example the horizontal position of the centre is found by dividing a finite number by the angle between the two n-lines, which is often zero...

Anyway, when you describe the suspension in terms of its n-line slopes, which still only takes two numbers in this simplified 2-D example (ie. 1 slope per wheel), all the calculations are much simpler. Any control-arm-force (= force acting along the n-line) is easily decomposed into horizontal and vertical components (see "Jacking Force" thread for graphic explanation...).

(Edit: Note that many experts (err, say, Danny...) manage to further cock-up the calculations by only moving the horizontal control-arm-force components to the R/PC, and forgetting to also move the vertical (jacking) components there as well. Naturally, the errors magnify enormously, even when the R/PC is only a few kilometres away! With the n-line approach all the force components can be kept at their wheelprints.)

Extrapolating to full 3-D analysis is also very simple, requiring just lateral and longitudinal n-lines at each wheelprint. And there is no possibility of confusion between a rolling MOTION about a "roll centre or axis", and the calculation of FORCES acting at some "force-based roll centre".

Finally, by far the biggest DISADVANTAGE of thinking in terms of R/P "centres", is that the above calculation problems have led people like Claude (and also Pat, and who knows how many other DJs), to pressure students, and possibly also OEM Engineers (?), to avoid the benign "sweet spot" of supension kinematics (horizontalish n-lines), and instead waste vast amounts of time "optimising" the kinematics to prevent "R/PC migration". All because of a lack of understanding of simple Mechanical problems.

Well, in fact, it all started with the failed education system... But until someone tries to fix it (Claude!) it is only going to get worse...

Z

(PS. Danny, just saw your post. Do you really need four spring/dampers to control two wheels? Note that you can control dive and squat by fitting almost-rigid corner springs. In fact, that is the way most race teams do it. Stupid, but true...)

Big Bird

11-21-2013, 04:51 AM

Actually, I have wondered that myself Z. Driving from Melbourne to Wangaratta this morning, I performed a little "experiment". I gently swerved from left to right, maybe 5-10cm either side a straight line, maybe 100m per steering input. My "yaw centre" was wildly crossing the country, zooming from Perth to Auckland (approx) each 5 seconds. Yet my beat up old 94 Mazda ute calmly continued on...

A distant rotational centre just means you are almost moving in a straight line. No big deal...

Tim.Wright

11-21-2013, 12:26 PM

Big Bird

11-21-2013, 12:49 PM

Sorry 'bout that Tim. I thought I'd lost it for a moment there. If you see any of my IC's out on the loose again, give them a stern warning and send them home.
Anyway, I'm off to the roll centre to get some lunch. Want me to grab something for you?

11-21-2013, 07:39 PM

Originally posted by Big Bird:
My "yaw centre" was wildly crossing the country, zooming from Perth to Auckland (approx) each 5 seconds. Yet my beat up old 94 Mazda ute calmly continued on...

Originally posted by Tim.Wright:
There I was minding my own business here in Germany when an instant centre flew past me at MACH 50 and knocked me on my arse.

For the record, I should point out the distinction between the "centres" that describe MOTIONS (as BB's above), and the "centres" where different FORCES can be added together (eg. at the intersection of n-lines (= "force-lines"), as in my above post).

Interestingly, in typical VD analysis the motion centres (strictly speaking, the 3-D "Motion Screws", or "ISAs") dance around a lot more than the force centres. This is because whenever a car is travelling in an approximately straight line, its "motion centre" (eg. of "yaw", as BB's above) is at ~infinite distance to one side of the car. A slight change of direction and the "centre" can be extremely far away on the OTHER side. Or it is above you. Or below...

In fact, these "motion centres" are somewhat like tiny little angels who have drunk far too much energy drink, and are now teasing ALL the photons in the Universe and calling them "slow-coaches".

By comparison the "centres" where forces are combined spend most of their time near the car, at the intersection points of n-lines. It is only when the n-lines go to parallel, and then past parallel, that the centres flit off to infinity on one side, and then come screaming back from infinity on the other side. Yippeeeee!!!...
~~~~~o0o~~~~~

But back to the big problem here.

Originally posted by Claude Rouelle (top of page 2):
4. The issue I have with no antidive and no antisquat is the lack of control of your kinematics pitch center. It is the same issue than front view with top and bottom horizontal wishbone; the kinematic roll center is on the ground but its lateral position movement is undefined and uncontrolled.

Note that all these "centres" are a fiction. They are like little angels that help our imagination when we have to calculate things.

But Claude says that we must imprison these helpful little angels. For, if they leave the confines of the car, ... then, ... well, ... they become THE DISCIPLES OF THE DEVIL!!! Disaster will befall us all! Death, destruction, and general mayhem will ensue. And all these bad things will happen, even though real cars seem to manage just fine whenever their various force and motion angels are hithering-and-thithering around the Universe.

I guess you students have the choice of superstitious beliefs in Voodoo and Black-Magic, or more practically testable beliefs in boring old-fashioned Mechanics... :)

(And BTW, while I have no problems with these little angels zipping around the Universe, I note that their travels complicate numerical calculations (too many zeroes and infinities), so hence suggest the n-line approach as being easier (with force components calculated at their wheelprint, as in the "Jacking Force" thread).)

Z

Tim.Wright

11-22-2013, 12:43 AM

Fair enough...

I'm still yet to hear an acceptable explanation as to why moving roll and pitch centres are a bad thing.

A couple of years ago we tested a racecar with dynamometric wheels. My job was then to do various analyses on this data of which I dont want to go into too much detail. But I can say that at some points we have seen that the kinematic roll centre was passing through the ground. Obviously when you have this situation there is a point just before and after passing through the ground that the lateral position is undefined (i.e. with road noise etc its going from pos neg infinity). The classic devil condition that we are taught to avoid.

The interesting thig is that not only did the car make the corner unscathed, but looking at the wheel forces, there was absolutely no apocalyptic behaviour. There was load transfer to the front axle under braking, load transfer to the outside wheels in the middle of the corner, and load transfer to the rear axle on corner exit. Quite boring actually.

The drivers comments about this condition: "the car feels so heavy with the dyno wheels..."

I had already decided that migrating pitch/roll centres were the bullshit of the century but this test for me was just proof.

Greg: grab me a roll mate, will fix you up later!

Tim

Claude Rouelle

11-22-2013, 11:15 AM

Tim,

Thank you for your positive and constructive sharing of experience. We have experienced the same thing in our vehicle test in Argentina in 2010 (it was fun and a great learning experience with 12 days of DOE tests with a car equipped with a big number of sensors, including 6 slip angle sensors and 4 WFT = Wheel Force Transducers) although I have to say that to locate the force based center (or the intersection of force lines as some people call it) we had to majorly filter the data because of the WFT noise. That is why simulation and K&C test benches (where you can isolate some other input, minimize the noise and make parametric study is still quite useful. It is the same issue with tire modeling; WFT will show you the tire forces and moments on track reality but it is sometimes so noisy that it is not always fully exploitable. On the other hand tire testing machine are not the reality (the belt grip is sometimes far way from the asphalt grip just to name one issue) but it allows to make specific, pressure, camber, load, etc.. parametric study. Modeling is not simulating. Ideally you need both. Same as CFD and wind tunnel.

But let me ask you a simple question and honestly compare our experience: if you change one or a few suspension X, Y, Z pick up point coordinates let's say 20 mm or even "only" 5 mm do you or don't you see car behavior difference?

My experience is that I have most of the time seen difference in tire wear and tire temperature distribution, grip, balance, stability and control, objective data measurements and driver subjective appreciations. I do not say and I never said hat kinematics is the nirvanah of vehicle dynamics; it is part of the puzzle at the same level than aero, springs and ARB, damping etc... However I also know and maintain (I already wrote this) that a wrong kinematics cannot be patch by so called "good" spring and ARB and damper and aero choices. And that we should work FIRST on the kinematics before anything else.

Examples? We have dozens experiences in different racing series where drivers complained of understeer when we remove front negative camber but if we keep the high front negative camber the drivers could not complete the whole stint distance because they were damaging the front tire inside shoulder, blistering the compound or even blowing up tires. We worked with the tire model and some simulation and found an appropriate VSAL to reduce the camber change in roll. We make sure that other things such as bumpsteer remained the same. The result were always successfully confirmed (if not the perfect amount at least the right direction) and then tuned with IR senors.

So Kinematics is certainly not negligible. Some F1 teams who believe that performance is essentially aerodynamics are still learning it the hard way.

This is what I know. What is your experience?

Tim.Wright

11-22-2013, 12:17 PM

The examples that you have given there seem completely reasonable to me. Our work with the dyno wheels (sadly) did not involve changing the kinematics. Only a few tuning parameters. Also (sadly) I have not had the time to look into the changes in detail. I was focused more on developing a way to calculate the wheel forces without the dyno wheels.

However, this is slightly off topic... Camber gain is a very real and measurable geometric quantity with very obvious effects on inclination angle. The argument at hand rather is about roll/pitch centers which are much much more abstract.

The 64 thousand dollar question is this: Has anyone ever had a problem on a car that was conclusively pinned down to excessive horizontal migration of a roll or pitch centre?

My experience (not a lot, I've been doing this for only 5 years) on the matter is this:
1. I have never reached this conclusion myself (as I mentioned previously, there was no problems with our test car as the RC went underground).
2. I also have not come across anyone else who have reached this conclusion based on either test or sim data
3. I've never even heard a satisfactory verbal or written explanation of why its a problem.

Tim

Claude Rouelle

11-22-2013, 02:34 PM

Tim,

You are right: I demonstrated the relative importance (at least the importance in the chronology of kinematics design in the car concept) with camber "gain" on roll and heave.

But as far as roll centers, my experience is that the roll centers altitude has an influence on tire wear and tire temperature (track variation). Also, in the corner entry, because of the suspended mass inertia, the "geometric weight transfer" = forces going though your suspension elements, plays a larger role that the "elastic weight transfer"= ARB, spring, damper then anywhere else in the corner.

Also with roll centers above or under the ground you will have different jacking forces and subsequently different dynamic ride height; not negligible on a aero car sensitive to ride height.

As far as pitch center I can tell you that I had the chance to change the pick up points of a rear suspension in F3000 and Indycar (the anti-dive) a few years and when you brake (or pull the hand brake with the rental car :) ) your laser sensor and suspension linear potentiometers show that you do not get the same dynamic ride height.

But hear me well, I did not say that kinematics made spring, ARB, damper, inertia and compliance irrelevant.

Tim.Wright

11-22-2013, 03:49 PM

I'm still agreeing with you here. In my opinion, roll centre heights and anti X geometry are extremely important because they affect A: normal loads on the tyres and B: body attitude.

I also agree that roll centre heights are important in corner entry (actually I would also add corner exit and any other transient situation). Though I don't necessarily see a problem with letting it go through the ground plane.

Where I'm not on the same page is when we start talking about horizontal migration of the pitch/roll centres. I.e. lateral roll centre migration and longitudinal pitch centre migration. They seem to be a pretty useless construct to me. As Erik has pointed out, their definition falls down when the roll/pitch centre heights tend towards zero. They become uncontrolled and move about wildly which suggests something drastic is happening. But when you look at the tyre forces (either calculated or measured) nothing interesting is happening at all.

Big Bird

11-22-2013, 04:48 PM

OK, I've just had one of my "let's investigate this with a spreadsheet" epiphanies.

Objective: investigate lateral IC migration on tendency of an object to accelerate rotationally
System: 10kg object, 1kg.m^2 MoI around its own centre, located 50cm above its virtual IC. Object experiencing 10ms^2 gravity, 10ms^2 lateral acceleration.
Method: Shift IC horizontally in increments, retaining 50cm vertical displacement. Recalculate total torque on system for each new IC location, also recalculate MoI of system for each new IC location.
Calculated output: Rotational acceleration of 10kg object around its IC (= total torque / MoI)

Findings:
Rotational acceleration when IC directly under mass = 14.2857 rad.s^-2
Rotational acceleration when IC offset 0.01 metres from directly under mass = 14.567 rad.s^-2 (approx. 2% difference from central point)
Rotational acceleration when IC offset 0.1 metres from directly under mass = 16.666 rad.s^-2 (approx. 16% difference from central point)
Rotational acceleration when IC offset 0.28 metres from directly under mass = 18.24 rad.s^-2 (approx. 27% difference from central point)

So we see that the tendency to rotationally accelerate changes, and it is increasing as we shift the IC laterally
Now, from here on in, things get interesting as we shift the IC further laterally. The MoI starts taking over, and rotational accel begins to diminish for increasing lateral shift

Rotational acceleration when IC offset 0.7 metres from directly under mass = 14.284 rad.s^-2 (approx. 0 % difference from central point)
Rotational acceleration when IC offset 1.0 metres from directly under mass = 11.111 rad.s^-2 (approx. -22% difference from central point)

Rotational acceleration when IC offset 2 metres from directly under mass = 5.74 rad.s^-2
Rotational acceleration when IC offset 3 metres from directly under mass = 3.74 rad.s^-2
Rotational acceleration when IC offset 4 metres from directly under mass = 2.75 rad.s^-2
Rotational acceleration when IC offset 5 metres from directly under mass = 2.17 rad.s^-2
Rotational acceleration when IC offset 6 metres from directly under mass = 1.79 rad.s^-2
Rotational acceleration when IC offset 7 metres from directly under mass = 1.52 rad.s^-2
Rotational acceleration when IC offset 8 metres from directly under mass = 1.32 rad.s^-2
Rotational acceleration when IC offset 9 metres from directly under mass = 1.17 rad.s^-2
Rotational acceleration when IC offset 10 metres from directly under mass = 1.04 rad.s^-2

Rotational acceleration when IC offset 100 metres from directly under mass = 0.1 rad.s^-2

Rotational acceleration when IC offset 1000 metres from directly under mass = 0.01 rad.s^-2

So for example if our IC moves 900 metres, from 100 to 1000 metres offset from directly under the mass, then we are getting no greater "delta accel" than if we moved the IC from directly under the mass to a point 3cm horizontally.

A roll centre migrating around close to the vehicle centreline might have some effect on our delta rotational accels. But the influence dissipates as we get further from the vehicle. A roll centre moving rapidly under the vehicle would cause problems, but a roll centre blasting through space at 10 times the speed of light, while I am driving here in Australia, is simply telling me that I'm approaching zero rotational acceleration. And that signifies to me a point that could be quite stable

Of course, if you believe that a roll centre that disappears into infinity to your left, and then reappears from infinity to your right, could only have passed under your car to get there - then this is all going to seem like the car is going to tie itself into a knot. But it doesn't.

IC's are constructs to help us visualize and conceptualize the rotations of objects. It is the motion of the objects we design that is priority, and I think we are closing ourselves off to potentially good solutions if we give the motions of our imaginary constructs a life of their own.

Cheers all, and enjoy your lunch Tim :)

Claude Rouelle

11-22-2013, 07:20 PM

Tim and Geoff,

Very good input. Thank you. That helps everybody to think. I have been right and I have been wrong in the past. I could be right and I also could be wrong again and I am ready to change my mind but the following is what I believe and what I have experienced....

Tim,

I do not have an issue to have a roll center going through the ground...except that at that time the lateral roll center migration will be difficult to control. But.... as Geoff remarks infinite left or infinite right, it is still a huge inertia so if you look at the transient behavior the effect is small if not negligible. Similarly I do not have a problem with a roll center crossing a tire (well.... I am a bit less sure about that) but I do have a problem when the front roll center goes towards one side of the car and the rear one towards another.

However when you take your tire model and your tire forces vertical, lateral and longitudinal loads ... (which are kinematics, AND spring and ARB and damper and inertias dependent), your compliance (that will screw up all your initial estimations), the "real roll centres" will be in different position so you have to decide how far you want to go in your simulation. In fact it depends on your input accuracy and relevance

I mention corner entry because that is where driver make the difference more than corner exit but you are right: corner exit is transient too.

Geoff,

You touch the most important point that I have experienced in my career as far as kinematics in concerned. Most of people think steady state and think about kinematics as a way to qualify and quantify things like camber variation in heave, pitch, steering, also bump and roll steer, anti-dive and anti-lift and anti-squat and roll and pitch centers (axis) etc... but my experience in both simulation and on track testing is that the biggest effect of kinematics is on transient.

In fact I can make 2 cars; one with a low roll axis and stiff front and rear ARB and a second one with high roll axis and soft front and rear ARB having exactly the same behavior, same roll angle, same cambers, same wheel vertical loads, same lap time IN STEADY STATE but I guarantee you that the car will be very different in transient.

Many people look at roll centers a) in simplified way in their kinematics definition b) in a bit more complete way (but still not perfect) as the intersection of "forces lines" but they ignored the Huygens (or parallel axis) theorem which will influence the rotational acceleration of both suspended and non suspended masses.

In fact in a simplified 2 D front view, you have a suspended mass rotates around a roll center, 2 suspended masses rotating around their own instant center and these 3 centers are not mechanically but kinematically linked.

***

There are simplified and therefore incomplete and inaccurate kinematics software on the market. That is why they are cheaper than tools like for example Adams. But one one hand I have met probably over 100 engineers using Adams and I only know 2 of them you can use it properly and these guys are close to Gods in both vehicle dynamics and C++ and have 20 + years of work experience. On the other hand, when our mother taught us our language she did not read us Shakespeare or Moliere or Goethe. I believe that the simple version of kinematics definition for students who are about to design their first car is more useful than a full system of second order differential equations. Simple then complicated not the other way around. That is what I have seen in my teaching experience.

When you have an Indycar at the entry of a banked corner on an oval where you put the front roll center between the front wheels and under the ground and the rear roll center above the ground and the right side of the car and that you see on the data that you have more load on the rear pushrods, more lateral and vertical G on the chassis BUT the rear dampers are in EXTENSION and the laser shows you that the rear ride height INCREASES (while the front goes down) and that you have experienced that the amount of these changes can be modified with slightly different suspension pick up points coordinates (ideally confirmed and refined with K&C measurements) then you realize that kinematics can help you to achieve specific dynamic ride heights in specific part of the corner IN TRANSIENT (as F1 active suspension were doing) then you think differently about kinematics.

Page 168 and following of the Maurice Olley book give a beginning of answer to these questions. The mix of yaw axis and roll axis is a topic that I do develop in the OptimumG 10 day advanced workshop.

Can't wait to see Bill Cobb, Doug Milliken, Dynatune comments on this subject. Every will be a winner.

Big Bird

11-23-2013, 05:00 AM

I've signed in around half a dozen times today to see if Bill, Doug or Paul had chimed in. Aaarrggghh. Suspense is killing me!

Geez these forums are quiet these days. Back when I was a young fella....

:)

Geoff

Big Bird

11-23-2013, 05:00 PM

Claude, I've always been of the understanding that roll centres were primarily about transient behaviour. Even the most basic treatments (e.g. the railway truck on a pivot) are presented in terms of dividing up the instantaneous and time-dependant weight transfer. Dunno why people would ignore this. Oh well.

Parallel axis theorem for MoI seems to be forgotten by quite a few as well. I cant remember who it was, but I read one theory where the author claimed you only need to consider the height of a roll centre, and that the horizontal location of the RC was negligible. I was rather troubled by this, horizontal location has a big effect on MoI. I do remember it was a respected dynamicist who said this, but cant remember who.

Other input, anyone??

Cheers,

Claude Rouelle

11-23-2013, 05:30 PM

Geoff,

Thank you again for your input

Roll center is primarily about transient: go in the paddocks, FSAE or Pro racing and ask. You will be surprised how many people take that into consideration.

The moment of inertia of an object such as the suspended mass is a function of this mass, the square of the radius of gyration of this mass and the SQUARE of the distance between this mass CG and its instant center, not the square of the vertical distance.

That means that the lateral movement of the roll center influences the suspended mass inertia, its stability, its response and the roll frequency, the need of roll damping.

I agree with you 100 m or 150 metres who cares it is so big inertia anyway. But 1.0 or 1.5 metres.... not negligible.

If you look at some very asymmetric Nascar front suspension with very different VSAL many think (even some crew chiefs of these cars) that it is about target of desired camber change in heave and roll. It is about it not only about that. It is also about transient tire load.

May I insist again that
- There could be huge difference between kinematic and "real" forces based roll centers, especially if the car has big compliances. In simple words the difference between kinematic and real roll center will probably be bigger on a passenger car with suspensions bushings and a LMP1.
- You need to look at the same time at the suspended mass rotating around its roll axis while the non suspended mass will be rotating around their own instant axis
- At the end you will never be spot on; your tire loads, tire model, kinematics, compliance will never be accurate enough. So you work in Delta.
- And at a certain time you need to stop the intellectual...gymnastics and go testing

Buckingham

11-23-2013, 09:53 PM

Most NASCAR Cup cars ride around for most of the lap fully engaged into their virtually rigid bumpstops such that the RF completely loses "kinematic" behavior. The compliance slopes will actually flip signs once the bumpstop engages. This is very easy to see on a KnC rig if you know what you are doing. There is no kinematic RC because the shock/bumpstop becomes an effective 5th link. Talking about the Front RC of a cup car riding around on a bumpstop is about as meaninful as talking about rear caster.

Talking about the rear kinematics of a Cup car is however extremely relevant. So much so that they put a hole thru the rear window so they can to adjust it during pitstops.

Tim.Wright

11-24-2013, 02:10 PM

I do not have an issue to have a roll center going through the ground...except that at that time the lateral roll center migration will be difficult to control. But.... as Geoff remarks infinite left or infinite right, it is still a huge inertia so if you look at the transient behavior the effect is small if not negligible. Similarly I do not have a problem with a roll center crossing a tire (well.... I am a bit less sure about that) but I do have a problem when the front roll center goes towards one side of the car and the rear one towards another.

This is the key issue I want to clarify. If we are speaking of a traditional geometric roll centre, then I think the lateral position is bordering on meaningless the closer that the roll centre gets to the ground.

Just to explain my point, this is a spreadsheet I made one weekend... Here is a suspension design that has the roll centre almost on the ground (1.2mm high). The roll centre is on the centreline of the car and the left and right force lines have slopes of 0.15% in the static condition. This means that if you have lateral cornering forces of 1000N on each wheel, there would be 1.5N of jacking force for each wheel (0.15% x 1000N = 1.5N).
https://lh6.googleusercontent.com/-CZARfksz7Vs/UpJQ7CJ79ZI/AAAAAAAAAaU/uvKLbxqQAnE/s0/RCy_000.jpg

This picture is the same suspension design with the suspension rolled to the left by 1mm at the wheel centres. The roll centre has now moved 804mm to the right and has moved down to the ground level. The left and right force line slopes are -0.12% and 0.43%. That means under a 1000N force, the jacking forces will be -1.2N and 4.3N for the left and right respectively.
https://lh6.googleusercontent.com/-TZtbNF_tdgM/UpJQfmxlipI/AAAAAAAAAaM/3EbZXFGzX2w/s0/RCy_800.jpg

So the lateral movement of the roll centre is quite drastic but in terms of the wheel forces, there was a change in the order of a only few Newtons of jacking force.

So I guess what I'm saying is this: the lateral roll centre movement is telling us something, but in a very clumsy way. It is representing the difference in the jacking "percentage" between the left and the right wheel. I.e. if the roll centre moves to the left wheel (while maintaining a constant height) it means that the jacking is increasing on the left wheel and decreasing on the right wheel. While I think this is an important effect to capture and understand, I think that expressing it as lateral roll centre movement is the wrong way to go. 800mm of lateral roll centre movement means something totally different if the roll centre is 1mm high compared to 50mm high.

What is important to look at, in my opinion, are the force line slopes as the car rolls. In my example above you see a change of slope of -0.27%L / +0.28%R which is nearly nothing. The roll centre movement suggests that something drastic is happening as it has shot off 800mm from the centreline. Which method do you think is giving a better representation of what is happening in the suspension?

Claude Rouelle

11-24-2013, 04:14 PM

11-24-2013, 07:58 PM

Some of the recent posts on this thread are pleasing, but most are disappointing.
~o0o~

Particularly pleasing is that Tim keeps trying to get back to the topic of...

Posted by Tim:
... talking about horizontal migration of the pitch/roll centres. I.e. lateral roll centre migration and longitudinal pitch centre migration. They seem to be a pretty useless construct to me...

Also that both Tim and Geoff are supporting their views with numbers and calculations, which are easily verified or disproved by any interested reader, unlike the usual racer's anecdotes of "We gave it an extra half-turn of xxx, and gained 2 seconds!". Thank you both very much for that. :)
~o0o~

Claude,

Particularly disappointing is that you keep trying to avoid the very straightforward questions I asked you back on page 3. Is a R/PC that has migrated horizontally 200,000 km, and is at an altitude of 2,000 km, in a GOOD or BAD place? And WHY?
~o0o~

And much more that is disappointing.

1. There still seems to be a lot of confusion between FORCES and MOTIONS. The above R/PCs are found by a kinematic analysis of the suspension linkage (ie. by finding the "n-lines", or "force-lines"). But this is done for the purpose of calculating the control-arm forces acting from-ground-through-wheelprint, and these forces' subsequent affects on the car body. VERY IMPORTANTLY, these R/PCs in NO WAY WHATSOEVER (!!!!!) characterise the instantaneous MOTION of the car. The car moves about a "centre" (in 2-D), or a "motion screw" (in 3-D) that is totally unrelated to these "kinematic/force-based" R/PCs. If you do not understand that, then you must go back to Square One.

2. Claude, you do not appear to understand the above point.

Posted by Claude:
I do not have an issue to have a roll center going through the ground...except that at that time the lateral roll center migration will be difficult to control. But.... as Geoff remarks infinite left or infinite right, it is still a huge inertia so if you look at the transient behavior the effect is small if not negligible. Similarly I do not have a problem with a roll center crossing a tire (well.... I am a bit less sure about that) but I do have a problem when the front roll center goes towards one side of the car and the rear one towards another.
Why the problem shown in bold? I suspect you are confusing "places where forces are combined", with a "motion axis". Is that why you often refer to these as "roll or pitch axes"? If so, then you must really learn more about the dynamics of bodies.

3.

Posted by Claude:
Many people look at roll centers ... as the intersection of "forces lines" but they ignored the Huygens (or parallel axis) theorem which will influence the rotational acceleration of both suspended and non suspended masses.
This is PURE CODSWALLOP!!! It does seem to confirm your misunderstanding above (ie. points 1 & 2). To restate the obvious, the car does not rotate about your "roll or pitch axes" (ie. the line that passes through the R/PCs). To assume this, and then want to invoke the "parallel axis theorem" to analyse the Dynamics, is sheer nonsense. As is much else in that post...

4.

Posted by Claude:
Roll center is primarily about transient: ....
...
That means that the lateral movement of the roll center influences the suspended mass inertia, its stability, its response and the roll frequency, the need of roll damping.
More bulldust of the above nature...

5.

Posted by Claude:
Believe me when I teach in India or South America or Asia to young students ... I can't rush to jacking forces in the first 3 day seminar; If I do I lose 90 % of the audience....
Simple then complicated, not the other way around.
Claude, you are teaching an unnecessarily complicated form of VOODOO and BLACK-MAGIC, with a large amount of BULLDUST rolled in. Much of what you are teaching above has no connection with reality. So the more the students learn, the harder everthing is for them to understand. So then you tell them..

- And at a certain time you need to stop the intellectual...gymnastics and go testing.
This is simply your excuse for not knowing what you are talking about. Your theoretical Voodoo-Bulldust is no predictor of car behaviour, so you have to "go testing". Enough practical trail-and-error will find the solution to any problem. But if you want to speed things up by using "theory", then old fashioned Newtonian Mechanics is much simpler, and infinitely more accurate, than the rubbish you are teaching. Jacking forces can be explained on Day One, in a very simple way. Please read the "Jacking Force" thread (http://www.fsae.com/forums/showthread.php?4063-Jacking-force&p=17492&viewfull=1#post17492), and if you have difficulty with it, then ask for clarification!

6.

Posted by Claude:
[Quoting Tim->] "What is important to look at, in my opinion, are the force line slopes as the car rolls." You are absolutely right. In steady state. What about transient? What about the effect of suspended and non-suspended masses inertias around their respective instant centers?
Once again, this seems to relate to your complete misunderstanding of Point 1 above. Claude, you must go back to Square One. In this case Square One is Newton's First Law, aka Galileo's Law of Inertia. Anyone who does not have a strong appreciation of the cause-effect relationship in this Law (actually, a "postulate", or "axiom") will never understand Vehicle Dynamics. Or, for that matter, any Dynamics.
~o0o~

Bottom line, all of this stuff is easily understood through the methods of very old-fashioned Mechanics. No "complete system of second order differential equations" is needed! (I think Claude suggested that, but I couldn't find the quote...). In fact, no "cogitatio caeca" (= "thinking blind", = analysis via algebraic equations) is needed!!! I still haven't got around to browsing my copy of Den Hartog's "Mechanics", written in 1930s? and recently bought off the Interweb for $3.99, but I am sure all that you need is in there.

Z

ben

11-25-2013, 10:10 AM

This is a great thread and discussion between three engineers I've had the pleasure of meeting in person (Tim, Claude and Geoff) and one I haven't in Z.

I have to say in Z's favour that his graphical explanation of jacking forces on the thread he mentioned is brilliant. It really opened my eyes and the fact that it can be explained so elegantly and visually means it would be better to start here than roll centres.

I can also understand how Z can rub people up the wrong way. Of course his prerogative but Claude is right to point out that teaching is about more than drilling facts into people. Inspiring the class matters, and Claude is a master at this.

Good debate as ever, but as Geoff pointed out it's just us old boys a lot of the time :-)

Tim.Wright

11-25-2013, 07:03 PM

Tim,

Excellent.

We do agree much more than we disagree.

3 additional comments.

1. Were you able to figure out all this the first or second year of engineering school or FSAE? Believe me when I teach in India or South America or Asia to young students who discover race car, race car engineering, and simply how a car works I can't rush to jacking forces in the first 3 day seminar; If I do I lose 90 % of the audience. They would be overwhelmed and discouraged. Z can say that the teaching is going down the drain (he could be right but what I do not doubt is about the hunger of the students to learn, wherever they are from) but I can't change the reality of their starting point. Simple then complicated, not the other way around. I go though more details and in our professional workshop and/or our university courses. At OptimumG we have created the transient version of the "magic number" (anti roll stiffness distribution percentage) where dampers, inertia and tire deflection, tire forces and moving non linear kinematic (with or without compliances) are taken into account. I have tried to present this in a 3 day seminar to young students who discover VD ...that is the time where you start to see the guys yawning, checking their SMS or going to the bathroom. Nobody wins at that time.
Good question. The actual mechanisms behind roll centres only clicked for me a couples years ago. I was having a discussion with someone about K&C data and they mentioned the usefulness of looking at Fz vs Fy during a lateral compliance test and it suddenly hit me like a truck. I thought f*** me, how did I not notice this before. This being:
The Fz vs Fy IS the anti roll (geometric load transfer) effect produced by ICs being above the ground.
In fact the dFz IS actually the jacking force and that is what is giving the anti roll effect. Turns out jacking forces they actually aren't evil at all.
Exactly the same mechanism is working longitudinally and is known as anti dive, squat, lift, raise

The pre-requisite being that I had previously proven to myself mathematically the the front/side view IC's are a valid construct and can (within limits) in fact work as an instantaneous "pin joint" between the body and hub. The geometric roll centre does no such thing.

I think the reason it took so long to sink in is that I had to unlearn everything I had read and heard about geometric roll centres in the 5 years before hand.

While we are on the point of education, I'd like to share my experience where I have been in the position in the last couple of years where I had to teach a couple of people (yes literally only 2) how suspensions work in a very short time period. They were engineers, but knew absolutely nothing about vehicle engineering. I spent about 1 hour explaining things like this:
I explained to them the concept of the (front/side view) instant centre (not a roll centre) and how it is just an application of four bar linkage theory.
I suggested to them to look at the front view kinematics as 2 instant centres which pin the wheel/hub to the chassis. I draw a swing arm axle on the page to start the discussion.
I tell them to imagine a cornering force at each wheel. At this point its blindingly obvious that the cornering forces are creating torques about the ICs such that the outside wheel is being shoved into rebound and the inside wheel into bump.
Explain to them that load transfer (Ay x CGh) is doing the opposite (outside in bump inside in rebound)
At this point they realise themselves that the lateral forces at the contact patches combined with the ICs above the ground work to counteract the rolling moment induced by load transfer.

And boom, they understand suspensions better than most people in the industry. In about one hour, the basics are in their head and I didn't mention the word roll centre once. Yes there are some simplifications but each step can be completely explained with a free body diagram.

Conversely, when I first read about geometric roll centres, It required a leap of faith because I didn't understand how drawing all of these lines nails down the point that the body rolls about. But I thought that its so complicated that it must be right. I ran with it for years but it never sat well with me.

Ironically I still use the geometric roll centres and my own spreadsheet (which is doing the classic LLTD calcs) to do baseline setups. Its not a bad tool for steady state setup, especially for road cars where you have more similar cornering forces left and right since you are well under the limit. I think the classic roll centre theory is a decent approximation for this job and the cost in computational time and man hours is practically zero. When you consider that the springs you choose will eventually be overridden by the driver's subjective feedback, there no point wasting a lot of time calculating the spring rate in any more detail than you need.

2. Look at a Nascar or a Grand Am or a good GT car or an LMP1 and observe the front splitter left and right ride height pretty good consistency despite the braking, acceleration, banking, aero effect of speed, lateral G, tire deformation etc... They are not on bump stops most of the time; that I know so. Their "pitch axis" is like an "hinge" quite often parallel to the ground which helps them to keep the best possible downforce. They then play with the rear static ride height, rear springs, the whole car kinematics dampers and ARB, inertias etc to give their car the attitude and therefore the aerobalance they want. For a race car performance or a passenger car crash avoidance (active safety) it is all about transient. Everything else are just pictures of a film that we stop so that that we can understand the steady state behavior. But we forget to move the film back to its real speed

I believe all of that. I just don't believe it has anything to do with a geometric roll centre which is the current discussion point.

In terms of controlling the roll motion have a read of this SAE paper (http://papers.sae.org/983033) and consider this:
When the outside damper is hard on the bumpstop. The outside wheel vertical movement becomes zero while the inside still moves. Your roll centre (as in the roll motion axis) is now the contact patch of the outside wheel.
Now consider the point just as the bumpstop starts to engage. The outside wheel rate is increasing and its movement is diminishing. The inside wheel continues to move into rebound unrestricted. The motion roll axes is then somewhere in betrween the CL and the outside wheel is it not?
Now forget the bumpstops and realise the a simple rising rate suspension is going to have this same effect.

Then ask how is it in any way valid to transfer your body roll inertia to the geometric roll centre with the parallel axis theorem???

ben

11-26-2013, 02:54 AM

Excellent post Tim. Thank you.

I think I had the same epiphany looking at Z's diagram of jacking forces. I now genuinely don't think geometric roll centres are a good teaching tool at all. They give a superficial understanding, which requires a lot of unlearning. Anti-roll as the angle of the n-line is much simpler and physically meaningful.

Ben

Tim.Wright

11-26-2013, 04:16 AM

Yea exactly.

I'm all for simplified models, but if you need to first unlearn what you've learned in order to progress to the next step then the tool is not ok in my opinion.

Buckingham

11-26-2013, 07:53 AM

Good question. The actual mechanisms behind roll centres only clicked for me a couples years ago. I was having a discussion with someone about K&C data and they mentioned the usefulness of looking at Fz vs Fy during a lateral compliance test and it suddenly hit me like a truck. I thought f*** me, how did I not notice this before. This being:
The Fz vs Fy IS the anti roll (geometric load transfer) effect produced by ICs being above the ground.
In fact the dFz IS actually the jacking force and that is what is giving the anti roll effect. Turns out jacking forces they actually aren't evil at all.
Exactly the same mechanism is working longitudinally and is known as anti dive, squat, lift, raise

Just remember, a K&C test is different because you have a constrained chassis. On the rig, jacking produces changes in wheel pad load. On the track, jacking produces a steady state change in suspension deflection with no net change in tire load.

However, in transent you have to get from State A to State B, so there will be a change in tire load based on the inertia of the system resisting this change in displacement.

Tim.Wright

11-26-2013, 10:33 AM

Just remember, a K&C test is different because you have a constrained chassis. On the rig, jacking produces changes in wheel pad load. On the track, jacking produces a steady state change in suspension deflection with no net change in tire load.

However, in transent you have to get from State A to State B, so there will be a change in tire load based on the inertia of the system resisting this change in displacement.

I don't totally agree. If you take say a front axle with an IC above the ground and a lateral force at the contact patch, you then have a jacking force Fz = Fy.tan(IC_angle). If you raise the IC, then you raise the jacking forces. If there is no corresponding change to the rear axle IC heights, then you do increase the tyre load due to an increase in load transfer on that axle.

If you instead raise both the front and rear ICs together, both the front and rear jacking forces increase but the elastic forces from the spring decrease. So in the end the total tyre loads might be unchanged but the jacking forces are definitely there and have increased.

HenningO

11-26-2013, 11:09 AM

For anyone getting started with vehicle dynamics the word roll center comes up in virtually any book/paper on the topic. Even SAE/ISO/DIN have definitions for what the roll center is, these definitions usually say something along the lines of: "point at which a lateral force applied to the vehicle body is reacted without causing suspension roll angle". That definition along with the name "roll center" can fool even the brightest student of vehicle dynamics (I agree with previous posters, the car doesn't roll around the geometric RC). Correct me if I'm wrong, but there is no "official" definition for n-line/jacking coefficient/ICz-ICy.

Assuming that RC isn't the way to go, why is it still used, here's my thoughts:
- It is easy to (mis-)understand for anyone getting started in the field.
- It is useful metric of a suspension's characteristics and it can be calculated no matter what suspension type/topology used (yes, the IC/n-line method would allow the same... but it's not presented in literature).
- The way it can be used in LLTD calculations are presented in many VD books.
- Vehicle dynamics/Race engineers can see the effects of moving the RC in both driver comments and logged data, allowing them to build up experience of how RC can be used as a tuning tool. These kinds of "rules of thumb" are used at most levels of racing where engineers have to make decisions based on limited data sets in short amount of time.

Now, to get a bit more technical, as Tim alluded to: If the lateral force distribution left/right is equal the RC calculation method will give you the same results as the n-line/jacking coefficient/ICz-ICy method. So if you don't have any information on how your tires generate forces and moments, you'll be guessing this. In this case, one could argue that using the RC method is the safer way to go.

Nonetheless, since I had the "epiphany" (the same one Tim and Ben mentions) a couple of years ago, I found it interesting how people seem to get the entire side-view anti-effect notion without any problems, but struggle to grasp the very same thing in front-view.

One argument that I feel is compelling against the use of RCs is the following: If we have an independent suspension, why does the position of the right side suspension affect the left side suspension? The RC position is a function of both sides and hence makes it a poor choice for analyzing independent suspensions.

Regarding the lateral movement of the RC, I would agree it is not a very useful metric. Vertical RC movement on the other hand is extremely useful for any car that sees different vertical loading (from either down force, inertial loads or just a whole bunch of groceries).

Finally, I prefer to look at the IC location (Y,Z coordinates) as opposed to the slope of the n-line. I agree that the geometric weight transfer as a function lateral tire forces only depend on the slope or ratio of ICz-ICy. But when you want to look at the non-suspended weight transfer, the height of the IC with respect to the non-suspended CG location becomes highly relevant. Additionally, when we start to look at Mx from the tire, then the ICy with respect to the contact patch location becomes highly relevant. All of these three effects will change the weight transfer and/or suspension deflection.

Moving forward, how do we better define the name of this method/approach? I think the notation: "n-line/jacking coefficient/ICz-ICy" isn't going to make it a hit...

Tim.Wright

11-26-2013, 11:26 AM

Moving forward, how do we better define the name of this method/approach? I think the notation: "n-line/jacking coefficient/ICz-ICy" isn't going to make it a hit...

The Z-bomb?

murpia

11-26-2013, 03:53 PM

However, in transient you have to get from State A to State B, so there will be a change in tire load based on the inertia of the system resisting this change in displacement.
This is number one on my list of transient vehicle dynamics effects to study should I ever get the opportunity to do some multi-body dynamics modelling. Drag racers use >100% anti-squat to generate traction this way and race car turn-in should behave the same way.

Number two on the list is proving / disproving the 'stiffer front bar for more turn-in' rule of thumb.

Number 3 is characterising front low-speed compression damping for turn-in tuning.

Number 4 is working out the correct relationship between front toe, front camber and Ackermann, to avoid front tyre scrub.

Number 5 is understanding the behaviour of static margin, during turn-in, to ensure stability.

Number 6 onward are all about transient behaviour of tyres...

Regards, Ian

Moop

11-26-2013, 04:01 PM

While the n-line/instant centre approach is a better approach(imo) to understand steady-state/transient geometric load transfer and how the geometric suspension forces result in changes in body attitude, I don't think that it tells the entire story.

I can't make a picture right now, but consider the classic 4 bar linkage representation of a vehicle suspension in front view. You have the chassis as one link and two swing arms + hubs as the other two links. The swing arms are pin jointed to the chassis and at the ground. We know that the instant centre of the chassis wrt the ground is at the intersection of these links, or the geometric roll centre.

Because the instant centre of the chassis is below the chassis link's centre of gravity, we know that when the chassis link rolls for whatever reason, it also must translate in Y for its motion to be consistent with its kinematic constraints. Basically, because of the kinematic constraints, the chassis cannot roll without also having some lateral motion. This effectively increases the inertia of the chassis to this combined rolling and translating motion and is exactly what the parallel axis theorem tells us.

Neglecting this lateral motion and just considering the forces transferred makes no difference in steady-state, but it will make a difference in transients/dynamics.

This is similar to the case of a simple pendulum, where you can either analyze its motion with 2 equations of motion and 1 kinematic constraint, or one equation of motion in the generalized coordinate. The only difference here is now the pendulum has a rotational degree of freedom and rotational inertia.

In terms of controlling the roll motion have a read of this SAE paper (http://papers.sae.org/983033) and consider this:
When the outside damper is hard on the bumpstop. The outside wheel vertical movement becomes zero while the inside still moves. Your roll centre (as in the roll motion axis) is now the contact patch of the outside wheel.
Now consider the point just as the bumpstop starts to engage. The outside wheel rate is increasing and its movement is diminishing. The inside wheel continues to move into rebound unrestricted. The motion roll axes is then somewhere in betrween the CL and the outside wheel is it not?
Now forget the bumpstops and realise the a simple rising rate suspension is going to have this same effect.

Then ask how is it in any way valid to transfer your body roll inertia to the geometric roll centre with the parallel axis theorem???
What happens here is the exact same thing that happens in side view that Z has talked about in the past/complained about how people don't understand it.

Consider a 2D chassis mass in front view(no wheels) with heave and roll degrees of freedom and no kinematic constraints from the suspension, and also suppose the chassis only has corner springs. In the case where the tracks are symmetric and the spring stiffnesses are equal, if you write out the equations of motion for free vibration, the mass matrix will have a diagonal of zeros and the modes of the system will be uncoupled, ie. pure heave and pure roll.

If you instead make one of the springs stiffer than the other or one of the half tracks larger, the mass matrix will be full and the modes of the system will be coupled. The modes will be a roll mode with a bit of heave and a heave mode with a bit of roll. This is the same idea that you're talking about in your first and second bullet points.

I'm not entirely sure how this all comes together when you put the kinematic constraints and asymmetric springing together though, but it would make sense if they continued to have the same effect they have when analyzed by themselves, plus some additional effects potentially due to their interaction.

The annoying part though is that, at least in the general case, to analyze it you have to solve the equations of motion and equations of constraint at the same time, which is into the realm of DAEs and multi-body dynamics.

11-26-2013, 08:49 PM

Posts are coming thick and fast now...

First (the last bit of) the disappointing stuff. Namely, just how hard it is to communicate these essentially spatial ideas here. In the ancient Greek forums they would all stand around a sandbox, and with long sticks they would draw lines, and curves, and point to various intersection points, and ask "Do you mean this 'centre' here?" So much easier, IMO... :)

Anyway, I'll do a sketch tonight, and then the scanner (ughhh, much profanity...), and the Picassa upload (groaan...)...
~~~o0o~~~

Thank you to Ben and Tim (and others) for supporting the idea that it is easier to look first to the wheelprint n-lines for this sort of analysis, rather than RCs and PCs. Here I should note two things.

1. To find a R/PC you must first locate the relevant wheelprints, and then from a kinematic study of the two independent suspensions you must locate the two wheelprints' n-lines. The last step of locating the intersection point of the two n-lines, which gives the R/PC, is largely redundant (not needed for calculations), except perhaps for use in loose conversation such as "It has a high Roll Centre" (vs "It has steep n-lines, err, lateral, that slope up-to-centre...")

2. Both Mark Ortiz and Bill Mitchell have been promoting the idea of thinking in terms of the "force-line slopes", rather than R/PCs, for at least ten years (possibly more than twenty...). This for reasons similar to mine above (and more below). "Control-arm-force-line slopes" might be an even more accurate term, maybe with "passing through the wheelprint" thrown in there as well. "N-lines" (aka "normal-" or "right-lines", because at right angles to path of motion) is the general term used throughout the wider field of Kinematics for the last ~200 years (eg. it is commonly used in robotics nowadays). So "n-lines" is a short phrase with very wide application.

(BTW, congratulations Ben on the new job...)
~~~o0o~~~

Some general comments.

The word "transient" can mean "whenever the "quantity of motion" of a body changes because of the forces impressed on it". (Nerd Note: Newton's definition of "quantity of motion" = our "momentum".) As such, a car at constant speed and radius on a Skid Pad is, in a very real sense, in a "transient" state of motion (its momentum vector keeps changing direction).

As is done on the Jacking Force thread this sort of "transient analysis" is easily done in a standard "Statics FBD", by using d'Alembert's Principle and having an appropriate "inertial reaction force" added to the other "real" forces to bring them to equilibrium. In the Jacking Force diagram the inertial reaction force is Fi (which implies "transient"), and it is in equilibrium with the two wheelprint forces and the gravitational force Fg.

The more common understanding of a "transient" in VD is when the body is wobbling about (in R/P), so that the spring-dampers are expanding/contracting. Such rotational "transients" are also easily analyzed using d'Alembert, and simply require the inclusion of some "inertial couples" (say, "Ti.roll") in an otherwise bog-standard, Static FBD.

I stress this here so that you students DO NOT THINK THAT YOU MUST HAVE a big, complicated system of 2nd order ODEs, plus Matlab+++..., to be able to understand these things.

The key point is that a force vector F, acting on a particle of mass m, for a period of time dt, will cause the particle's current momentum vector P, to change in length and direction in proportion to F. Essentially, you just add dP, which has same direction as F, and length proportional to F.dt, to the end of old P, to get your new momentum vector P+dP, at the end of that particular timestep dt (ie. NII is F = dP/dt, NOT F = mA!).

For extended bodies (ie. not point-like particles) you change their angular momentum vector L each time step in a similar way to above, but by using the couple T acting on the 2nd MoIs of the body. This is very easy in 2-D, but requires knowledge of the body's "inertia tensor" in 3-D, and is perhaps best done using Euler's Rigid Body Equations (still quite easy).

All this is very graphical and much easier to explain on a blackboard (or a stick in the sand...).
~~~o0o~~~

HenningO,

I see the biggest problem, by far, in talking about Roll or Pitch Centres, is that there is a very strong implication that these are "the centres of the motion". Almost everybody seem to fall for this, including Claude a lot of the time... I will post a sketch showing how wrong this is soon...

Both these "centres", when taken as "points where forces can be added together", could equally be called the "Heave Centre". This is because with non-horizontal n-lines, and unequal n-line forces, there is a vertical control-arm-force-component passing through this point that acts to lift the car. Of course, if you start thinking in these "Heave" terms, then you must not forget to also include the horizontal CA-force-component acting through this point as well!

The SAE/ISO definitions of the RC are especially disappointing because they only define it in terms of its height, so not really a "centre" at all. By omitting the horizontal location of the RC these definitions do not account for the vertical ("jacking") force's affect on body roll, and so, indirectly, have resulted in much heated debate on these pages! (Aaaarghh, ... how many words!!!???) Also, depending on L/R split of the wheelprint Fy forces, the same suspension can cause the body to Heave EITHER up OR down. This seems to be totally ignored... I have another sketch (in another box) that explains this, that I will try and post soon.

"But when you want to look at the non-suspended weight transfer, the height of the IC with respect to the non-suspended CG location becomes highly relevant. Additionally, when we start to look at Mx from the tire, then the ICy with respect to the contact patch location becomes highly relevant. All of these three effects will change the weight transfer and/or suspension deflection."

Agreed, and these effects were covered to some extent on the Jacking Force thread. The Wheel-Assembly ("non-suspended") mass effects can also (IMO quite easily) be analysed by considering the suspension's n-lines passing through the WA's CG. Gyroscopic effects (also on the JF thread) can also be understood using n-lines, but require a few more of them. As noted earlier, n-lines are an old and reliable concept used throughout Mechanics.
~~~o0o~~~

Lao Tzu (old Chinese philosopher writing in "Tao Te Ching") "There are so many names, is it not time to stop?"

I'll try and post some pics soon...

Z

DougMilliken

11-27-2013, 05:58 PM

On the question of terminology, MRA vehicle dynamics simulations can be traced back to work done in the 1960's and early 1970's at CAL and GM (see history chapter of RCVD). The vehicle input deck (originally on punch cards!) calls for the side view kinematics as "longitudinal force anti" (different directions are anti-squat, anti-lift, etc). It also carries that naming scheme to the front/rear view case with "lateral force anti" for independent suspensions. I guess this is now sometimes called "force based roll (or instant) center", but we never used that terminology.

The underlying analysis (several large, detailed reports) is designed to work with K&C output. GM developed piece-wise K&C capability in the 1950's and 60's and then designed/built a comprehensive rig called the Vehicle Handling Facility (VHF) in the early 1970's--located at the Chevy Engineering Lab in Warren MI. Tom Bundorf has put together a slide show with pictures of many of the special test rigs--we host it at:
http://www.millikenresearch.com/VehicleDynamicsAtGMByRTBundorf.pdf (bottom of the Olley book page).

Back to "longitudinal & lateral force-anti" -- one of the standard K&C tests that was developed was to clamp the chassis to ground, apply longitudinal or lateral forces (or combined) to the tire footprints and measure the change in load. For drive wheels (ie, anti-squat on rear wheel drive), the longitudinal test is run with the driveline locked. With the brakes locked the longitudinal test results were reduced using the F/R brake bias. Tests could be repeated at different ride heights if desired. The change in vertical load (delta), resulting from the horizontal force was curve fit (linear and nonlinear options), and that became part of the vehicle description for the computer simulation.

jpusb

11-28-2013, 05:04 AM

This thread went crazy.
May I contribute with a question, or should I say, an additional point of view. A lot of talk in this thread has been going on about (and maybe it didn't touch this enough) if going far in the car model, and what it accounts for, is worth the time in FSAE (and this is not yet clear to me). Going back and remembering this is FSAE, I will add my question which adds the driver variable to the pot. What are your thoughts on RC (or n-line slopes, IC, anyone you like fits the question) values and value fluctuations with lateral accel. and their importance or relevance when you put an amateur driver on the car?. What do you think the average amateur driver works best around?

If I may, I can rephrase that question to a more practical one (and this includes not just the end-view case but also lateral or longitudinal case). Claude gave an example in which two different designs can give the same Steady State (SS) results (roll angle, weight transfers, etc) but behave very different in transients. To that, I think we can all agree (I hope so), but, where (in this thread, and in the design process) do you think we should input the driver? Of course testing is where the driver has more input and where you get to know what he likes or works best for him, but a good design should have input the driver in the design process (and many people don't). A particular setup may be faster (clock time around a lap, sector, whatever) in SS and simplified TS analyses but the whole real package includes a driver that can read/feel these things and a vehicle that is rarely in SS (FSAE again) so, thoughts?. Will an amateur driver get an advantage from an anti-whatever setup? Do you think it is better if everything goes through the shocks? Of course in this topic, previous experience is precious, but initial thoughts and reasons may lead to a good initial setup.

JP

11-28-2013, 07:37 PM

ROLL CENTRE vs INSTANTANEOUS MOTION CENTRE.
============================================

Earlier I said that a car's "Kinematic Roll Centre" (same as its "Force-Based Roll Centre", namely the intersection point of the car's lateral n-lines), is in no way related to the car-body's "Instantaneous Motion Centre". Why so? And how do you find the IMC?

Firstly, it is important to note that when doing calculations of FORCES acting on the car-body, such as the FBDs shown on the Jacking Force thread, you only have to consider the ONE SINGLE BODY. That is, you only need one reference frame, with, say, only one origin, and only one X, Y, (and Z) axis.

BUT!!!, when studying MOTIONS you MUST consider at least TWO DIFFERENT BODIES (or, equally, two different reference frames). It is completely pointless to try to describe the motion of a single body with respect to ... itself (?), or with respect to ... nothing much at all (???). A motion is always that of BODY-A, WITH RESPECT TO, BODY-B (or referance-frame/cartesian-axes-A, wrt, frame/axes-B, etc.). Maybe this is why some people find Kinematics harder than Statics.

Here we consider the motion of the "car-body" with respect to "the-ground" (= the road, the Earth, Terra Firma...).

Secondly, as noted ad nauseam elsewhere, the 3-D motion of car-body wrt ground is best described via the concept of the "Motion Screw" (aka ISA). That is, at any instant the car-body translates parallel to, and rotates around, an Instantaneous Screw Axis that resembles a momentary "screw joint" between body and ground. (Important note: this "screw joint" serves only to describe the relative MOTION. It is incapable of transmitting FORCES.)

However, this 3-D stuff is a bit difficult to explain clearly with just text and simple sketches, so here I dumb it down to 2-D. Because of this dumbing-down you should ignore all motion in-and-out of the plane of the sketches, and any rotations other than those entirely in the plane (ie. around an axis perpendicular to the plane).
~~~o0o~~~

So, at the top-left of the sketch (below) is an end-view of a car, notionally cornering to the right (ignore the fact that this "cornering" cannot happen in this 2-D picture...). As a result of the leftwards centrifugal force acting on the body, the body has rolled a little bit to the left, and ALSO the whole car has slid some distance to the left (remember, all this is WRT the ground).

From a simple analysis of two points (A and B) on the car-body, at two successive moments in time (0 and 1), we find the 2-D Instantaneous Motion Centre of body wrt ground (well, it is only really "instantaneous" when dt -> 0). In this case the "IMC" is a long way under the car. If the car's Roll-stiffness was greater (ie. less roll-angle), or the road was slipperier (ie. more sliding), then the IMC would be even further underground.

(In case you are wondering why the car has moved left, even though it is "cornering" to the right, this is due to the dumbing-down process of moving from 3-D to 2-D. It might be better to think of a strong wind blowing from the right, which pushes the car to the left. This wind represents the centrifugal force and acts along a Line-of-Action that happens to pass through the car's CG. Or perhaps think in terms of the SAE RC-height test where a lateral (leftward) force is applied to the car body...)

https://lh4.googleusercontent.com/-w15wnI59bJU/UpfrNdJvY0I/AAAAAAAAAOo/TM4YQuVM2BM/s800/MotionCentres.jpg

At the right of the sketch is a car similarly cornering to the right (or being blown leftward). But this time its outer (left) wheel has hit a curb that has briefly given the tyre very high lateral stiffness. With stiff Roll-mode springs, softish Heave-mode springs, and an above-ground KRC/FBRC (ie. n-lines sloping up-to-centre), the car body has only rolled leftward a small amount, but it has "jacked-up", or "heaved", quite a lot. The end result is that the IMC is now a long way to the left of the car. Less body-rolling, and/or more body-heaving, puts the IMC even further to the left.
~~~o0o~~~

So is there any time when the IMC can be coincident with the KRC/FBRC? Certainly, yes, and this is the case that many (most?) people consider to be the norm. But this only happens IF BOTH WHEELPRINTS ARE EFFECTIVELY PIN-JOINTED TO THE GROUND!!! Buckley's chance of that ever happening! :)

Putting this into 3-D terms, each wheel would have to be rolling along some sort of track, such that each "wheelprint" could not move laterally or vertically wrt the track. (Check the fairground rides for hints on how to do this - ie. extra "guide" rollers). Anyway, what we are talking about here are tyres of infinite lateral (ie. cornering) stiffness, which is unrealistic.

Unfortunately, in static tests, such as the SAE RC-height test, the tyres ARE approximately "pin-jointed-to-ground" because of static friction (as long as you neglect sidewall lateral deflection). I reckon this is one of the main reasons that Heave motions are so often neglected in VD analysis (ie. the "RC" should be called the "Heave/Roll Centre"). As seen at the right of above sketch, Heave motions typically require the track dimension to change, so can NOT happen when the tyres are pin-jointed-to-ground.

Note that a real car with normal tyres, an above-ground RC, and TOED-IN wheels, will heave upwards even when travelling in a straight line. You can try this by strapping on some ice or roller-skates, get some speed up, spread your legs like the n-lines, then toe-your-feet-in. Or try toeing-out ..., ouch!
~~~o0o~~~

So now we can reconsider Moop's post above (and many of Claude's on other threads), where it is suggested that in this end-view the car-body can oscillate in a roll-motion with the centre of this motion being the KRC/FBRC. The implication is that, if this is so, then the "parallel-axis-theorem" has to be used to increase the effective MoI of the system, above the "normal" MoI of the body about its CG.

My view is that the above is only possible if the wheelprints are (unrealistically) "pin-jointed" to the ground. More likely is that a roll oscillation will take place about a Motion Centre that is very slightly below the body's CG. As the top of the body rolls leftward, the KRC (which is significantly below the MC) moves rightward and pushes/pulls the wheelprints rightward. Body rolls rightward, KRC pushes/pulls the wheelprints leftward. The wheelprints follow an "S-shaped" path along the road.

Furthermore, as the tyres follow this S-shaped path, their Fy forces, which are always opposed to the direction of side-slip, act to damp the roll oscillations (dampers always exert a force opposing their motion). Thus, if you are calculating the amount of roll damping needed to suppress roll oscillations, then not only is the roll-mass-MoI number smaller than in the parallel-axes/roll-about-RC case above, but there is also extra damping from the tyres thrown in for free.

Note that this is a very different situation to a car SUDDENLY entering a corner, with the tyres suddenly pushing the car-body's KRC towards the inside of the corner, and thus causing sudden, and significant, body-roll. This is a very different problem to "roll-oscillations", and requires a different amount of damping to completely absorb the energy (ie. to stop the body-roll at its steady-state angle).

Bottom line, there is a big difference between a "centre" which describes a point at which forces can be added together, and a "centre" which is used purely to describe relative motions (with NO FORCES acting there). Sometimes these two different types of "centre" might share the same spot (eg. pin-joints in 2-D linkages...). Often they do not (eg. RC and MC above).

Enough for now...

Z

(PS. Why did my scanner give the black-and-white sketch a blue background? Did I ask for that? (NO!!!) Why is the file ~200 kb when it should only be about 50 kb?? Why is the grumpy old-fart starting to foam at the mouth??? :()

11-28-2013, 08:40 PM

Jpusb,

Brief response to your questions.

1. Effectively "suspensionless" cars have won FSAE (ie. very stiff springs). So "optimising" the kinematics (RC height/migration/etc.) vs spring-stiffnesses, etc., is NOT of the utmost importance (in FSAE!).

2. The best thing for your drivers is lots of seat time. When they become reasonably good they will drive around any smallish faults in the car setup.

3. So bottom line is to get something built quickly so the drivers get a lot of practice. Roughly modifying an old car so that it has highly adjustable suspension (ie. lots of cutting-and-butting of frame++) at the beginning of the year is a good way to give your drivers seat time in "different" types of car. I would bet that a good driver in a non-optimal car will easily beat a novice in the "most optimal" car.

That said, IMO horizontal n-lines (end-view and side-view) cause the least problems.

Z

ben

11-29-2013, 03:48 AM

Great info as ever from Z.

To answer the anti-dive question from Jpusb in my own way; in my experience anything above 25% anti-dive tends to be perceived negatively by the driver. This is another version of Z's comments about relatively horizontal n-lines being least problematic.

Unfortunately "Ben said" or "Z said" are not acceptable design tent answers (rightly so...). In this situation testing with a real driver is infinitely preferable to trying to infer what the effect of the anti-geometry will be from a simulation with a poor (or no) driver controller and a non-existent transient tyre model ;-)

Ben

murpia

11-29-2013, 07:44 AM

...

1. Effectively "suspensionless" cars have won FSAE (ie. very stiff springs). So "optimising" the kinematics (RC height/migration/etc.) vs spring-stiffnesses, etc., is NOT of the utmost importance (in FSAE!).

...
Z
Nor in many other formulae. One thing that does need reinforcing though is the ability of the suspension to distribute forces accurately. Non-engineers (or poor ones) will equate lack of suspension _movement_ with lack of suspension (period). Hence I mildly object, Z, to your phrase 'suspensionless'... Lack of suspension movement does indeed diminish the role of kinematics and easily understood 'static' parameters start to dominate: 'static' camber, 'static' toe, etc. By 'static' I refer to what you would measure on a 'flat patch'.

Lack of suspension movement does not however mean loss of control over suspension forces. Controlled & tunable roll stiffness distribution front to rear is still present in a car with 'stiff' suspension while obviously absent in a car with 'no' suspension. Even if to the naked eye on the track the motion of the chassis relative to the wheels will appear similar. F1 cars appear to have 'no' suspension save the tyres, but in fact have exquisitely tuned, ultra-low friction, low-compliance suspension designs specifically to control the roll stiffness distribution as well as possible.

If you decide to adopt stiff suspension to diminish the importance of the kinematics, you need to place additional effort in characterising the effect of the other compliances in the system on roll stiffness distribution. Chassis torsional stiffness get the most attention but there are lots of others. There is a handy shortcut though, if you can fit pushrod loadcells like any serious formula car would have. They neatly eliminate all the compliance effects (including the tyre vertical stiffness) and report the truth. If your suspension design model, or (Claude's 'magic number' spreadsheet) is telling you your car has 55% front roll stiffness but the pushrods are telling you 51.5%, believe the pushrods...

Regards, Ian

Claude Rouelle

11-29-2013, 12:28 PM

The difference between the basic Excel 1st magic number (the one of the ratio of the front / total "elastic" weight transfer) spreadsheet and the "reality" (that is as much as you trust your load cells and pushrod strain gauge) is effectively due to
- tire stiffness being one of them (change only the front or only the rear tire pressure and you won't have the same ratio)
- the chassis torsion stiffness
- the chassi torsion stiffness distribution (for the same total Nm/deg you can have the front stiff and the rear soft ... or vice versa... think about how Go-Karts are tuned)
- the chassis torsion damping
- all other suspension, rim etc...compliances
- possible asymmetrical aero effect (in the car setup, and/or side wind, and/or aero yaw angle)
- of course the damper and inertias.

In any case whatever the car, the tires, the driver, there are too many parameters to be spot on in your calculations; you work in delta, in quantified effect of the variation of one of the parameters (spring, ARB, weight distribution etc...) on this "magic number"

I have been using this spreadsheet AND the pushrod strain gauges on several race cars. Every time we changed one of the car setup parameters we saw pretty much the same VARIATION (not the same absolute number) on the both the excel spreadsheet and the strain gauge data. It makes you both more self confident (after a while you know which direction a given setup change will influence the car behavior why and by how much) but also humble because you know there is a difference between real number and reality, mainly from compliances and damper.

11-30-2013, 08:17 PM

To anyone reading this thread and wanting more information, here is a link to a post on Migrating Roll Centres (http://www.fsae.com/forums/showthread.php?4063-Jacking-force&p=118099&viewfull=1#post118099) that I just put on the Jacking Force thread (ie. similar to above discussion, but perhaps more appropriate on the JF thread).

Z

Tim.Wright

12-01-2013, 05:10 AM

So is there any time when the IMC can be coincident with the KRC/FBRC? Certainly, yes, and this is the case that many (most?) people consider to be the norm. But this only happens IF BOTH WHEELPRINTS ARE EFFECTIVELY PIN-JOINTED TO THE GROUND!!! Buckley's chance of that ever happening! :)

I just realised this too during the week. I had always wondered why the kinematic roll centre isn't valid since it is merely the application of 4 bar linkage theory twice.

The reason why its not valid is because like Z mentioned, the tyres aren't pin jointed to the ground at the contact patch. Therefore, if you use the geometric RC as a motion centre about which you recalculate the body's roll intertia, its not just a simplification but IMO a massive error.

Silente

12-01-2013, 10:11 AM

first of all thanks to all the contributors of this thread. It is really a great and informing one.

Going through all your comments pushed me to go again through some "theory" that i realized i had never fully understood (although the ipothesis behind certain ways of threating anti concept were clear in books like Milliken RCVD, although explained maybe a bit too quickly). And now i feel i know/understand much more about the problem.

Z, thanks also for your contribution, it has really been useful!

On my side and my experience, what Claude calls "magic number" has always been a great tool to have an idea about the actual car configuration, more than something to precisely calculate the final loads. I am totally with him also about the deltas concept, since normally what you are interested in are the variations effect, not the absolute value.

It has always been a very useful way to compare two setups, for example a very soft and a very stiff one, or to have a ballpark idea of how to initially set a new car still using the same tires in use in another one.

On the other hand, from a pure calculation perspective, all this discussion enforces once more the idea that proper load calculations are (maybe) only possible using multibody's approach. Then you have no need for RC/PC any more.

dynatune

12-04-2013, 05:25 PM

Gentlemen, may I suggest to all of you who have given their 0.02 cents on the issue to read the book from Wolfgang Matchinsky written in the year 1986 and translated into English somewhere in 2000 under the name of "Road Vehicle Suspensions" The book is very tough literature but in the 25 I have been using it as my personal refrence bible I have not been able to find one single flaw in it. The best thing thing about it is the purely scientific and mathematical approach on it. I did create in 1989 already a full 3D kinematics suspension analysis program on it (in FORTRAN and TURBO PASCAL) and just recently in EXCEL for the sake of it http://www.dynatune-xl.com/new-dsdm-70.html . The man is my pope !

Cheers
dynatune, www.dynatune-xl.com

Fra881

11-21-2015, 06:17 PM

I revive this interesting post because it's not clear to me why you say the anti features make the suspension stiffer.

let's take a 100% anti-squat (so that there isn't even the problem that the wheel moves forward in bump, but it actually moves backward so it should take bumps better).

the reason why the suspension doesn't move under longitudinal weight transfer is an opposition of internal forces, or the absence of a moment. why should that influence the reaction of the suspension to an additional external instantaneous force? the internal forces are in equilibrium, the addition of the upward force induced by the bump should just be taken through the spring, exactly as it is if the car with 100% anti-squat is still, and I pull up a rear wheel with my hands.

Ahmad Rezq

11-22-2015, 08:50 PM

Fra881,
Assuming for example a 100% anti-squat suspension in an accelerating car + an additional vertical force (F) acting on the rear wheels (0.5F per wheel)
IMO What will prevent the spring from reacting this force ?!
Like the static case when there is no external forces acting on the car expect the weight also what will prevent the spring to deflect to support the weight ?!

858
F_S: Force on the spring
F_CA: Force on control arms
tan (theta): F_CA line of action slope.
h: CG height
L: Wheel Base

I didn't read the discussions over here. I will do in the weekend to understand the topic better.

((Correct me If I'am Wrong))

12-20-2015, 09:22 PM

I revive this interesting post because it's not clear to me why you say the anti features make the suspension stiffer.

Fra881,

Sorry, I didn't have time to reply to this post earlier. Hope the following helps.

1. Large Anti-Squat at the rear of car is the one place where large "anti" (namely, steep non-horizontal wheelprint n-line slopes) can be useful. As you say, this typically makes the wheels move backwards in bump, so improves ride comfort and general tyre-grip on rough roads.

It also forces the wheel down onto the ground during hard acceleration, giving much higher levels of tyre-grip, so is often used by drag-racers (often at well over 100% A-S). But the extra Fz tyre load is only there because the car-body is being accelerated upwards. The resulting jacking-up of the CG is no problem in drag-racing, but it might be a problem if your car is accelerating out of mid-corner in circuit racing, because ... posible rollover!

Note that "wheelprint n-line" slopes are calculated differently for inboard and outboard drive/braking. For inboard, the wheelprint slope is the same as at wheel-centre. For outboard, you can have a very steep wheelprint n-line slope, while also having a horizontal n-line at the wheel-centre.

(Edit: Found this Figure that explains above.)

https://lh5.googleusercontent.com/-cTTmIQ9a27Q/TvHQ24CoryI/AAAAAAAAAH8/VJHR8n6dYvo/s800/MechAntiFg7.jpg
~o0o~

2. Large Anti-Roll, namely large slopes of lateral n-lines in end-view of suspension, is generally NOT good for independently sprung cars because of the "jacking-up" of the CG (see the "Jacking" thread linked to elsewhere on this thread). Note that with Beam-Axles, steep lateral n-lines (= high "RC"s) do NOT jack in the same way, so high "RC"s are OK with Beams (on smoothish roads).
~o0o~

3. Large Anti-Dive (the title of this thread) on the front suspension really does make the suspension "stiffer", and can thus lower tyre-grip on rough roads.

... the addition of the upward force induced by the bump should just be taken through the spring ...

The key point here is that the bump does NOT hit the very BOTTOM of the wheel and just push it VERTICALLY upwards.

Rather, the bump hits the wheel somewhere forward of the bottom of the wheel, and it pushes the wheel upward AND REARWARD. Picture side-view of the wheel as a clockface, front of car to right, with the bump hitting the wheel somewhere between 4 and 5 o'clock. This is always the case with sharp edged bumps, such as the kerbs on sides of racetracks.

(Edit: Also, even if the bump hits the wheel only at its bottom, and assuming the wheel is close to locked and sliding during braking, then the increased Fz force means the tyreprint can sustain a larger Fx force (because Fx = mu x Fz), and the resultant "bump" force (Fz+Fx) is again up-AND-REARWARD.)

A high-Anti-Dive suspension carries this up-and-REARWARD force as if the suspension is an almost rigid column in the direction of the force. Hence rougher ride and bigger variation in grip. Less Anti-Dive allows the bump-to-wheel contact-zone to move in the direction of the up-rearward force, so the spring can absorb and "spread out" (in time) the impact force, and the damper can then dissipate the energy.

Note that this effect also works the same in end-view Anti-Roll, namely during cornering. See the cornering car at right-side of "Where is the MC?" sketch on previous page. As the car corners it slides slightly to the left (because of "slip-angles", etc.). If it has high Anti-Roll (= high "RC"), and the outer-wheel hits a step in the road as shown, then the suspension has little ability to absorb the impact with that step. So the impact is transmitted very harshly to the car-body, the body can jack up severely, and rollover can be initiated. NOT GOOD! This problem is worst for rally cars and desert racers.

And, of course, large Anti-Dive will also jack-up the car during braking for corner entry, thus raising the CG, so not good for the subsequent cornering phase...
~o0o~

Bottom Line: Of the various "Antis", large Anti-Squat at the rear of the car is the one that can be useful, and has the least disadvantages. But end-view Anti-Roll, and side-view Anti-Dive of front suspension, are best kept to low values, say, less than ~30%. Especially on bumpy roads.

Z