Direct Acting Spring Damper

[ritwikdas18]

Hello everyone,
I am Ritwik Das from BITS Pilani,India.Our team ran a Pushrod and Rocker system for their last car. However this year we are considering to use a direct acting spring damper system. The reasons to consider so are as follows:
1.Low Weight
2.Less Complexity
3.More Testing Time
4.Bellcrank geometries are not usually changed very often to give different wheel rates.
5.Multiple shock pick up points may be used to give adjustability instead.
6.No significant aero drag on an FSAE car.

We have spring dampers that we have used for the last car. I need to determine whether these will be suitable for the direct acting setup.

What I know & what I have
The force and velocity curves specified by the manufacturer ,so my guess is that the damper shock forces seen in the curve should be less than we would face when the spring is directly mounted.I am also attaching the calculations used for determining spring rates and force velocity curves which were used last year.
We currently have the Quarter Midget Series Damper from KAZ Technologies. This is the link
http://www.kaztechnologies.com/quarter-midget/

[Mitchell]

It's unlikely that you will be able to achieve that MR with direct acting. Work out how the shock will package first and see what range you have available. What are the shocks? Some teams run an extended spring hat to allow them to have more options for motion ratio.

[ritwikdas18]

We are not trying to achieve a MR of 1 but less than 1(MR = spring-damper-change-of-length/wheelprint-vertical-displacement), the reason being to lower the effective unsprung mass.
We currently have the Quarter Midget Series Damper from KAZ Technologies. This is the link
http://www.kaztechnologies.com/quarter-midget/

[ChristianChalliner]

For some context, for a ride frequency of approximately 2-2.2 Hz at both ends of the car (for a 288kg car with driver included) 45/55 Weight distribution and motion ratios of 0.5 at both ends of the car i was going to purchase the damping code (compression 3, rebound 2) for both front and rear.

Maybe you could compre with that and see how your shocks suit the situation. Personally i would have liked ratios higher than 0.5 but my situation prevented me from doing so. 0.7 would be fairly good if you could reach that i think.

[JSR]

It's unlikely that you will be able to achieve that MR with direct acting. Work out how the shock will package first and see what range you have available. What are the shocks? Some teams run an extended spring hat to allow them to have more options for motion ratio.

For a example, here is what Delft used to do when they where using direct acting.

[ritwikdas18]

Is that(the above picture) a custom made spring damper with the rod extension for the team ? The valve code currently obtained using new calculation doesn't match the old ones. So can the dampers be revalved to change the force we want OR do we have to buy completely new dampers because the latter is too costly. Also I came across this note by a member of Monash FSAE

"Also regarding adjustability on our car we have multiple shock pickups on the chassis, each point has a 10lb/in wheel rate change from one another. This means I have 4 sets of spring and the range is from 100lb/in to 220lb/in in 10lb/in increments. It also has coupled damper adjustments so if I need less damping but the same wheel rate I drop the shock to a lower position and put a different spring that makes it equivalent, this is done with a pin that is retained by an R-clip and this change can be done in less than 2 mins. When I did the design for the 06 car I found it was easier get the different motion ratios I wanted from the direct system than from bell cranks. Multiple bell cranks would need to be made or long bell cranks with multiple hole without the sort of adjustment I wanted, this system worked out far better."

[ritwikdas18]

Is that(the above picture) a custom made spring damper with the rod extension for the team ? The valve code currently obtained using new calculation doesn't match the old ones. So can the dampers be revalved to change the force we want OR do we have to buy completely new dampers because the latter is too costly. Also I came across this note by a member of Monash FSAE

"Also regarding adjustability on our car we have multiple shock pickups on the chassis, each point has a 10lb/in wheel rate change from one another. This means I have 4 sets of spring and the range is from 100lb/in to 220lb/in in 10lb/in increments. It also has coupled damper adjustments so if I need less damping but the same wheel rate I drop the shock to a lower position and put a different spring that makes it equivalent, this is done with a pin that is retained by an R-clip and this change can be done in less than 2 mins. When I did the design for the 06 car I found it was easier get the different motion ratios I wanted from the direct system than from bell cranks. Multiple bell cranks would need to be made or long bell cranks with multiple hole without the sort of adjustment I wanted, this system worked out far better."

[bob.paasch]

If you will take a look at the photos from 2015 FSAE Michigan, you'll find the car that was 1st in both autocross and endurance used direct acting dampers (Rejoice Z!! We've been listening to you! Brown gokart!). GFR used rod extensions to get the motion ratios we wanted. The use of multiple shock pickups on the chassis is a great idea, and could be coupled with exchangeable extensions of different lengths to maintain ride height.

Ritwik, GFR uses non-adjustable dampers. Our vehicle dynamics folks have learned to re-valve them, it's not hard.

[Ahmad Rezq]

bob.paasch
When I saw your car on facebook page with direct acting, the first person came into my mind was Erik .

[ritwikdas18]

This is really a silly question so please bear with me ,
Ours is a steel spaceframe chassis , so to have multiple shock pickups won't we need multiple tabs on the chassis at different heights which have to be welded. Also how can rod extension alone create a variety of motion ratio without coupling it with multiple pick-up points since Motion Ratio is defined as "sin(Spring Angle)*B/A where B= Distance of CA mounting point on the chassis to the shocker mounting point and A=distance of CA pivot point to the chassis. So how does using rod extensions of different length without changing the spring angle affect the motion ratio. What am I missing?
Sorry again for the silly question but I can't just figure it out.

[Z]

Bob,

Yes, I did notice that! :)

(Other Teams might take GFR's Michigan-2015 results as reasonable evidence that while DASDs might not, in themselves, make the car much faster, they certainly DO NOT SLOW YOU DOWN!)

BUT (!!!) ... I am not sure I even want to contemplate this ..... but ... was it the DASDs that cost you 45 points in Design Event??? :(
~~~o0o~~~

Ritwik,

DASD Spring-Rate and Damper-Rate calcs are done the same way as with Rocker-Activated-SDs. But you must take account of the MR correctly. Which means MR must be used "right-way-up" (which depends on how you define it), and it is squared.

Some rough rules to get you in the right ballpark...

1. A DASD acting "vertically" above the wheelprint has MR = 1.
So SR and DR calcs can be done without consideration of MR. Call these the default, or "Wheel-", Spring-Rate and Damper-Rate.

2. A DASD acting at ~45 degrees to a ~horizontal Control-Arm (eg. wishbone) has MR = ~0.7, and MR-squared = ~0.5.
So in this case you use a SR and DR that is TWICE the default Wheel-Rates. Given that in your earlier post (calc'd with MR = 1) you had selected damper-valving between the softest and second-softest available DRs, means that in this MR = ~0.7 case you ONLY have to go up to about the middle-stiffness valving, from that graph. Also, buying a Spring-Damper with stroke of only ~40 mm is enough to give the mandated 51+ mm suspension travel, so a potentially lighter damper can be used.

3. A DASD acting at ~30 degrees to a ~horizontal CA has MR = ~0.5, and MR-squared = ~0.25.
So in this case you use SR and DR that is ~4 x the default Wheel-Rates. From your earlier post's graph, you would need the stiffest, or second-stiffest, damper valving. But this suspension would then also be good for ~130 mm (5"+) of suspension travel, so good enough for Baja+! Countless real cars over the years, both production and racing, have had MR = ~0.5. So NOTHING UNUSUAL here.

4. The car company best known for the most comfortable ride, and excellent grip on rough roads, and hence "really good suspension", is Citroen (with the DS and 2CV being the standouts). On these cars the typical MR = ~0.3, or less. Draw your own conclusions.

5. Worst case, on smooth-track racing such as FS/FSAE, "...any suspension will work, if you don't let it...".
So, worst case, stiffen your DASDs up as much as you want, and still OK. But I strongly recommend keeping the "main" SRs and DRs at the softer end of the range, and then make good use of your BUMP RUBBERS for the worst cases! These are squishy-rubber- (or better, foamy-polyurethane-) doughnuts-around-damper-rod. The dampers should bottom-out on these at least once per lap, at the worst road-bump, car-wobble, whatever.

6. The big decision here is really a structural one, related to the overall car/frame design. Pick a chassis mounting point for the top of the DASD that best suits the overall frame design, NOT because of some obscure "spring/damper equations". Roughly speaking, the DASD should have its lower-end pointing at the wheelprint, and its upper-end mounted at an already strong frame-node, and pointing roughly towards the car's CG. This way the DASD takes a large part of the worst-case wheel forces, making life a little easier for the other control-arms.

7. Finally, KEEP IT SIMPLE (but well executed)!!!

Z

(PS. Ritwik, Bob said that "...multiple shock pickups ... could be coupled with exchangeable [rod] extensions...". My guess is that GFR didn't think this extra complication was necessary to win AutoX and Enduro. It wasn't...)

[Kevin Hayward]

Bob,

ECU guys beat you to it, but with maybe a little less speed :) The guys really like the simple approach to cars and we chuffed to see GFR go down the same road.

I agree that rebuilding the dampers is pretty easy. Feasible to do it track side in a half an hour or so. ECU keeps two full sets for this reason. Although two full sets of the Penskes still costs less than one set of the Ohlins.

Like Z, I am also curious as to how it was received by the judges. Our team was criticized in Oz for not having enough adjustments.

Car did 3.8s Accel, 2nd in skidpad by almost nothing, 2nd fastest car in Autocross, and quickest car in Endurance. Big dynamic improvement over the previous car that had more damper adjustments. However with the DASDs the students had to learn a lot more about rebuilding.

Kev

[Mitchell]

Bob,

ECU guys beat you to it, but with maybe a little less speed :) The guys really like the simple approach to cars and we chuffed to see GFR go down the same road.

I agree that rebuilding the dampers is pretty easy. Feasible to do it track side in a half an hour or so. ECU keeps two full sets for this reason. Although two full sets of the Penskes still costs less than one set of the Ohlins.

Like Z, I am also curious as to how it was received by the judges. Our team was criticized in Oz for not having enough adjustments.

Car did 3.8s Accel, 2nd in skidpad by almost nothing, 2nd fastest car in Autocross, and quickest car in Endurance. Big dynamic improvement over the previous car that had more damper adjustments. However with the DASDs the students had to learn a lot more about rebuilding.

Kev

Kev,

Do you use a known shim for particular rates or just go up/down in shim thickness as required? We have been using the 7800s for over 5 years, new shocks are out of the question ($$). This is the first year they will be direct acting and I was hoping to avoid multiple trips to the 3rd party shock dyno.

Mitch

[Kevin Hayward]

Mitch,

I couldn't tell you what the guys ended up putting in the car, or even too much of how they chose the final settings. I agree that 3rd party dynos can be a pain. However, we have a small damper dyno in the workshop, which makes things a lot easier. A heck of a lot of testing and shim stacks can be tried in a solid day or two of testing. The Penskes are a really good solid little damper. My choice would be with them over the Ohlins twin-tube jobs just about any day of the week. The Ohlins are good, but fundamentally the oil has a much more torturous path on the latter.

When at UWA as a student we put together our own damper dyno. Work was mainly done by one of the 3rd years. It was a bit cooler than the Roehrig at ECU because it was a hydraulic controlled job that we could program all sorts of inputs. Put together with stuff lying around the uni.

For these little 1/4 midget shocks you could put together a pretty good dyno pretty easily. Don't really need to bother with a scotch yoke mechanism. Just hook the damper to a crank attached to a speed controlled motor. Hook the other end to a load cell. Linear pots are pretty cheap now. DAQ through your ECU, a little USB DAQ device, or any logger you might use already. I bet an arduino could do a heck of a lot. All in a sturdy frame of big steel sections. If you are really tight do the speed control through a belt drive with a fixed speed motor. Most of the time the guys just test the damper at one motor frequency. While it helps to alter the frequency to investigate inertia effects you can go a long way through development without bothering. If you cant afford a load cell then make your own with a few strain gauges and an amplifier.

Don't need to move it too quickly. The Roehrig we have hits 284mm/s at the stroke we run, but that is significantly higher than where the damper spends most of its time on the car. Temp is also not too critical, as most fluids are quite good with density and viscosity stability in the temp range you will see, and the dampers don't tend to get too hot on the cars. Monitor it though if you have a spare tyre temp sensor. Even better just use a simple thermocouple and dont even worry about emisitivity.

Kev

[ritwikdas18]

Z Thank you very much for your response to this thread , but my question(silly) still remains unanswered.
Q1.The chassis shock pickup point has to be on a node , so in order to have multiple shock pick-up points won't I need to have Multiple Tabs/Clevis on the member of the chassis.Its very unlikely that shims can provide adjustability of more than 0.1 change in MR . So what exactly does it mean?
Q2. I also didn't quite get exhangeable rod extensions since when you fix the chassis pick up point and point on the LCA the motion ratio is fixed. So how will exhangeable rod extensions help in varying motion ratio?
I am also attaching an image of our current car's chassis design with front and rear damper pick up point and also monash's Direct acting Setup.
623
624625626627

[Swiftus]

...thus very different wheel rates if spring rates are around the same values...

So why not get some different springs?

[ritwikdas18]

Yes , thats what we would do.

[Mitchell]

Mitch,

I couldn't tell yo

...

emp range you will see, and the dampers don't tend to get too hot on the cars. Monitor it though if you have a spare tyre temp sensor. Even better just use a simple thermocouple and dont even worry about emisitivity.

Kev

Thanks for the great info Kev.

[Z]

Ritwik,


... my question(silly) still remains unanswered...
Q1. ... multiple shock pick-up points ...?

"Multiple shock pick-up points" are NOT NECESSARY. Repeat, they are an UNNECESSARY COMPLICATION!

Find ONE suitable "shock pick-up point" (per corner), with MR anywhere between 0.5 and 1.0, that best suits the chassis structure.

Then calculate suitable spring-rates and damper-rates. Be honest, and call these your "first best guess". Buy said springs and damper-shim-kits. ALSO buy 10-30% SOFTER springs/damper-shims, and 10-30% STIFFER springs/damper-shims, in case your "first best guess" was not close enough. Now you have THREE suspension set-ups for each corner -> soft, medium, and hard.

Next do lots of testing. Importantly, DO NOT buy any other springs/shims. Just work with what you have got (but see below *). Adjust tyre-pressures, and do more testing. Adjust toe-angles, and do more testing. Adjust camber-angles, and do more testing. Learn how to drive fast with the spring-dampers you have (ie. either S/M/H).

Maybe, just maybe ... next year you might buy some more springs or damper-shims. Other things are much more important. Weight-distribution! Aero! These might require new spring-dampers, but that is next year...
~o0o~


Q2. ... how will exhangeable rod extensions help in varying motion ratio?

They DO NOT. But exchangeable, or length-adjustable, rod extensions are good for easy adjustment of ride-height relative to damper-stroke. So USEFUL. The conventional threaded adjustment similar to the ends of many push/pull-rods is a good enough way to do it.
~o0o~

* To repeat what I said earlier, IMO the easiest way to improve the "suspension related" performance of FS/FSAE type cars is to have fairly soft springing, but back this up with appropriate BUMP-RUBBERS. These might appear to be stupid-low-tech, but they make a big difference!

The biggest performance improvements come from better weight-distribution (less total mass, and lower and more rearward CG), stickier tyres, and MORE AERO! Soft springs with damping-ratio = ~0.5 -> 1 x "critical" keep everything well-planted, and the bump-rubbers take care of the occasional excesses (ie. a big-bump, or too-much-body-roll-from-sudden-turn-in).

Z

[ritwikdas18]

OK Thanks for all the valuable information Z. I need a bit of help in my ride and roll rates calculations which I have been doing over the past few days. This is the excel sheet that I am using for my calcs

https://docs.google.com/spreadsheets/d/198hFUW9CvfZ3Winln0uxclNfQIKhFADuWrn9XSKRM58/edit?usp=sharing
My procedure was in the following way
Static Deflection->Ride Frequency,Ride Rate->Wheel Rate(Taking into Account Tire Rate)->Front Spring Rate,Rear Spring Rate,Front & Rear Roll Rate ->Roll Gradient -> Front & Rear Lateral Weight Transfer-> Dynamic Front & Rear Wheel Travel -> Total Front & Rear Wheel Travel -> Check whether you are bottoming the car or not.Check whether I have the desired roll gradient or not-> If not change Static Deflection and go on.
1.Also I had a constraint that the chassis was already designed(This should not have happened.I know.) . So my front and rear motion ratios were fixed to around 0.766 and 0.5 respectively.
2. Also another mistake was the chassis ground clearance is around 2.5 inches (Very high I know). It should have been ~1.5 inches since the minimum usable travel is only 1 inch.So any suggestions to damage control this mistake in ride rate calcs or any other way?
3.I don't quite know how to to decide my Lateral Load Transfer Ratio.I know that I can control the under/oversteer of my car by doing so, but how do I arrive at a ballpark number with rough calcs according to my requirements.
4. I have also not yet decided on the roll gradient of my car, but will do so in a few days.
5. Lastly For calculation of roll rates Optimum G neglects Tire Rates and uses wheel rates in their formula. I think this should be Ride Rates . Am I wrong?
6.Any iteration can be done on the excel sheet by changing the static wheel deflection and everything else changes accordingly.I have also mentioned all formulas used while calculation and I assure that I understood all of them by the classical mechanical methods before using them.
Z I must thank you that although it may seem trivial to choose static wheel deflection or ride frequency first since they are interrelated but I personally can make more tangible sense out of static wheel deflection than ride frequency.

[Z]

Ritwik,

Only time for short comments...

1. Excel file has TOO MANY NUMBERS!!! Redo to ONLY SHOW 3 SIGNIFICANT DIGITS per entry ... AT MOST!

2. Your Static-Deflections of Xf = 1.6 cm, and Xr = 1.9 cm look OK. They are at softer end of workable range and could work well if backed up by good bump-rubbers.

3. Consider that if you corner hard enough to just lift inner-wheels, then outer-wheels get double their static load, and hence outer-suspension deflects an additional Xf,r = ~1.6 - 1.9 cm (assuming ~horizontal n-lines, = ground-level-"RCs"). Since half-track = ~57 cm, this gives Roll-Angle = ~1.7 degrees (in Steady-State, but maybe more in "transients" if underdamped...). So, if you only want 1 degree maximum SS-Roll-Angle, then fit springs with 1 cm Static-Deflection. (<- See how easy this is to work out! :))

4. Length adjustable damper-rod-extensions are a good idea, so that you can LOWER YOUR STATIC RIDE-HEIGHT. Down to 4 cm (1.5") is good.

5. Final LLTD will depend on the type of diff you have, and other real-world issues. Generally, start at 50:50, then soften the springs at the end that needs more grip. If spool-diff + difficult-turn-in, then stiffen rear-springs and dampers. But much bigger changes can be made by adjustments to tyre-pressures, toe-angles, camber-angles, and ... driving-style... All this best addressed in testing. MUCH TESTING...

Anyone have other comments? (I haven't had time to check the equations...)

Z

[Jay Lawrence]

Quick, change your significant figures before Z reads it!!!!

Anyway,
1. Not sure why your chassis design will cause your MR's to be fixed? Just design a different rocker/change S/D direct mounting (sorry, can't remember which)
2. Relates to 1
3. Your LLTD can be decided by you by plucking a number from your head and tuning your car from there. You could start at 0.5 if you wanted. This is then tuned via ARBs/spring rates
4. Your roll gradient (assuming you mean degrees body roll per g of lateral acceleration) is determined by your static wheel deflection expectation at your maximum lateral acceleration
5. -
6. -

There's nothing wrong with using wheel deflection. Frequencies are only really required when you are trying to specify your damping characteristics, but it doesn't look like you're there yet.

Well, Z beat me to it

[Claude Rouelle]

Ritwik,,

I suggest you make a simple Excel spreadsheet of the variation Vs time of your front and rear chassis ride height variation (or wheel travel Vs the chassis if you want) after a sudden impact of a vertical force at the wheel. You will have a nice sinusoidal output with logarithmic decrements.

Make it simple by using a damping coefficient with is linear (same at low and high speed) and symmetrical (same in bump and rebound)

I suggest 1.5 G of vertical acceleration of initial impact. Ignore the tire deflection in a first calculation and input it in a second one.

Even at the first period, the amplitude will be significantly smaller once you include your damping.

With a damping coefficient of 0.3 the first amplitude variation (without tire) will be about 65 % of your amplitude without damping.

With a damping coefficient of 0.9 the first amplitude variation (without tire) will be about 40 % of your amplitude without damping.

You can then repeat the same calculation in pitch and roll.

Choosing your static ride heights or your springs stiffness or your suspended mass frequency on the simple criteria of suspension deflection without taking into account your damping will lead you to either to too high cars or too stiff suspensions or both.

[Z]

Claude,

I think you should re-think above comments. Your very long signature at the bottom of your posts implies a responsibility to get these things right.
~o0o~


With a damping coefficient of 0.3 the first amplitude variation ... will be about 65 % of your amplitude without damping.

Much, much less! From memory a Damping-Ratio of 0.3 gives an amplitude reduction, per cycle, of something like x 20%, or less. So, each successive peak is one-fifth as high as the preceding peak (or less?).
~o0o~


With a damping coefficient of 0.9 the first amplitude variation (without tire) will be about 40 % of your amplitude without damping.

Huh!!!??? It should be obvious that 0.9 is quite close to 1.0. And, BY DEFINITION, a system with Critical-Damping (ie. DR = 1.0) NEVER returns to its zero position! It has NO "first amplitude".

In practice there is always some extra friction involved, above that of the dampers themselves, and usually of the "stiction" type, and this is usually enough to also prevent a system with DR = 0.9 from getting back to its zero. So abovequoted "40%" should be more like 0%!

A good rule-of-thumb for fast return to zero with negligible overshoot is DR = ~0.7. Less than 0.7 gives faster return, but with some small overshoot. More than 0.7 gives very sluggish return, and may not actually get back to zero. (Simple, 1 DoF, Spring-Mass systems being considered here.)
~o0o~


Choosing your static ride heights or your springs stiffness or your suspended mass frequency on the simple criteria of suspension deflection without taking into account your damping will lead you to either ... too high cars or too stiff suspensions or both.

This whole sentence makes no sense to me, at all. It seems to be an irrational slur against the use of "suspension/static-deflection" as a simple number used to characterise a simple suspension's stiffness.

So, Claude, can you please explain your thinking here in more detail. Advantages, disadvantages, etc. Please be as clear and rigorous as possible.

Z

[Goost]

"impact" i.e. "impulse"

[Z]

... make a simple Excel spreadsheet of the variation Vs time of your front and rear chassis ride height variation ... after a sudden impact of a vertical force at the wheel. You will have a nice sinusoidal output with logarithmic decrements.

Make it simple by using a damping coefficient with is linear...

Even at the first period, the amplitude will be...

Goost,

Your graph confirms that the logarithmic decrements, per period (= cycle), are as in my last post. Damping-Ratio = 0.3 reduces the amplitude to ~15% per period, and DR = 0.9 reduces amplitude to well under 1% per period. Also evident is that DR = 0.7 gets back to zero quickly, with only small overshoot. (FWIW, the "logarithmic decrement per period" = ~ e^-(2 x Pi x DR).)

BUT (!), if Claude's post was suggesting that the heights of the first peaks in your graph (ie. after one-quarter period) somehow imply that DR = 0.9 is better than DR = 0.3, because of the lower peak for 0.9, then I say that is VERY BAD ADVICE!

Why? Firstly, note that your graph is NOT representative of "chassis ride height variation" under Claude's assumed conditions. It should be quite obvious that a suspension with very stiff damping will transmit to the chassis much more of the "sudden impact ... at the wheel", than a more softly damped suspension. The stiffer damping launches the car higher into the air whenever it hits a "sudden vertical impact", and generally makes a real mess of the tyre Fzs. This drastically lowers grip.

Secondly, you might wonder why damper manufacturers started fitting all those expensive blow-off valves to their dampers, all those years ago. In short, on a racetrack with "sudden impacts", linear dampers with DR = 0.9 are a disaster. Softer damping (via blow-off) and good bump-rubbers are a much better solution. Of course, on millpond smooth FS/FSAE tracks you can get away with excessive damping, because "Any suspension will work, if you don't let it ... Although in FS you should dress it up with some pretty (but irrelevant *) graphs, so it looks like you know what you are doing...".

(* Your graph is of an upward "unit impulse" (= finite delta of momentum) applied to the wheel with the chassis rigidly fixed to inertial space (= infinitely massive). This is hardly realistic.)

Z

[Goost]

Z,

I'm pretty sure mass (+ maybe coulomb friction) is the only thing that transmits the 'sudden impact ... at the wheel' if we're being pedantic.
The other effects all require motion.
But, I know what you meant and won't sit and argue about that because it's childish.

"stiffer damping launches the car"
Maybe so, maybe you meant friction? There are plenty of reasons high damping rates are better than lower ones...

~~~

chassis fixed? No. Why do you say that? the mass in the model is the chassis, not the wheel. "Ignore the tire deflection..."
So the impulse is either a force on the chassis, or a displacement at the ground, which have the same response (ratio) for impulse.

Anyway you are of course right that it's "hardly realistic" , though it's certainly not "hardly useful"

If you want to get into a more complex model, how about set up some conditions in a model or provide one?
Here's a one wheel bump (like rolling over a broomstick with one tire) in a 7-dof ride model with nonlinear damping curves and friction to boot.

Now please tell me from this what damping I should change based on what you see here?
(and perhaps to stay on topic, why should I use direct-acting spring dampers instead of whatever this is?)

This picture is near useless unless I provide every spring rate, damping rate, what anti-roll/anti-pitch springs I use
which are of course useless without mentioning track, wheelbase, inertia properties, wheel-set mass
which are of course useless without damper hysteresis since this is small amplitude
which are of course useless without mentioning the input Amplitude since we are now nonlinear.
And of course not forgetting all the assumptions about rigid suspension linkages, no temperature dependent springs and damping, etc. etc.

I think there is much more to see in my previous plot for discussion, even if it is 'hardly realistic'.

~~~

I'm worried the concept of 'static suspension deflection' is going to confuse people if we include friction, because then the term isn't quite right, is it?
Even if we were accounting for preloaded coil-overs the 'static suspension deflection' won't quite be the static suspension deflection.

[ritwikdas18]

Sorry for the late reply . Some health issues.
Anyways back to topic

1.Claude, I am attaching the response functions for my Front and Rear Ride Frequency(~3.9 & 3.6 Hz respectively) along with a 2 Hz to see the difference with DR of 0.3 and 0.9 .The amplitudes on the y axis are not the same as Goost because he assumed (Zo=1) while I took the Boundary Conditions as y(0)=0 (or z(0)=0) and y''(0)=1.5g(verified it be around 65% and 40% respectively).Will do in roll and pitch and update this post in a while.
641

2.Claude, I know that I have to also consider damping in roll,pitch and heave while calculation of ride and roll rates. So I made an excel sheet and combined them both.This is the excel . The figures in there are not final yet and have to be finalized . These are the initial calcs

https://docs.google.com/spreadsheets/d/10QgOUKDsDnOEnRCymudTnSBz1C4v10yGlFrfU1_Yg3w/edit?usp=sharing

3.Since I already have posted , that we are using a Quarter Midget Damper(http://www.kaztechnologies.com/quarter-midget/). So the knee speed is fixed (not a damping speed as in the excel sheet) and we have to choose among 25 options(5 compression and 5 rebound) for valve code.I am attaching the valve codes again (so that you don't have to go to the first page).
642
4.For Roll Gradient a value of around 1.76 is deemed okay.I aim to tune it in testing.
5.For LLTD as per my calcs I am starting at 50:50 and aiming to tune in testing too.
6.To help the above cause I have thought to buy 20% softer and stiffer springs than that calculated both for each corner of the car.
7.Jay the MR is fixed because the points on uprights are also fixed .Although MR can be changed by offsetting the shock point from the LCA upright point but that would only result in additional bending moments in the X direction due to Fz and in the Z direction due to Fx without achieving any significant change in MR.
So therefore MRf=0.766 and MRr=0.5
8.Z you told that the given static deflections/frequencies are on the softer side, but the typical FSAE frequencies are in the range of 2-4 Hz. and mine is 3.5+Hz. So its stiffer right ? with the obvious implication of less mechanical grip but better response in transient and lower ride height and center of gravity.




On another note , Claude you said that you are planning to hold a seminar in India (and also asked us about the price that we maybe willing to pay).I hope you have not forgotten that Sir because it is not included in your seminar calendar for 2015.

[SNasello]

Ritwik, I just wanted to point something out to you which might be a slight problem.
You have in your spreadsheet a rear Spring rate of ~180N/mm which is over 1000 lb/in. Do you know if you can buy such stiff springs that will fit on your damper? If not have you tried contacting someone that will make custom springs for you in about 150mm free length with 35mm diameter? I don't think it will be that easy.

Your values look correct to me, just ask yourself it would be easier to change the rear motion ratio than work with what you have. You have all the information to make an initial guess at what damping codes you need as an initial guess. Pick one based on the information you have and go testing. Learn and repeat.

[Z]

I see that "blind thinking" (cogitatio caeca) is strong here. Just pick an equation, any equation...
~~~o0o~~~

Goost (Austin),


Here's a one wheel bump (like rolling over a broomstick with one tire) in a 7-dof ride model...
...
This picture is near useless unless I provide every spring rate, damping rate, what anti-roll/anti-pitch springs I use
which are of course useless without mentioning track, wheelbase, inertia properties, wheel-set mass
which are of course useless without damper hysteresis since this is small amplitude
which are of course useless without mentioning the input Amplitude since we are now nonlinear...

I agree 100%! Useless!!!

And far too much of that nowadays. Too many vaguely chosen examples, which are specified in sloppily worded sentences and presented in contextless plots, and which are in no way representative of a given problem, yet nevertheless emphatic conclusions are still drawn from their useless results.



I think there is much more to see in my previous plot for discussion, even if it is 'hardly realistic'.
[with this plot referring to]
... the mass in the model is the chassis, not the wheel. ... So the impulse is either a force on the chassis, or a displacement at the ground, which have the same response (ratio) for impulse.
[and]
... I'm pretty sure mass (+ maybe coulomb friction) is the only thing that transmits the 'sudden impact ... at the wheel'...

I still have no idea how your above model/plot is supposed to represent Claude's original problem of "... ride height variation ... after a sudden impact of a vertical force at the wheel."

No, "mass", in itself, CANNOT "transmit the sudden impact".

Rather, any force or motion at the wheel is transmitted to the chassis through the spring-damper (neglecting "jacking-forces" acting through the control-arms). The springs transmit the force as K x extra-spring-deflection, so giving greater "impact" with greater K (and this K includes bump-rubber at end of stroke). The dampers transmit the force as C x damper-velocity, so greater "impact" with greater instantaneous-C (or Damper-Ratio, "zeta").

So, assuming a car travels at a given velocity over a given sized bump, and all-else-equal, it is bleeding obvious that the car with greater DR (eg. = 0.9 vs 0.3) feels the greater "impact". That is, its chassis launches higher into the air, which is the OPPOSITE of your plot. This, of course, is the whole point of fitting suspensions to cars in the first place (ie. softer-suspension = better-bump-absorbancy)!

It is really disappointing to me that so many young "engineers" can produce so many pretty plots without seeing the obviousness of their flaws!


I'm worried the concept of 'static suspension deflection' is going to confuse people if we include friction, because then the term isn't quite right, is it?

The use of "Static-Deflection" to characterise a suspension's "stiffness" was originally brought up as an alternative to "F&R Ride-Frequencies" (<- on a thread of same name).

Both concepts are flawed in that they both rely on many, usually unspecified (!), simplifying assumptions. For example, "SD" implies linear spring-rates, which is usually only approximately the case. But the "RF" concept is even more ridiculous, because its simplifying assumptions are almost NEVER met in practice (eg. CG position and Pitch-MoI must both be "just right", which is very rare, +++).

Neither concept says anything more about the suspension than the other. Well, except that, since SD = (M x Ge)/K, and RF = sqrt(K/M), the SD concept implies that you are on a planet with same gravity "Ge" as Earth. So for "moon-buggies" you must use Gmoon...

But, bottom-line IMO, is that "RF" is used nowadays for no other reason than that it sounds oh-so-clever. By contrast, "SD" is so-stupid-simple that I calculated suitable numbers without even using pen or paper (see earlier post).
~~~o0o~~~

Ritwik,


Z you told that the given static deflections/frequencies are on the softer side, but the typical FSAE frequencies are in the range of 2-4 Hz, and mine is 3.5+Hz.

Despite claims to the contrary, many FSAE cars have run, at the actual competitions, with effectively RIGID suspensions. This is obvious from seeing the cars visibly "bouncing on their tyres", because no suspension movement to damp-out such bouncing. Such cars have, in fact, been outright winners of FSAE competitions. These cars would have been even faster if they had softer suspension.

Your current spring-rates are about right.
~~~o0o~~~

Claude,

I see that Ritwik has included calculations for Pitch and Roll inertias and frequencies based on "the parallel axis theorem". As you may be aware, I believe that approach is deeply flawed.

So, rather than you spending so much time "Politeness Policing", in insisting that Forum-newbies properly introduce themselves, could you please spend some time clarifying why you think the parallel-axis-theorem is necessary? FWIW, I think I know why you use that approach, but it has NOTHING to do with "oscillation frequencies".

After all, isn't this Forum supposed to be about discussing technical engineering issues, rather than enforcing unnecessary social protocols? (For example, I have had countless very polite, very long, and very interesting technical discussions with people at racedays, etc., without ever exchanging any names! Only if future contact is sought is it necessary to exchange names, business-cards, etc.)

Z

[ritwikdas18]

1.Z, I checked with the radius of gyration of typical FSAE Cars in pitch and roll (which is 600mm and 300mm respectively) .My values may be wrong(infact they are wrong) but its close to actual values.All these values are anyways ballpark values because I don't even know the exact CG height and weight distribution of my car . Infact I don't even know the exact unsprung or sprung weight of my car.
2.Z & Claude , Could you comment on my damper code selection? Its at the lower right corner of the damping rate spreadsheet.
3.Also one more thing I wanted to say that I plotted a graph of motion ratio variation v/s bump travel. Now its within 1.5% at the front and within 3% at the rear (for 1.6 inch of travel in compression and rebound). Its degressive obviously but as far as I think it wont make much of a difference because the above numbers would lead to a 3% and 6% increase in roll rates at the front and rear(due to (MotionRatio)^2)) respectively and that too when we assume that the travel is 1.6 inch in compression and 1.6 inch in rebound which is too much for my case(<.5 inch dynamic wheel travel). So any comments on the above?

[Tim.Wright]

Non linear motion ratios aren't necessarily bad, but you need to keep an eye on how they change the LLTD balance in function of roll angle. These non linearities will likely change the balance of the car between the linear and limit ranges.

[CWA]

My interpretation is as follows:

Goost's original plots / Claude's descriptions (pretty standard 1DOF oscillation) depict 'wheel travel' change during a bump event; this is unsprung mass motion in relation to the chassis. They are not wrong. But neither are they describing the actual motion of the chassis through space. Or perhaps more accurately, Goost's plots show displacement of the chassis w.r.t to a bump profile, but not the rest of the road.

As Z says, the motion of the chassis through space (hence ride height variation) should correlate with the vertical forces acting on the chassis, which are generated during these 1DOF oscillation events. With a stiffer car (more springs per mass, or more damping per S/M), yes the wheel travel (w.r.t chassis) is less. But the forces acting on the chassis will be much greater during the event, so doesn't it make sense that the chassis moves more in space? That stiffer suspension gives more ride height variation?

If we picture this in terms of net displacements in space, this also makes sense. Greater wheel travel (remember above definition) motion during a single bump event does not correlate with a chassis ride height increase as I believe is being implied. Is the opposite not true; 'bump profile height' minus wheel travel gives overall (upwards) chassis displacement (w.r.t to the rest of the road). So less wheel travel, means more overall ride height variation.

Consider a front axle and the chassis above it, of a car travelling along a flat road which has a sizeable bump in it. If the front suspension is soft, so soft that the wheel travel seen during the bump event is equal to the bump profile in the road, neglecting phase lags, doesn't the chassis remain flat w.r.t the rest of the road? Doesn't the soft suspension facilitate minimal ride height variation?

Now consider a much stiffer variation of the same car conducting the same event. During the event, less suspension travel means the chassis follows the profile of the road more, and is displaced this much in space (w.r.t to the flat road) too (with all associated uncomfortable accelerations). This second, stiffer case of course has very low wheel travel (unsprung mass motion w.r.t chassis), but the instantaneous ride height variation (of the chassis w.r.t the rest of the road) is going to be much higher than that of the first, softer case.

As ever I welcome correction on this, but this is what I believe to be true.

Perhaps what Goost is trying to imply is that on a bumpy surface, the pitch orientation of a race car (which aerodynamics are perhaps more sensitive to?) is less controlled with a softer car. I don't necessarily disagree with this, but I think it should be made clear that calling this a chassis 'ride height change' issue is perhaps not descriptive enough, and the plots posted above to try and make this point are misleading. To make this point, don't you at least have to consider 2DOF side-view coupling of modes (with pitch inertia considered) rather than 1DOF SMD, as Z has suggested?

If I have misunderstood the issue and this is redundant input then apologies.

[Flight909]

Non linear motion ratios aren't necessarily bad, but you need to keep an eye on how they change the LLTD balance in function of roll angle. These non linearities will likely change the balance of the car between the linear and limit ranges.

Also is important the effect when you have downforce because you can change balance with speed. With more compression of suspension the stiffness will change (maybe even balance front to rear), this can also be good if the tires are not well balanced when you load them.

[Claude Rouelle]

For people who do think that there are no need for suspension...... I just want to remind that damping is the #1 parameter that influences tire temperature (and therefore tire grip) is the suspension damping.

Two years ago I simply jumped on one FSAE car at Formula North. Feeling and looking at the decrease of the amplitude for a given input, stepping on or out the car, I did not need any sensor, just a bit of experience, to know that the dampers were far away from critical damping. I went with one of the team members through some basic calculations and found that their damping ratio was about 0.2 or 0.3 (I do not remember exactly). I suggested that they tune the damper to go to 0.7. They had the damper curves and they were able to do that. The lap time improvement was really significant: 2.5 seconds (again if I remember well) on a circuit of 45 seconds (again, if I remember well) but the tire temperature increased something like 20 or 25 degrees C.

Later on, that winter, that team tried different front and rear roll centers and the difference in the lap time and the car feedback was HUGE. According to the team comments just raising one roll center 10 mm and the other 30 mm made the car 1.5 seconds quicker

I am sure that those team members will recognize themselves in this thread: up to them to comment and confirm with exact numbers.

As a consultant I once had a high end single seater car customer who told me that damper did not count. OK I said and I move the bump and rebound knobs and significantly changed the damper curves. The driver went out on new tires and completely destroyed the tires in 4 laps while he normally was able to run 50 to 60 laps with the same sett of tires. You draw the conclusions.

0.7 of damping ratio in heave is not a bad idea to start with on a FSAE car. But often that will give you way less that 0.7 in roll and pitch. Up to you to decide what damping is the most important.

[ritwikdas18]

Claude Sir, I would like to say one thing ,
For equal ride rate and damper coefficient front and rear and cg at (1/2) of wheelbase, the ratio of (damping ratio in heave / damping ratio in pitch)=Pitch dynamic Index=(Radius of Gyration in Pitch)/(Wheelbase/2) For FSAE cars or race cars for that matter the pitch dynamic index is much lesser than 1 . In my case it is ~0.78 . So Damping Ratio in Pitch =(1/0.78)*Damping ratio in Heave.=> Damping Ratio in Pitch for Race cars/ FSAE is usually greater than damping ratio in Heave. This is however based on the assumption that I have but it stands that damping ratio in Pitch is greater than that in Heave, my excel figures also tell me the same.

https://docs.google.com/spreadsheets/d/10QgOUKDsDnOEnRCymudTnSBz1C4v10yGlFrfU1_Yg3w/edit?usp=sharing

Also Claude ,
I don't want you to give me the answer but could you comment on my Damping Rates (Damper Code Selection) & Ride and Roll Rates.

[Z]

Ritwik,

Per your numbered points top p4.

1. Your estimated Pitch and Roll Radii-of-Gyration are close enough. The Yaw-RG will be a bit bigger than P-RG. It is the one that all Teams should work hardest to reduce (ie. for "agile" car), but it is also NOT relevant here...

2. Damper code selection? This depends on money available, but I would be inclined to buy 4 x sets of damper-shims in each of the #3, #4, and #5 ranges (Edit: Oops, see correction at bottom of post...), for low/high-speed and bump/rebound. If you can afford this, then there is no problem having many shims sitting in a cupboard in the workshop. If this too expensive, then buy 2 x sets of (Editted->) stiffest and second-stiffest, and mix-and-match between F&R. But see note* below.

3. The best FS/FSAE driver in the world will not be able to feel any change from your 3% MR change. Especially not when this 3% is over your quoted +/- 4 cm range. As you note, the practical movements during cornering will be more like +/- 1 cm, giving ~0.007 x MR change. And since these changes are likely same F&R, then NO change to "SS-balance".

* Your most important goal now is to get a well-detailed and ROBUST car built ASAP. Then straight into MUCH TESTING and DRIVER TRAINING. A moderately well-trained driver will make much bigger gains (ie. be much faster!) than any amount of tweaking that you now do to the "theoretical" suspension design.

During said testing you will also have a great deal to learn about the best (???) tyre-pressures, toe-angles, and camber-angles. Only AFTER you have these three working sort-of-OK should you try a different set of carefully chosen spring-rates and damper-shims. Then new tyres (you should have already worn out one set) ... and more adjustments to tyre-pressures, toe, camber...

Once again, most important to fast competition times is much driver training time.

Which reminds me, what competition are you going to?
~~~o0o~~~

CWA,

Your understanding is the same as mine. :)

Z

(Edit: Just checked Kaz-damper-graph... It seems they have shim-kits #s 1-2-3-4 getting progressively STIFFER, but then #5 is the SOFTEST!? So, in my above comments read it as "buy the three STIFFEST ranges of shim-kits"...)

[ritwikdas18]

We are an Indian team and we get the dampers from US.US dollars when converted to INR are really too much to afford, Maybe, maybe we can afford different spring rates if more money comes in at the end but not now. Also about the event:
We are required to complete the CAR and submit its running video with pictures showing that we abided by the rules by 5th October .Without that deadline completed we will not be allowed to participate/register at the event.We are participating in Formula Student India 2016.(FSI). So obviously everyone at the event will have tested their car a lot simply because the rule requires them to do so. About our past result ,In FSI 2015 we stood 8th place among ~40 teams .

[Goost]

No, "mass", in itself, CANNOT "transmit the sudden impact".
Again, I said arguing over this is not fruitful. If you apply a practical force to anything in real life, the only way it can truly instantaneously react is by its mass. A spring requires displacement, a damper requires velocity.

The important part of this is that this input IF A Force only adds a certain amount of energy to your system.
But ah now you agree we mean displacement...

where a known input Displacement occurs at the wheel, and it's fast enough you might even say it was 'instantaneous'.
In this case, yes stiffer spring with same displacement means More Force (or more energy).
If the car had no dampers - would the spring rate change the amplitude of oscillation at all? or would the stiff one 'launch'?
If it 'launches', you have got to say what part of the model was ignored before anyone can move forward.

~~~


By contrast, "SD" is so-stupid-simple that I calculated suitable numbers without even using pen or paper (see earlier post).

Yea I saw that. unfortunately, thanks to these calculations Ritwikdas18 has springs (as of the time of this post) that are about twice as stiff as a 'good soft' FSAE car.
Oops.

~~~


Goost's plots show displacement of the chassis w.r.t to a bump profile, but not the rest of the road.

This is what I mean if you like. Really it's just an under-damped, second order spring-mass-damper.
I think ~85% of automotive springs and dampers can be explained with that model.
The things happening in the plot are useful when discussing these concepts, but useless presented to someone wanting to argue.

It also shows Claude and Z were both right about the response with varying damping (the initial question), only Claude's description was more practical at that point in the discussion.

The rest of this is right, which we all agree on and I probably didn't need to repeat in section at top.

~~~

Trying to get to the point...

Claims that the chassis 'launches' anywhere with stiff springs make crummy points for argument as it's a hyperbolic example.
Choosing the softest spring that doesn't bottom out in your suspension to soften ride - depending on the travel you have that's either hyperbolic or simply wrong.

Neither of these examples gives a criteria for choosing anything about your springs and dampers!

it doesn't bother me a bit to ignore the chassis at first and just look at the tire contact with the ground.
There you will find an actual spring rate and damping rate that are ideal for the things you cannot change about your suspension - the Masses and the Tire Spring Rate.
But what defines 'ideal'? I know how I do it, how do you? whether it 'launches' over a bump is not my criteria.

Some things I have learned from this reasoning;

1) why (non-aero) racecars have stiffer springs than road cars, but often not by much (it isn't suspension travel)
2) that choosing a damper somewhere between 'ideal for ride' and 'ideal for grip' is a very good compromise
3) The actual effects of unsprung mass
4) why automotive damper 'curves' are not symmetric in compression/rebound
5) why low speed damping ratios > 1 are completely reasonable

~~~

ritwikdas18,

it would be much easier for anyone to give you advice about your dampers if you reduce it into damping ratios.
You will want the ratios later anyway.

My advice without more numbers would be to let your supplier help you choose them, they are always helpful and usually know what they are doing.


Damper code selection? This depends on money available, but I would be inclined to buy 4 x sets of damper-shims in each of the #3, #4, and #5 ranges
How can you say something like this after discussing for weeks about how to properly pick springs??

[ritwikdas18]

@ Goost
1.The damping ratios are included in the lower right corner of the damping rates sheet. You can also check the damping ratios in heave,pitch and roll similarly and leave a comment
2. Also if you go on the ride and roll rates sheet , and decrease frequency to make it a good soft car then my roll gradient increases a lot to around 3-4 deg/g .The only way to decrease that would be to use ARBs. A lot of time will be used up in designing and manufacturing one because we have never used it before.So I prefer not to use them given that 5th October is the deadline of car completion. If there is mistake in my calculation or anything that you feel is wrong / too stiff then I would request you to suggest me what to do with them. If you feel ARB's are absolutely absolutely necessary given my vehicle information , then please tell me the same. One more reason the spring rates are high are due to low Motion ratios of 0.76(F) and 0.58(R) (I changed the rear one).Since ride rates/wheel rates are important , could you comment on them?
Thanks a lot.

[Goost]

Ritwikdas18,

I did mean 'wheel rate' or 'ride rate' when I said it was too stiff. Really I didn't even look at your installed spring, just know that your natural frequency is too high, by ~50% from my experience.

I can't see your calculation for the damping ratios on mobile.
If you use the slope of the high speed line to compute them, I think that's not good - Better to use the average damping.

Anyway, the code you show (4?) is still far too soft damping ratios. Probably because the motion ratio is outside the range these are designed to accommodate.

Sorry not to explain fully, would rather not have a one sided conversation about this:
I would suggest damping ratios between 1 and 1.5 low speed, and effective ratio of ~.65 to .85 high speed.

~~~

Don't recall you saying - can you change any suspension geometry? Could you raise the roll centers to stiffen roll?
That's not as adjustable, but easier to implement sometimes than ARBs.

[Goost]

Here are some optimization results to get you thinking.

'Stability' should be called 'ride' probably.
'Grip' in this case is almost, but not quite, related to tire deflection.

Please be careful as they will be different for different sprung/inspiring mass ratios, and for different tires.

The parameters are however on the right range for a reasonable lightweight FSAE

These charts Do Not explain low speed damping or asymmetric damping rates rebound/compression.

[MCoach]

3. The best FS/FSAE driver in the world will not be able to feel any change from your 3% MR change. Especially not when this 3% is over your quoted +/- 4 cm range. As you note, the practical movements during cornering will be more like +/- 1 cm, giving ~0.007 x MR change. And since these changes are likely same F&R, then NO change to "SS-balance".


Coming from a softly sprung car this year, I'd like to note: Yes, you can tell the difference in single digit motion ratio changes. The difference for us was a linear rate curve vs a single percentage progression. Both drivers who tested the change noted the difference, agreed on it, and preferred the same setting. It's noticeable.

[JT A.]

Coming from a softly sprung car this year, I'd like to note: Yes, you can tell the difference in single digit motion ratio changes. The difference for us was a linear rate curve vs a single percentage progression. Both drivers who tested the change noted the difference, agreed on it, and preferred the same setting. It's noticeable.

That's interesting, but I'd still be really skeptical about getting a placebo effect. How did you swap out bellcranks without the driver seeing what you did?

[Claude Rouelle]

Ritwik,

You do not know for sure if you car will be "born" with a U/S or O/S characteristic, correct. No simulation, tire model and driver adaptation skills will give you that spot-on combination that early.

So you need to give your self the opportunity to adjust your balance (or your TLLTD if you want), depending upon your driver comment, tire temperature, analyzed data etc... But you have no ARB? So what will you do? Change the springs probably, front, or rear or both. But then you will destroy your ride frequency target. You will fill a hole by creating another one

Also you will need to switch to a spring of let's say 125.0 lb/in to a 137.8 lb/in ... that doesn't exist. It will be 125 or 150 lb/in. While with a good front and rear ARB design, you can have a wide range of roll stiffness choice AND TLLTD choice too.

Up to you but in my opinion front and rear ARBs should have been part of your initial design.

That being said nothing prevent you to design, install, test and develop them after the video submission deadline.

Cheers,

Claude

[Claude Rouelle]

Changing you MR 3 % will change you wheel rate to about 6 % which is in the zone that a) will be sensed by any decent driver (including FS / FSAE decent average driver) and b) will change your tire temperature enough that it will affect your car performances one way or the other

[Z]

Goost,


... arguing over this is not fruitful...

Grooaaannn... :( [Commence Rant ->]

It is the process of discussing things in a WELL-REASONED way that has, most "fruitfully", given everyone here their oh-so-easy lifestyles (...too easy!). In the hope that just a few of you eventually learn this process, I repeat its bare bones here.

The essence of this process was chiselled above the doorway to Plato's Academy, namely "Let no one ignorant of geometry enter here!" This approach to thinking about a subject, any subject, was later epitomised in Euclid's Elements, which became the only textbook that all schoolchildren HAD to have for the next two thousand years. Sadly, the Elements were dropped from the curriculum about a hundred years ago... And now we have the nonsense that fills these pages.

Very briefly, this "geometric" way of thinking consists of a 3-step process.
1. Start with clear DEFINITIONS of all important terms (these should be at the very beginning of any discussion, but are almost nonexistent these days).
2. Make very clear statements of what is ASSUMED to be true (again, rarely seen nowadays).
3. Use a rigorous process of DEDUCTION to find consequences that stem ONLY from the Defined and Assumed "truths", with no drawing of wild and unjustified conclusions (as is so common nowadays).

It is an easy process!!!



But the above approach has been too successful. Nobody has to get anything right these days, yet everyone can eat their fill, and more! So the school-kiddies are nowadays taught such codswallop as, "There are NO wrong answers. Everyone's opinion is equally valid. Ah..., yes little darlings, you are all winners. A gold-star for everyone!".

And when these kiddies grow up and go to University, they learn how to make their magic-boxes draw colourful little pictures. And they believe that all their pictures must be good, and true, and worthy of yet another gold-star. And their ideas must never be subject to any sort of criticism, for "... there are NO wrong answers..."!!!

Worst of all is that said attitude has now spread throughout the whole education industry.

So "teachers" can now peddle pure poppycock, with NO clear Definition of terms, and NO clear statements of Assumptions, and NO rigorous Deduction of consequences. And when said "teachers" draw wild conclusions, such as "Pitch and Roll inertias must be calculated using the Parallel Axis Theorem...", they expect the students to swallow this nonsense WITHOUT ANY RATIONAL EXPLANATION, at all.

So now we have an Education system that is founded on nothing more than "Appeal to Authority" (ie. "I am the Teacher, therefore I am right!). This is regardless of how nonsensical are the teachings. And this is no different to any primitive, superstitious, faith-based belief-system, such as voodoo, black-magic, or [insert the stupidest religion/ideology you can think of here].

[End Rant] ...for now. Or until Claude attempts to rationally explain his teachings (eg. "Parallel Axis Theorem" above).
~o0o~


... unfortunately, thanks to these [static-deflection] calculations Ritwikdas18 has springs ... that are about twice as stiff as a 'good soft' FSAE car.
Oops.

As Ritwik noted, he is NOT following the flock and fitting ARBs, so his 4 x wheel-springs take all the cornering loads. And as I noted, it is a historical fact that cars have won FSAE with NO suspension movement at all.

But, for all you students who are firm believers in voodoo, black-magic, etc., ... you had best do what you are told and fit those ARBs...
~o0o~

Many other poorly reasoned arguments, but moving on...


How can you [Z] say something like ["Ritwik should buy 3 x sets of damper-shims for his rebuildable, but non-adjustable dampers"] after discussing for weeks about how to properly pick springs??

The Kaz site has Ritwik's chosen non-adjustable QM dampers at $279 each, the "top-of-line" 7800-Double-Adjustables at $750, and 4-way-adjustable Ohlins at $650.

Why does Kaz sell the adjustable models at such (exorbitantly!) high prices? Presumably, because a great many Teams buy them!

To spell this out, obviously a great many Teams think they might need to adjust their dampers AFTER they finish designing/building the car. A reasonable idea IMO, given the large number of unknowns involved. (Egs. Just how heavy will the car be? How bumpy, and grippy, will the track be? ...).
~o0o~


I would suggest damping ratios between 1 and 1.5 low speed, and effective ratio of ~.65 to .85 high speed.

Why? And, very importantly, what does your suggested "damping ratio" apply to (ie. sprung-mass-heave, -pitch, or -roll, or single-wheel oscillations)?

IMO, in FS conditions, only one of those oscillations needs to be heavily damped (ie. at DR = ~1).

But suggesting a DR for an [B]unspecified oscillation, ... is meaningless. :(

Z

[Z]

MCoach, Claude,

To repeat, Ritwik's quoted 3% MR change was over his total suspension movement of ~8 cm!

The MR change is obviously much smaller over the much smaller suspension movement the car experiences in cornering. And if the MR changes are similar F&R, then even less change to "balance". And if the car is set-up to "work well" with whatever MRs it happens to have in the corner, then it is pointless quibbling about any trivially small changes away from the "central" MR.

The FS/FSAE Rules are such that these cars have the potential to be MUCH, MUCH FASTER than they are now.

It is beyond me why so many people here are trying so hard to maintain the status quo with their pursuit of over-complicated, slow, and boring "Mini-F1" cars.
~~~o0o~~~

Ritwik,

Having only now checked the prices for high-speed (= "blow-off") shim-kits on the Kaz site, I agree that they are ridicuously expensive.

The main damper-rate you might have to adjust is the low-speed bump and rebound. It appears (?) that both of these are controlled by the same slots on the "Bleed Discs", visible half-way down the "Shock Parts & Tools" page. At $85 these may be a more reasonable option for you to buy. Personally, I would be quite happy to modify these Bleed Discs by blocking or enlarging their slots, to custom-adjust the rates.

Perhaps Kevin, or ECU team-members, can comment on ease of adjustment of these Quater-Midget dampers?

Z

[theTTshark]

As Ritwik noted, he is NOT following the flock and fitting ARBs, so his 4 x wheel-springs take all the cornering loads. As I noted, it is a historical fact that cars have won FSAE with NO suspension movement at all.

But, for all you students who are firm believers in voodoo, black-magic, etc., ... you had best do what you are told and fit those ARBs...


Welcome to magically fairy voodoo land! Where being different is better! ALL THE TIME! Different different different! If you're not doing something different you're an idiot! Rah rah rah, hey hey hey!

Oh right! I forgot, everybody in the vehicle dynamics world are idiots except for you! Shoot, I probably forgot that because I'm an idiot. Or wait, are we just idiots in the threads where you know you can just keep derailing the conversation by being a prick all the time? Because I have yet to see your response to me in the moment diagram with weight transfer thread where you were saying we were idiots for using MMM diagrams and how there was no way any of us stupid peasant vehicle dynamicists could be using it for anything useful. You are simply here to try and make people believe you're the one true savior of VD because for the last 3 or so years I have yet to see evidence that you are here to have conversations and potentially change your opinion based on other people's experiences. I learned a lot from your posts where you aren't trying to constantly compare your e-dick to everyone else who disagrees.

Moving on. You can get away with running no ARBs some of the time, you can't get away with running only springs all of the time. I would say that a lot of top series in fact run no rear anti-roll bar, but at most places have to accept the front arb as a necessary evil. Why? Because we have constantly changing balance due to conditions, drivers, tires..etc, some tracks are incredibly bumpy and some smooth and everywhere inbetween and they all require different compromises, and time constraints that don't allow spring changes every time you need to make A LLTD change. I don't have to rescale and reset ride height when I make a bar change (assuming we're road course racing). Yes, racecars do exist in the real world and not just as hypothetical objects on a forum.



The FS/FSAE Rules are such that these cars have the potential to be MUCH, MUCH FASTER than they are now.

It is beyond me why so many people here are trying so hard to maintain the status quo with their pursuit of over-complicated, slow, and boring "Mini-F1" cars.


And your brown go-kart is the answer right?...All hail the VD messiah! For you are the only engineer ever to think they found the answer to make a racecar faster! We will build all racecars in your image!

You're the fifth person this month I've heard that thinks they have the answer. If only racing was that easy.

[MCoach]

That's interesting, but I'd still be really skeptical about getting a placebo effect. How did you swap out bellcranks without the driver seeing what you did?

The bellcranks are already designed with the mounting holes in them, so we only had to take the down time of moving one bolt on each side of the car. I was one of the drivers, so placebo effect may be in effect, but consistently higher lap times don't lie.

[Claude Rouelle]

Trent,

About Z

Some good observations about Z here. There is long time I have decided to ignore his comments and respond to his useless sarcasms. Too bad he lacks some respect and basic manners because some of his engineering perspectives could be useful.....although to my knowledge these perspectives have never been confirmed by success on race car he built or teams he worked with. That is true that to successfully work in a racing team you need some positive attitude and some communication skills.....

To all,

About front and rear ARB

Ask your driver about the car balance; let's say he tells you he has is O/S. You need to increase your TLLTD by either stiffening your FARB or softening your RARB. Which solution will you choose? To make that decision, ask your driver a second question: is your car too nervous or too lazy? If it is too lazy you need to stiffen your FARB, if it is too nervous you need to soften your RARB. Now what do you do if the car oversteers and is too nervous and you do not have any RARB? You will need to soften your rear spring and that will destroy you ride and pitch targeted frequency. That is a reason why most race and passengers cars have frotn and rear ARBs

About high speed damping and blow off

Lest' be serious and practical: what do you call damper low speed and high speed? At what damper speed do your damper reach the blow off speed and where on the circuit? If you do a damper histogram you will see that on a FSAE you probably spend 80 % of the time between - 50 mm/s and + 50 mm/s. That is usually what damper guys call low speed. There are a few little bumps at FSAE tracks but these are not Sebring bumps, chicane curbs at Long Beach or WRC special stage junps....

Claude (in Le Mans; I have a blast!)

[Ahmad Rezq]

Claude
Excuse me. But many of Erik posts which I read are related to basic engineering skills.
Example : The time when many professional engineers offered complex ways to explain anti features or jacking Z offered Simple free body diagrams which were more useful.

I don't know if Erik has developed new theories in VD or not but his contributions in many topics always make sense.

[Z]

Trent,

Tough day at the office? Given that you now work in NascarLand, you have my sympathies.


I have yet to see your response to me in the moment diagram with weight transfer thread...

I was hoping more people would post on that thread with more specific details of how they use those MMMDs (which is what JP, Silente asked for). I have suggestions for how those diagrams could be made more useful, and I may get around to posting them... But should I bother?


You can get away with running no ARBs some of the time, you can't get away with running only springs all of the time.

I have NEVER seen a good off-road racecar with ARBs, and they are all about good suspension. Oh, yes, ... they are different to FSAE. And so is Nascar...

Horses for courses...


And your brown go-kart is the answer right?

See bottom of post...
~~~o0o~~~

MCoach,

The above discussions about "changed handling from changing MRs" is a good example of the SLOPPINESS of thinking that I was getting at. So, one more time, putting it as simply as I can...

Ritwik's rear suspension has a Kinematic layout of DASDs and suspension-arms that gives a 3% variation of instantaneous MR, over the 8 cm range from full-droop to full-bump. (This is my understanding from Ritwik's post top-of-p4, which is quite well written, given that English is probably not his first language.)

IMPORTANTLY, on Ritwik's car:
The MR at static-ride-height NEVER CHANGES.
The MR at full-droop NEVER CHANGES.
The MR at full-bump NEVER CHANGES.

It is simply that the MR at these different suspension positions is (very slightly!) different to the MR at the other positions.

But (!), on your car,

The bellcranks are already designed with the mounting holes in them, so we only had to take the down time of moving one bolt on each side of the car.

From which I assume, you are CHANGING THE MRs OVER THE FULL RANGE OF SUSPENSION POSITIONS, from one "car-set-up" to another "car-set-up"!!!

Unfortunately, you have not bothered telling us how much your MRs changed, so NO meaningful conclusions can be drawn!

Anyway, I hope you can see the huge difference in the above two cases (ie. they are NOT comparable!).
~o0o~

Bonus points for anyone who cares to post calcs for "How much handling-balance-change would come from Ritwik's above suspension, and in what circumstances?".

Hint: By my reckoning, and based on my Assumptions written on this scrap-of-paper, because LLTD would only change as peak cornering-Gs change (because of more body-roll, to spell it out), even a doubling of cornering-Gs would give a change in LLTD so small as to be imperceptible. This imperceptibility because the tiny change in LLTD would be against a background of the car going ~40% faster, having ~double the aero-drag (hence more R-load), ~double any aero-lift/downforce and associated shift in balance, ~double the toe/camber++ compliances, and massively changed loadings/temps++ of the tyres...

It is entirely possible that, because of all those "other" changes, the car would move from, say, mild-US to massive OS, even though the (tiny!) LLTD change would be in the opposite direction.
~~~o0o~~~

Back To DASDs.
=============


Posted by Claude:
... observations about Z...
... to my knowledge these perspectives have never been confirmed by success on race car ...

My intention when first promoting DASDs was to give the smaller and less well resourced Teams a potential advantage. By tossing their previous year's Rockers+++, they could save time, money, and weight, and thus put more effort into the much more important parts of the project (eg. aero, driver-training, reliability, more aero+++). Oddly, it seems that it is mostly these lower-half-of-the-ladder Teams that still see DASDs as the work of the Devil.

On the other hand, and contrary to Claude's claim above about "... never been confirmed", it seems that the Teams towards the top-of-ladder think differently. Some of them like DASDs! Hmmm, maybe that is because these more successful teams are using that ancient approach of thinking things through, "Plato's Academy style"...???

Claude, check the ladder. And when you have time, try thinking through that Parallel Axis Theorem... You know, with clear Definitions, Assumptions, etc.

Z

(PS. Ahmad, Thanks. No new VD theories. I am still trying to learn what all those clever people from hundreds/thousands of years ago gave us...)

[Z]

... you need to give your self the opportunity to adjust your balance ... But you have no ARB? So what will you do? Change the springs probably...
... you will need to switch to a spring of let's say 125.0 lb/in to a 137.8 lb/in ... that doesn't exist. It will be 125 or 150 lb/in...
...
Up to you but in my opinion front and rear ARBs should have been part of your initial design.

Ritwik,

There was some discussion on the Lincoln-2013 thread (in Competitions section of Forum) about how to make fine adjustments to DASD-rates (see Matt Davis's posts bottom-p6 and p7).

Hopefully this link to Uni of Cincinnati's Facebook page will show you how it was done...

https://www.facebook.com/bearcatmotorsports/photos/a.305398312844316.79876.142023435848472/544556405595171/?type=1&theater

The idea is simply to insert some rubber spacers between the spring coils, thus taking a half-coil or more "out of action", and thus slightly stiffening the spring-rate.

Also UoC's 2013 car was originally intended to use ARBs, but after initial testing they found them to be UNNECESSARY! :) And searching around for other recent competition winning FS/FSAE cars shows ... same result!

Z

[Claude Rouelle]

I uses bump rubbers a lot.

I fact I remember testing over 2000 of them on a specific test machine for Indycars several years ago. Nascar team play that game too. They are very useful fora very ride height sensitive heavy aerodynamic cars.

But their utilization can be misleading

Bump rubbers are not bad ideas but
a) It is an engineering competition; you will need to come with bump rubber curves Force Vs displacement
b) Bump rubber have huge hysteresis. Will you take this into account?
c) Bump rubber are not only stroke sensitive but also peed and temperature sensitive
d) In a perfect world, as their stiffness are not linear you need a damper which is stroke sensitive
e) If you use bump rubber with clearance at the moment the bump rubber is "engaged" the force variation is negligible and then progressive, that is what you want from a bump rubber. BUT the stiffness variation is HUGE. If you look at the SLOPE of the Force vs movement you realize that you suddenly add another spring. Your damper is not adequate anymore. I have been working on many race cars that worked well when the bump rubber was on ( we tired to do that in straights) or off (we tried to avoid using them in corners) but got crazy amplitude and frequency once the bump rubber was active Passengers can test driver can really feel that issue too: bumper rubbers on passengers cars are usually used for curbs "to avoid getting the tire in the inside face of the fender" but sometimes if the bump rubber become active in roll or in pitch the heave and roll or pitch frequency chance so much that the driver can have difficulties to control his car.

I don't say you will need to measure all this but you need to be aware of all this and ideally quantify bump rubbers advantage and limitations.

While you will have that finished you may will reconsider rockers with variable motion ratio. But, on the other hand, only a few FS/FSAE team have shown their ability to do so.

Make it simple yes but know how simplistic it could be.

[ritwikdas18]

Thanks everyone for guiding me. I would like to put my final decisions, instead of reiterating over and over.
1.Z I couldn't go with using high stiffness springs because of the following reasons

a.My low motion ratio and high spring rate put my rear compression off the chart as is shown in the figure
667
b.Therefore I had to change the frequencies to get it within the curve. Now my front and rear static deflections are 3.4cm and 3.9 cm with frequencies of 2.7Hz and 2.5Hz
https://docs.google.com/spreadsheets/d/1W_6i1c3-6RdbP01oaf8D4tLAHoa9a6Ll7R5aDgUaUH0/edit?usp=sharing. [ Please ignore the damping decisions section only . They are mentioned below]
Goost,I raised the roll centre to decrease the roll gradient but still roll is compromised due to low ride rate compared to former.All of the above 2 changes led to the following:
668
2. The final damping decisions were taken as follows
Front Compression: Code 1
Front Rebound: Code 1

Rear Compression: Code 4
Rear Rebound: Code 2


669
3.Due to the above frequencies there is now more travel, but it is still within the range I feel as Z pointed out[This is because when I increased by rear MR to 0.62 , the rod extension length increased as well, thus lesser change in MR with roll ].I would go through that forum page you told.
4.My spring rates are as follows:
Front: 29.8N/mm
Rear:~50N/mm
Custom Spring Manufacturing in India is more costlier than in US. Since I am getting the front springs from KAZ, and many people in this forum are from US,I would like if someone could post here a reliable custom spring manufacturer in the US.{The spring would be defined as follows:-

Material: Chrome Silicon

ID: 43.18mm (1.70 inch)
OD Max: 56mm (2.20 inch)
Free Length Max:127.00mm (5.00 inch)
Spring Travel-70mm (2.75 inch) (This may not be required)
End Coil Configuration- Closed(Squared) and Ground
}

On another note I would like to thank Z for saving so much time of my team with these DASD setup. He is right in saying that he wants to give small resourced teams a potential advantage.I may lose some points in design since I do not have ARBs and bellcranks, but hoping to cover them up in dynamic events or physical data from car testing.Some facts
In the recent FS India event ,there was/were
1 Car in Acceleration
3 in Skidpad
5 in AutoX
7 in Endurance(out of which only 2-3 completed)
out of a total of 40 cars.
Claude was there at the event as Design Judge. FSAE Michigan results also were more or less the same.

I will post more here when we go through the testing phase.

[Z]

Ritwik,


... I raised the roll centre to decrease the roll gradient but still roll is compromised due to low ride rate ...

This is a BAD DECISION, albeit quite common in FS/FSAE.

It is the type of decision made by students with limited understanding of the many issues involved, and usually made simply because of pressure from other people who perhaps have even less understanding, but who doggedly believe that certain meaningless metrics must be met (eg. "ride frequency", "roll gradient", etc.), regardless of the details. Nevertheless, and all things considered (ie. you are competing against other students), you might get away with it....

More important is to realise that it is VERY EASY TO INCREASE YOUR SPRING-RATES (see below). But do not worry about this now! Instead, get the car built and running as soon as possible. Use your above chosen spring and damper-rates to start with. Preferable is RCs close to ground, say 5 cm or less (especially so with soft springs!). DO NOT have RCs above 10 cms!.

Anyway, once you have worn out a set of tyres on your above roly-poly car (ie. with static deflections = ~3 - 4 cm), you can try stiffer springs. Please look again at the link I gave above to Uni of Cincinnati's Facebook page. See how they have pushed some spacers between the coils of the springs. All you need to do TO STIFFEN YOUR SPRINGS is PUT SPACERS BETWEEN THE COILS.

Alternatively, you can REMOVE COILS by cutting them off, and REPLACE WITH A SOLID SPACER (say, aluminium or even plastic). For example, cut the spring in half, put one half on shelf, add a spacer of appropriate length to the remaining half, and you have just DOUBLED your spring rate! Again, note that with your MRs you get the full ~5 cm suspension range with only about 3 cm of spring movement. So you only need about HALF-LENGTH of the springs you are currently specifying.

Also, I am sure that there are NNN millions of motorbikes in India that have springs about the right size for you. You know the inner-coil-diameter you need (ie. ~44+ mm, but can be bigger), so now work out suitable wire diameters and number of coils to achieve your desired rate. Then go looking through the bike stores and wrecking yards for suitable springs. A longer spring is fine, because you just cut it shorter to suit (and use oxy + angle-grinder for "closed and ground end"). Then, if necessary, machine some aluminium (or steel or plastic) seats so it fits neatly onto your QM dampers. <- All this is very easy to do, but do it AFTER first lot of testing...

Your selected dampers are good enough for now. Any damper issues you might have in testing can be addressed later (also easy to fix).

Z

[ritwikdas18]

I can change the RC's back again till the suspension clevises are put during manufacturing , not an issue but what shall i do with the increase in roll gradient that comes with it? What value of roll gradient do you think that must be met?
Thanks for the advice on springs . My current RC heights are around ~2.7 inch and ~3 inch Rear currently with previous heights being around ~1.7 inch and ~2 inch.

[Z]

Ritwik,


My current RC heights are around ~2.7 inch and ~3 inch rear ... with previous heights being around ~1.7 inch and ~2 inch.

Your "previous" RC heights are preferable (ie. 4 and 5 cm). Your higher "current" RC heights are bordering on DISASTER, especially with soft springs. Because..., when cornering fast the rear of the car will suddenly "jack-up" in mid-corner, the inner-wheels will suddenly lift off the ground, the driver will suddenly soil his nice new racesuit..., and the driver will certainly never try cornering that fast again!

The standard solution to this problem is simply to lock the suspension up rigid, because "...any suspension will work, if you don't let it...". But there are better ways...
~~~o0o~~~


...but what shall i do with the increase in roll gradient that comes with [lower RCs]? What value of roll gradient do you think that must be met?

"Roll Gradient" is one of the meaningless metrics I mentioned last post. In FS conditions a rigidly sprung car (ie. RG = 0) can go quite fast. A well suspended car (ie. with non-constant RG!) goes even faster.

IMO, "well suspended" means quite soft and (relatively) overdamped in the small range around ride-height, say, +/- 1 cm. Outside that soft range bump-rubbers add increasing (non-linear!) spring-rate in the bump direction, and there is not much need for any extra droop movement, so the suspension can be droop-limited below -1cm (though this depends on circumstances).

So, rather than thinking about some magical, constant, roll-gradient, it is more helpful to think about the peak-roll-angles you expect to see. These are obviously related to the peak-camber/inclination-angles your tyres will see, via the suspension's "Camber-Compensation", etc. That is, with 0% CC you want small peak-roll-angles (ie. stiff springs), else you simply wear the outside shoulders off your tyres. With 100% CC, say from beam-axles or short-lateral-swing-arms, you can have large roll-angles (ie. soft springs) with no adverse effects on camber-angles, or resultant tyre-grip.

Peak-roll-angle can be controlled by:

1. Spring stiffness over its linear range. Obviously, softer springs = more peak-roll-angle, and vice versa. But nothing stays constant, or "linear", forever...

2. Non-linear springing. For example, soft springs in centre of range (= good for grip over small ripples), then bump-rubbers and droop-limiting to set peak-roll-angle. The right sort of bump-rubbers are important if the track has many bumps ... so NOT important in FS! Overall, in FS, the droop-limiting is more important, because otherwise narrow-car + large-inner-wheel-droop leads to -> jacking -> CG rise -> rollover! Or driver too scared to drive fast.

BTW, droop-limiting is usually set by spring-seat-height (on the threaded-damper-body) together with the ride-height-adjuster that you should add to your DASD (see bottom of U-Cincinnati's DASDs for hints). Typically, in FS conditions, the wheels should be allowed to droop downwards NO MORE than ~2 - 3 cm from "static", at which point the dampers reach full extension and set the "droop-limit". Droop-limiting can also be done independently of the dampers, say with adjustable seat-belt-like nylon-straps. These are absolutely essential in off-road racing, else (standard) dampers get ripped to shreds.

3. Stiff low-speed damping. This can be enough to limit peak-roll-angle in short corners such as slaloms and chicanes. In FS, or general road-racing conditions, it is usually best to put most of the low-speed damping stiffness on the rear-wheels (ie. fronts are kept soft). This "loosens" the car (ie. moves handling balance to OS) on corner entry, making it "turn in" better. It also "tightens" the car (more US-ish) on corner exit, allowing better acceleration out of corners.

Again, all of the above is "fine tuning". Build the car and do a lot of driving with whatever SDs you choose to start with. Then think about above issues, and try a bit more-or-less stiffness, here-or-there... Always easy to add more stiffness...

Z

[mdavis]

It's good to see UC's 2013 car still being relevant. As the suspension guy for that car, it makes me happy.

Having skimmed this thread, I think there are a couple of good view points in here, Claude and Z, who are approaching this from 2 separate angles, and I think they could end up in a similar space (faster cars and better engineers).

Having hung around with a bunch of autocrossers for the last couple years, I can definitely say that a pretty significant majority have no idea how to make their car faster (via setup tweaks), or why their current setup is fast. FSAE teams would be wise to seak out the few that do know how to make a car fast and pick their brains. An FSAE course at Lincoln (MIS is a little different, no real comment about European courses, since I've never driven them) has a similar number of driver inputs that your local autocross has for a big car. Go find the top 5 guys in PAX at your local SCCA events (for US teams, teams from other parts of the world, find whatever local motorsports applies to you), and try to co-drive with them. We did this in 2011, and it was very eye opening. We put a fast driver in our car, and he put 3 seconds on our drivers on a ~30 second course. Until you can keep up with nationally competitive autocrossers, your drivers are slow. There are only a few FSAE drivers that I think I've ever seen that I would consider "fast". Plenty drive fast cars, but few drivers are actually fast. This is going to be a key component to a good dynamics score. PS, it's fun to drive. Get out there and do more of it. It's much better than working in the shop or on the computer... I would also suggest going to a few dirt go-kart races, if you want to learn to be smooth with steering inputs.

Now, if I were still involved in FSAE (namely, the last 2 years after our 2013 campaign), I would be working on the following things: correlation of simulation to on-car results, finding the weak points in the design of our car (there were several in 2013, and anyone on our team will admit it) to improve for the next iteration. We started doing some simulation in '13, and it went alright. We didn't have any idea how the graphs from the computer tied to the car's performance. That was our issue. Given 2 more years (hell, 1 summer), we would have had a much better idea of why the car did what it did on track. Someone asked me if it was car or driver that had us ~6-8 seconds back of GFR in autocross. I think it was 50/50 car and driver. There was plenty we could have improved on both fronts...

Our 2013 team leader told Claude that if we had our team together for 2 more years, we would be challenging for top 5 overall at MIS. I think this is a 100% factual statement. Here's how the timing breaks down:

Year 1:
1. Build a reliable car
2. Tune the hell out of it (we were sort of on the right track, and the Lincoln pace showed it)
3. Train your drivers
4. Develop some kind of MMM (or similar) simulation to show what the car is doing on track

This gives good dynamics scores, for a reasonable overall finish. This is important for building sponsor base for the next 2 years.

Year 2:
1. Continue 2 and 3 from above
2. Correlate the simulation from Year 1 to on track performance
3. Simulate a variety of set-up options and validate them on track
4. Make very very small tweaks from Year 1 car, improving only the biggest 2-3 errors

This starts down the path of predictive simulation, and closes the loop on the simulation developed in Year 1. With the good drivers and a faster car, you're looking at a better dynamics finish, and with better understanding of what the car is doing, you're probably doing better in Design as well.

Year 3:
1. Use simulation that was designed in Year 1 and correlated in Year 2 to design new car, using plenty of sense checks along the way
2. Build car early
3. Test often
4. Repeat 3, a lot

By this point, your drivers should be very capable, and your car should be quite fast because you're on your 3 iteration of the package.

This takes time, and is probably why it's not followed very often. Please note, if you make any major changes (engine, tires, aero/no aero, etc.) you basically start this process over again... This is what we would have done, had our 2013 team been back for 2014 and 2015.

If anyone has any specific questions about our 2013 car, post them up, I'll be happy to answer them.

-Matt

PS, Z is giving away some autocross setup secrets in his last post. These were learned by running softly sprung cars on DOT race tires. The cars would be very close to dragging their mirrors from all of the body roll, but they were very very fast around an autocross course. Those cars were running DASD's with stock (generally small) or potentially 1 large anti-roll bar. If I were to build an A-Modified for SCCA use (basically, a ~180hp, 900lb FSAE car with lots of sprung mounted aero), I would use DASD's, and potentially no ARB's.

edit: Having watched all of the afternoon session of Endurance at FSAE-M this year, the GFR car was incredibly fast. Did DASD's cause this? No, they have been fast for years now. Did DASD's make them slower? It sure didn't look like it on track. Good job to the GFR team, your car was very well put together. As mentioned by the Kansas adviser in your paddock, I wonder how many teams show up with DASD's next year, because you guys showed they are not holding a car back... We only showed that it was feasible. GFR showed the real speed potential.

[ritwikdas18]

Also Z , I have 2 last year's springs from KAZ with spring rate of 27.2 N/mm .Given that I require only around ~32mm of travel on spring , I can cut those in about half or as much as required to get me to ~51 N/mm . Infact I think better would be to cut it to give me around ~45 since I can always increase stiffness with spacers but cannot decrease it. So perhaps I would not have to search for motorbike springs.

[JT A.]

edit: Having watched all of the afternoon session of Endurance at FSAE-M this year, the GFR car was incredibly fast. Did DASD's cause this? No, they have been fast for years now. Did DASD's make them slower? It sure didn't look like it on track. Good job to the GFR team, your car was very well put together. As mentioned by the Kansas adviser in your paddock, I wonder how many teams show up with DASD's next year, because you guys showed they are not holding a car back... We only showed that it was feasible. GFR showed the real speed potential.

I think the lesson to learn from GFR's performance with pushrods vs DASD is that any time spent debating pro's and con's & desiging different suspension layouts is wasted time that should've been spent improving your aero and drivers.

If you've never built a car before, sure, go with DASD because it will get you driving sooner and developing wings sooner.
If you've already built a car with bellcranks, it will probably take you more time & effort to redesign your car for DASD. Just copy over your previous suspension design and get to work on your wings & drivers.

[Z]

Ritwik,


... I have 2 last year's springs from KAZ with spring rate of 27.2 N/mm .Given that I require only around ~32mm of travel on spring , I can cut those in about half or as much as required to get me to ~51 N/mm...

Yes, but...

IF you cut them in half, then you must also,
1. Make some spacers to fill the missing gap, so the total length of spring+spacer is as required. Spacers can be steel, aluminium, or even plastic.
2. Learn how to do "closed and ground ends". This involves cutting the spring so that you have almost one full coil extra, using oxy-torch to heat a small section of the wire to red-hot about 80% of a full coil from the end, bending that last coil to "close" the end, then grinding that last coil so the end is "square".

Probably an easier way to begin with is to keep the original springs at full length. Then insert spacers between the coils, as per the Cincinnati picture. You can make these spacers out of any plastic such as nylon, HDPE, etc, or any hardish rubber. I would try making them as many little roughly ~1.5 cm cubical blocks with "grooves" on opposite faces, with radius of groove same as of spring-wire. Then push the blocks between the coils so that the coils sit firmly in the grooves and prevent the spacers from falling out.

Fill the gaps between half the coils on the original spring, and you double the spring-rate. Or adjust the number of "blocked out coils" for any other rate you want. When you eventually settle on a good spring-rate, you can get custom springs made that have just the right number of "working coils", for a neater and (very slightly) lighter job overall. But maybe leave that for next year...

Z

[P^squared]

673
Hey, everyone. The thing about the connectors between the dampers and length extensions is that the damper needs to fit fairly well in the connector, or else any compliance there will lead to bending loads.
Also the other end could be threaded to allow adjusting the right height on the go by rotating the extension tube.
Any suggestions about the connector design ?
I hope the pic gets uploaded

[ritwikdas18]

P squared , I have planned the same. The connector would simply be a threaded rod right?
and it must be attached to the shaft of the damper after removing the eyelet which is held tight with Loctite in my case .The shaft should not be damaged while doing so.Also since the eyelet is threaded the same thread is to be copied in the rod after removing it.

Could anyone (Z??) give a list of do's and dont's if I missed anything because we are doing it for the first time?

[P^squared]

The MTB dampers we have don't have eyelets(similar to female rodends ??) which could be screwed out.
Instead I plan on manufacturing a bolt-on connector, which would bolt onto the damper,at the opposite end of the reservoir, where a bellcrank would originally. Then the inner walls of the connector would be tasked with keeping it in place.
674 The dampers are similar in design, albeit shorter in length.

[Z]

Could anyone (Z??) give a list of do's and dont's if I missed anything...

Ritwik,

Refer to:
http://www.kaztechnologies.com/wp-content/uploads/2014/03/Penske-7800-Technical-Manual.pdf

Pages 2 and 3 have a Parts List and drawing, with
part #25 = the damper-rod, called "Shaft" and possibly with 3/8"UNC(?)-male-thread, and
part #28 = the female-threaded-lower-rod-end, called the "Eyelet".

At the bottom of page 4 are instructions for removing "Eyelet" from "Shaft". This requires special shaft-clamp-tool "TL-78SC", which I assume is just some soft jaws (eg. aluminium blocks with V-notches to grip the shaft?). At worst, use a hot-air-gun to heat the Eyelet and soften the Loctite...

The Spring-Damper-Extension can be made something like this "ascii drawing" below (with bottom/wheel-end of damper at the left, and "<" or ">" indicating direction of male>female thread...).

(Eyelet<3/8")<-(3/8"<steel-hexbar-with-male-thrds>12mm)->(12mm-locknut)(12mm>alum-female-thrds<3/8")<-(3/8"<Shaft.....)

So the "Extension" fits between the Eyelet and Shaft. The Extension is made of two main parts:
A steel-piece, machined from ~16 mm hex-bar, with male threads at each end, and is Loctite-fitted to the Eyelet (shown at left above).
An aluminium-piece, machined from ~30 mm round-bar, with female threads at each end, and is Loctite-fitted to the Shaft (at right above). This part must also be machined at its "top/right" end to carry the Spring-Retainer (#26).

The steel-piece then screws into the aluminium-piece via a 12 mm thread (x 1.5 mm pitch), which should overlap about 50 mm, and is locked with the 12 mm nut. This 12 mm threaded section is where you do "ride-height-adjustment". Only a single right-hand thread is needed here because the damper-shaft can rotate freely in the damper-body (some adjusters need LH and RH threaded sections). The other two threaded joints (ie. 3/8" to Eyelet and Shaft) should be "semi-permanent-Loctited-joints", done similarly to a steel-stud into aluminium.

Hope that makes sense.
~~~o0o~~~

P^squared,

Because you have a damper-adjuster-screw at the bottom of the rod (from what I can see in the Elka image) you probably have to do something like your CAD image, or better yet like Cincinnati's extension piece.

https://www.facebook.com/bearcatmotorsports/photos/a.305398312844316.79876.142023435848472/544556405595171/?type=1&theater

Note how C's extension has one bolt through the ball-joint-hole, and a SECOND bolt closer to the spring, with this second bolt CLAMPING the C-shaped extension hard around the damper. This is a preferable way of doing it because it eliminates any sloppiness. Alternatively, you can use grub-screws to eliminate slop between the extension and damper.

Also note how Cincinnati use shim-spacers to adjust overall SD length, and thus adjust ride-height. I would be inclined to use the threaded adjustment described above, but either way would work.

Z