Rates Again !

[Karam Atteia]

On calculating the Ride/Roll Rates for our suspension, my reference is RCVD and Optimum G Tips

I attached my calculations, to know if i'm on the right way or not.(excuse me it is hand written)

http://img14.imageshack.us/img14/766/dsc01484qf.jpg

http://img89.imageshack.us/img89/531/dsc01485ty.jpg

but still have two problems, can i go with the front ride frequency more than the rear or i should make the rear more to catch up the front when hitting a bump??

second: when should i add the magic no., in the front/rear ARB stiffness or in the front/rear roll rates ??

[Claude Rouelle]

Karam,

You need to come to the OptimumG seminar.

You will keep asking questions on this forum and I guess you will have my different answers most of them right but for different reasons. It is not that nobody wants to help you but it will be long to get a complete answer on a forum. There are many good books there ... but you can't ask questions to a book. That si why we have been doing seminars for 16 years

Just a few comments about the front and rear ride frequencies and front/rear frequency ratio....
- The way we do it in our presentation is a simplified method where we look at the front and the rear as if they were not attached to each other. Ideally you should look at your differential equations taking into account the pitch inertia.
- And yet this method do not take the kinematics neither the compliance into account...
- The frequency choice depend on
- The driver: Skilled drivers can better master smaller reaction time / high frequency and lower amplitude
- The bumpiness of your track.


Examples

[Karam Atteia]

Claude.

thanks for your attention, i really appreciate that cause no one helped me 5 days ago.

please i have some questions and i hope u can give me a hand.

1-For the magic number, when to use .. is at the start of my calc. when assuming the roll rates front/rear. or when i am finishing it and use it at the front/rear ARB ??

2-In the equation of (Total ARB roll rate needed ...... ) in the last of the OptimumG tech tip no.2 . The term Kw (wheel rate).. means which wheel rate front or rear or what ??

3-on calculating the rates .. the rear wheel rate was smaller than the front .. please comment on that.

4-on damping .. how can i caculate the velocity of the sprung mass resonance and the velocity of the un-sprung mass resonance ??

[Charles Kaneb]

Karam,

I think at this point you've got a good idea of what you're doing, and need to concentrate on building a car and designing good experiments to find out what works best for your FSAE car. You've understood enough from RCVD so far that a careful re-reading should give you the right answers.

You can change roll rates with a different swaybar tube thickness. You can change ride rates with a different set of springs. They aren't completely independent - a bigger swaybar will provide a higher rate in one-wheel bump, a pair of stiffer springs usually increases roll resistance - but you're doing most of the calculations right. A good driver will tell you REALLY quickly what's wrong with the car's handling!

-Charles

[Claude Rouelle]

Karam,

2 . The term Kw (wheel rate).. means which wheel rate front or rear or what ?? What do YOU think?

4 -on damping .. how can i calculate the velocity of the sprung mass resonance and the velocity of the un-sprung mass resonance ?? What the heck is the "velocity of a mass resonance"? What the heck is that? What do you mean? What are you looking for? What is your goal?

[Claude Rouelle]

Oops, I clicked on "Send" too quickly

Karam,

You need to come to the OptimumG seminar!

You will keep asking questions on this forum and I guess you will have my different answers most of them right but for different reasons. It is not that nobody wants to help you but it would be too long to get a complete answer on a forum. Too many parameters to be consider. There are many good books there ... but you can't ask questions to a book. Student want Q&A and a direct interaction with experienced engineers and e/ or experts. That is why we have been doing seminars for 16 years.

Just a few comments / traps about the front and rear ride frequencies and front/rear frequency ratio....

- The way we do it in our Excel spreadsheet and on our website is a simplified method where we look at the front and the rear as if they were not attached to each other. Ideally you should look at your differential equations taking into account the pitch inertia. Defining your spring center and your heave center is a good thing.

- And yet this method does not take the kinematics neither the compliance into account...

- The frequency choice depend on
- The driver: Skilled drivers can better master smaller reaction time / high frequency and lower amplitude
- The bumpiness of your track. Bumpy track require softer suspension
- The track grip and track temperature: low grip and low temperature often requires stiffer springs
- The tire "quickness" (relaxation length if you want). "Lazy" tire needs more suspension stiffness.
- And we are not speaking about damping yet
- Most of passenger car have a rear ride frequency 10 to 15 % higher than the the front one while in race car with heavy aerodynamics you can have front frequency 30 % higher than the rear one;
That is because of ride height (especially front) sensitivity on aero-balance

Each of the item I just wrote would raise the legitimate question :why? That is why the seminar is 36 hours! (In fact this year we have two 36 hours seminars: one on vehicle dynamics applied to race car design and development and one data driven seminar focusing on on-track testing, simulation and data acquisition)

A few more comments...

You do not win race on your suspensions; you win them on your tires. Your tires touch the ground, not your damper, not your ARB, not your spring. Calculating the front and rear suspended mass frequencies is a good thing but it is not sufficient. You need to integrate your non suspended masses (and inertia) and your tire stiffness. That is what 7 post rig (virtual of real) are for

- You need to treat the car as a holistic system with ride, roll, pitch frequencies and damping together. Most of the students know what their front and rear suspended mass frequencies are but 90 % of them do not know their roll and pitch frequency. They do not know their roll and pitch inertia either(indispensable for frequency calculation)

- Basic issue I often see in FS / FSAE is the confusion in the calculation of the effect of the ARB stiffness at the wheel. ARB stiffness can be measured in N /mm or N/m (a force measured along the tangent to the movement of the ARB blade end) and this case the ARB Motion ratio will be wheel mvt / ARB blade mvt). Or the ARB stiffness can be measured in Nm/deg or Nmm/deg and the ARB motion ration will have to be defined as roll angle / vs ARB twist angle. Your choice but do not mix them!

As you can already see there is see the a matrix of n dimension with a lot of parameters interaction and human interface. Not easy. If there was a prefect way to define spring and ARB stiffness, ideal damping, ideal kinematics, ... I would be rich!

That is also the reason why you need to test and you cannot make a competitive car unless you test for at least 4 to 5 months (except, maybe, if you are already experienced team)

The important thing is for you to master these calculations then go on the test track make good objective data and subjective data analysis.

[Karam Atteia]

Mr. Claude.

again thanks for your attention, you helped me so much but still confused a little.

I'm from Egypt - Helwan Uni., we arn't an experienced team at all since we started to participate in this competition in 2010 . We have no test track nor capabilities to do such things. No faculty advisor nor any one that could help !.

2-still confused, I tried to find it out many times but i can't.

3-the term (velocity of resonance frequency) is mentioned in Kaz Tech Tips. when talking about the knee speed and the comp./rebound ratio in damping.

[Claude Rouelle]

Karam,

2-still confused, I tried to find it out many times but i can't.

Give be at least a beginning of what you think the answer could be! There must be some "light at the end of the tunnel": come on Karam,

3-the term (velocity of resonance frequency) is mentioned in Kaz Tech Tips. when talking about the knee speed and the comp./rebound ratio in damping.

I am unfamiliar with this expression. Then you need to ask KaZ.

Just be a bit smart: what do you really think the velocity of a frequency could be? Think.

I prefer to give answer to people who ate least TRY to give a beginning answer on their own with some level of objectivity, even if they are completely wrong. How do you think an teacher feel when a student says "I don't get it, I have no clue" Really what do you think it is going to the teacher / design judge at that time?

Intelligence is the ability to find a solution to a problem you never met before. That is the case for you right now. Do not mix intelligence with knowledge (what you learn in books and at school) and experience (what you learn mainly bu making mistakes). One more observation: inability to develop intelligence comes from inability to master your emotions (I said emotion not sentiments). I stop here;this is not a psychiatric forum http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Karam Atteia]

Mr. Claude.

2-here we need the total ARB roll rate needed to increase the roll stiffness provided by ride springs to achieve the desired roll gradient.
So, the part which make me confused was the total roll stiffness provided by the front + the rear. then i think this wheel rate (Kw) is the sum of the front Wheel rate (Kwf) and the rear wheel rate (Kwr). http://fsae.com/groupee_common/emoticons/icon_smile.gif i did my best


3-"Velocity of sprung mass resonance" ... really i can't understand the meaning of this term !!!. But the velocity of the frequency ,, i think when a body hits a speed bump at low or high speeds that would effect the body frequency . http://fsae.com/groupee_common/emoticons/icon_smile.gif agin did my best

[Claude Rouelle]

Karam,

3-"Velocity of sprung mass resonance" ... really I can't understand the meaning of this term !!!.

Me neither so you will have to ask the author

Will come back later with additional comments /info

[Claude Rouelle]

Karam,

You have 2 unknown and a system of 2 equations.

The 2 unknown are your front and rear ARB antiroll stiffness

Your 2 equations are
1. Roll angle = roll moment / antiroll stiffness
= roll moment / antiroll stiffness of (front spring + front ARB + rear springs + rear ARB)

- Your roll moment is your suspended mass x your lateral acceleration x the distance from your suspended mass CG to your roll axis.

- Most of the time the roll axis inclination is small so in first approximation you can only worry about the vertical distance between your suspended mass CG and your roll axis.

- You already choose your front and rear springs. You decided that thanks to your targeted front and rear suspended mass frequency, your suspended mass at each corner and your motion ratio. I remind you are the targeted frequency is not only a matter of science: it is also matter of testing as these “ideal” frequencies are tire steady state and transient behavior , car other characteristics, driver skills , track shape and bumpiness, environment conditions (temperature mainly) dependent etc… and … I forgot if you have aerodynamic downforce and how much. A good target is between 2.5 and 3.5 Hz (with no wings) for a Formula
Student but again nothing is written in stone and you have to try. Also, you can get the” ideal” springs but if you do not accompany them with a smart choice of damping you could get the car easily 5 seconds of the pace. I have seen that happened too often

- For cars without wings or no significant ground effect (a bit like passenger cars) it is common to get 10 to 15 % more rear ride frequency depending your speed and your wheelbase Too long to explain here.

- However with a suspended mass distribution close to 50/50 and a lower frequency front you will get you will get a similar or even bigger ride height change front than rear on bumps. On a car with front wing and ground effect every mm of front ride height variation will change the aerobalance 2 to 5 times more than each mm of rear ride height variation. That is a killer on terms of car control. If you need 2 to 5 times more rear than front ride height variation (that ratio depends on your car pitch sensitivity; you need a wind tunnel / I do not believe in CFD unless you have a minimum of 100 million cells for ground effect cars and rotating wheels simulation and bodywork details … welcome to biiiig computers) then you will necessarily have a front natural suspended mass frequency bigger than the rear one. We have been working with big aero cars with suspended mass frequency close to 7 Hz front and 6 Hz rear. But those are driven by real drivers, not FSAE students… ( me provocative…? ? )

- You roll angle (deg) or your roll rate (deg / G) or your roll stiffness (Nm/deg or Nmm/deg) is also matter of choice and experience. Try something around 0.5 deg / G for a FSAE (it is 0.05 deg/G on a LMP1). Do not forget that this is the roll angle on suspension only. There will be additional roll angle due to tire and compliances (most of FSAE cars have ugly, huge compliance). In reality if you choose a theoretical 0.5 deg / G you can have in fact 1 deg / G if not more on the race track.

- Your antiroll stiffness comes from 4 causes: your front springs, your front ARB , your rear spring and your rear ARB.

- You know the total antiroll stiffness you want and you know already the antiroll stiffness from the front and rear springs. So you can calculate the antiroll stiffness of the total of front and rear ARB. What you do not know yet is the distribution of front and rear ARB stiffness.

2. “Magic number” or antiroll stiffness distribution (in %) is 100 * antiroll stiffness of (front spring = Front ARB) / antiroll stiffness of (front spring +front ARB + rear spring + rear ARB)

- Do not forget that the antiroll stiffness you are looking for is the one at the wheels not the springs or the ARB stiffness itself. You need to take the Motion Ratio into account.

- Most of the time students are confused by the way the ARB stiffness is measured and how effect of the ARB stiffness at the wheel is calculated

- You can have your ARB stiffness in Nm/deg or Nmm/deg and in that case you will need to use a motion ratio defined by chassis kinematic roll angle /ARB twist angle

- Or you can define your ARB stiffness in N/mm or N/m as the force to move the edge of one ARB blade on the tangent to its trajectory (the other blade being fixed on the test bench) and in this case the MR will be wheel mvt. / ARB blade edge mvt.

- You can always measure the ARB stiffness at the wheel itself with a dummy damper on one side, no spring on the other and a load cell and a basic scissor jack or, better, simply weight hanging on the hub (so your car won’t lift up and you won’t need to attach it to the ground) and in you r calculation wheel / ARB MR in this case will be 1.

- The target for “magic number” or antiroll stiffness distribution is something you can simulate with good tool like the Yaw moment Vs. lateral acceleration method and reliable, realistic tire model. But once gain even with tire models and good simulation; it won’t be spot on as there are many different parameters to measure and to input with enough accuracy.

- I suggest you start with a percentage quite close to the suspended mass distribution, may 1 % higher to give a bit understeer, safe tendency. Then find out on the track what the driver, the stopwatch and the tire like.

- Remember that this “magic number” (suspended mass elastic weight transfer distribution) is not the only one: you will need to work on geometric weight transfers and non-suspended weight transfers distribution, then include your tire stiffness, then maybe your compliance, your damper and your inertia in transient. Just make it simple and useful first.
So you know from (1) what the total ARB (front and rear) stiffness should be. From (2) you know the distribution between (front springs + front ARB) and (rear spring + rear ARB) needed. 2 unknown and 2 equations. Basic. Questions?

Other considerations

- Be careful: you will know the need of ARB stiffness at the wheel, not at the ARB itself. You will need to recalculate the ARB stiffness using the Motion Ratios.

- Make sure that your ARB material does not go into the plastic zone! I have seen to many ARB broken (at FSAE and on professional race tracks; bumps and fatigue were not taken properly into account)

- You can make a car with no ARB but in order to achieve 0.5 deg/G you need very stiff springs and there is a good chance that you will be over 4 Hz of vertical ride suspended mass frequency which could make the car not easily drivable.

- In a simplified way, without taking many parameters (like the effect of kinematics and compliance camber variation in roll) into account, if you have understeer in steady state you need to either soften the front ARB or stiffen the rear one. You ask the driver if the car is too lazy or too responsive. If you have understeer you and the car is lazy you will stiffen the rear ARB. You can stiffen your rear spring but that will destroy your front/rear ride and pitch “harmony”. If the car understeers and is too nervous you will soften the front ARB.

- Similarly if your car oversteers you will soften the rear or stiffen the front depending if the car is too nervous or too lazy.

- That is why I want to see FSAE cars with both front and rear ARB; because you never know what the car will be and what the conditions will be: track temperature, rain dry, abrasive or smooth asphalt

- Most of the time cars which as no front ARB and a rear ARB have rear roll center lower than the front. To avoid at all cost.

Karam (and others) I hope this helps.

[Karam Atteia]

Mr. Claude.

Really again thanks for ur attention and time.

[Claude Rouelle]

ur?

[Karam Atteia]

sorry, (ur=Your)

[Z]

Originally posted by Claude Rouelle:
...
Karam (and others) I hope this helps.

Claude,

Since FSAE is supposed to be about "engineering education", and not "motor racing", I have two main questions relating to your above post.

1. "... your targeted front and rear suspended mass frequency, ...
... the targeted frequency is not only a matter of science: it is also matter of testing [and a whole lot of other trial and error]...
A good target is between 2.5 and 3.5 Hz ... but again nothing is written in stone...

Given that it was pointed out on another thread that these "front and rear ride frequencies" have little connection to the real physical behaviour of the car (ie. the F&R do NOT oscillate at those frequencies), why do you keep encouraging the students to think in this misleading, unengineering, unscientific, way?

Talking in terms of those unrealistic "frequencies" does not help their education. And as you point out in the above quote, it doesn't even get them very close to a starting point for suspension stiffness.

As an alternative, why not use the older concept of "static deflection" of F&R springs? For example, rather than your misleading and largely irrelevant "between 2.5 and 3.5 Hz", above, why not say "A good target is static deflection between 4 and 2 cm..."? This way the students see a direct connection with their "target deflection" and the amount the springs deflect as the car's weight acts on them (and this assumes only that the springs and MR are linear, unlike all the improbable frequency assumptions).

Furthermore, if during horizontal acceleration there is a load transfer of, say, 50% of the cars's static load onto, or off, some springs, then the students can immediately estimate the amount those springs compress or extend (namely 50% of the static deflection) This then makes calculations of "roll gradients" and the like much easier.

To sum up this question, why are you encouraging the students to mindlessly "talk the talk of motorsport", rather than helping them get a good physical understanding of their cars?
~~~~~o0o~~~~~

2. "Your antiroll stiffness comes from 4 causes: your front springs, your front ARB , your rear spring and your rear ARB.
... I want to see FSAE cars with both front and rear ARB..."

Why (!!!) ???

Surely by now Claude, after all the discussion on this forum about the different possible ways of interconnecting wheels with springs, and the UWA students explaining it to you in person, you should realise that ARBs are one of the worst ways to do it! (FWIW, they stiffen the Twist(=Warp)-mode just as much as they stiffen the Roll-mode, and, generally, stiff-Twist = bad .)

In the above quote, you, who are a major influence on the students' decision making, are telling them that they should (MUST!?) fit two of the worst types of spring-wheel interconnections to their cars, simply because that is how it is done in motorsport. And why is it done that way in motorsport? Because, the witless drones will tell you, "...that is the way we have always done it!".

So, to sum up this question, will you start teaching how UWA-style fully-interconnected suspensions work in your next seminars?
(Note to students: UWA's car uses only 3 springs, rather than the 6 springs Claude is promoting.)
~~~~~o0o~~~~~

Or will you stick to the same-old-same-old in your future seminars, with "F&R frequencies" and "ARBs are a must", as you do in the above post?

Z

(PS. Do "magic numbers" really belong in an engineering education???)

[Charles Kaneb]

Z,

Adjustable front and rear ARBs are typically provided so that you can change the lateral load transfer distribution radically, and change roll rates almost independently of two-wheel heave.

I think suspension frequencies are useful to compare cars based on how stiffly sprung they are in heave. It's also a nice "sanity check" to see if one end is going to be too stiff in heave compared to the other.

[Z]

Charles,

"Adjustable front and rear ARBs ...

There are much better (and indeed, simpler!) ways to adjust LLTD. Your support for ARBs is because you know no better. But will Claude ever start teaching you students the better ways??? (I have asked many times...)
~o0o~

"I think suspension frequencies are useful ..."

You can do exactly the same with "static deflections". The difference is that "deflections" are more realistic, whereas "frequencies" just make you sound like you are smarter than you really are... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

PS.
Static Deflection = (G/4*Pi^2)/Frequency^2, or roughly,
SD = 0.25/F^2 (with SD in metres, F in Hz).

[Charles Kaneb]

Originally posted by Z:
Charles,

"Adjustable front and rear ARBs ...

There are much better (and indeed, simpler!) ways to adjust LLTD. Your support for ARBs is because you know no better. But will Claude ever start teaching you students the better ways??? (I have asked many times...)
~o0o~

"I think suspension frequencies are useful ..."

You can do exactly the same with "static deflections". The difference is that "deflections" are more realistic, whereas "frequencies" just make you sound like you are smarter than you really are... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

PS.
Static Deflection = (G/4*Pi^2)/Frequency^2, or roughly,
SD = 0.25/F^2 (with SD in metres, F in Hz).

If you want to try anything different from previous years' cars - be it a different type of suspension, a different tyre, a different weight distribution, wheelbase, or track, or experimental aerodynamic features, you will want to be able to change LLTD rapidly over a broad range. "It might've worked if we'd had a bigger front bar, but the car was too loose to tell anything else" is not a satisfying conclusion to a test.

We've got a different solution to the LLTD adjustability problem this year. You'll see it at competition if it works.

[Tim.Wright]

Originally posted by Z:

"I think suspension frequencies are useful ..."

You can do exactly the same with "static deflections". The difference is that "deflections" are more realistic, whereas "frequencies" just make you sound like you are smarter than you really are... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

PS.
Static Deflection = (G/4*Pi^2)/Frequency^2, or roughly,
SD = 0.25/F^2 (with SD in metres, F in Hz).

While agree with you about the anti roll bars, on the subject of "wheel frequencies" I really don't get why you push such an insignificant point.

1. If you have a mass and an elasticity you will have oscillations. Maybe not when you push the car down in the garage, but if you excite a suspension at this frequency say on a kerb or ripple strip, you will have a large response. So its good to know what this frequency is.

1a. Saying a frequency notation makes no sense because you never see a full oscillation is like saying measuring speed in km/h is useless unless you are actually driving for at least an hour.

2. You have demonstrated that static deflection and frequency tell the same story. The industry has chosen frequency. Why the push to speak in a language that no-one are used to?

[Z]

Originally posted by Tim.Wright:
... static deflection and frequency tell the same story. The industry has chosen frequency. Why the push to speak in a language that no-one are used to?
Tim,

The thing is, in the olden days a car's supension stiffness was described by talking about its static deflection (which combines mass and spring stiffness in a single number). It has its limitations, but it is a simple concept to understand, and is easily extended to describe "roll or pitch gradients".

The annoying part about "frequencies" is that these days the quite useful "pitch and bounce frequencies" described by Rowell and Guest (in 1920s, and discussed on other thread) are being ignored. Instead, the concept of "frequencies" has been dumbed down and is now just a fancy, but misleading, synonym for static deflections.

The buzz words are getting more sophisticated, but the understanding is being flushed down the crapper.

Z

[Z]

Originally posted by Charles Kaneb:
... "It might've worked if we'd had a bigger front bar, but the car was too loose to tell anything else" is not a satisfying conclusion to a test.

Charles (and anyone else with the above problem),

So, you've got a simple spring-at-each-corner car, your (cough...) "F&R ride frequencies" are within the range that Claude wants, but the car is too "loose" (not enough rear grip, or too much front grip (huh???)). Anyway, for handling balance you decide that you need to reduce front grip by shifting LLTD forwards.

Now, Claude is already going to carve you up in Design for not having ARBs, and you don't want to lose more points by fitting stiffer front springs, because that gets you out of Claude's "I want to see..." "frequency" zone (see his post mid-previous-page). So, what to do?

I would use the fact that all spring rates are ultimately NON-linear. That is, the linear range necessarily runs out when you reach the bump and droop stops (ie. the "spring" becomes stiffer, or "rate" increases...). So I would allow the rear springs to have, say, +/- 3cm of softish travel between their bump and droop stops. I would then adjust the front spring/dampers so that they only have +/- 1cm of travel between stops.

End result is that your (cough, again...) "front frequency" is still low (because soft springs in middle of range), but during cornering the outer-front suspension sits on its (carefully shaped) bump stop, and the inner-front is lifted up by the droop stop (possibly the damper, but can be other device). So TLS reduces front grip, and the car is nicely balanced. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Bottom line is that there are lots of ways of making a car fast, other than copying the most complicated, expensive, "real racecar" designs. Oh, and also suspensions that are soft in the middle of the range, but also use their bump and droop stops every corner, can work very well indeed...

Z

[Tim.Wright]

I'd prefer to change the springs and not worry about where the frequencies end up. As I've mentioned before, the ride frequencies should be used only to give you a starting point. In fact, in any literature which present these frequencies, there is always a disclaimer saying exactly that "this is only a starting point" After that, the car will tell you what it wants.

Not sure I like the idea of running on the bump stops as a first port of call either. Then you will have a car which tends to oversteer in the linear range and understeer in at the limit. I'd imagine that it would be quite difficult to drive.

Couple that with the fact that the track is not perfectly flat you you will need X percent of the travel to account for bumps in the track, and I think the end result will be that its difficult to ensure your bump stops contact only as you approach the limit.

I know it's been done sucessfully before, but I think its something that needs progressive bump stops and a lot of tuning.