Matlab for tire data/suspension design

[Jersey Tom]

Another big item is validating how accurate that tire data or fit is.

If you can't vouch that the fit accurately matches the actual test data... just be designing a suspension around junk.

[Belatti]

Another big item is validating how accurate that tire data or fit is.

If you can't vouch that the fit accurately matches the actual test data... just be designing a suspension around junk.

Indeed, thats a big

The data is from 2004

[fastback33]

So, someone please correct me if im wrong, but....I've found you want the RC somewhere below ground, ...

...

Which is why im uncertain if i should ever use tire data, and how common is it in a racings eries for them to use the data for their design?

The STATIC roll center should not be below ground. That way the rolling moment grows too much. You have to place it somewhere between the Cog and the ground. How to go with the position the roll center takes dynamically is something Im on with.

Here, racing teams do not use tire data and I could get some tire data test report, so Im looking for a team who wants to hire an engineer and invest some time and money to design next years car

man i've been busy, finals are over FINALLY! Anyways, how do you know that you want the static RC above ground but below the CG? Hell, i feel like i should just start over. Can someone explain to me Roll Center all over again?

JT - string calculator = CS Paper Doll Game

[RideRate]

If you want to design a suspension without the racecar-fairy-tale blinders on then you will forget you ever heard of a rollcenter. Rollcenters are an old-school idea that have no foundation in being applied to today's dynamic systems and race car suspensions. Put some kinematic, dynamic, and even statics thought and analysis into your suspension and you should soon see the fallacy that is the rollcenter (ESPECIALLY for a dual a-arm configuration). Where the rollcenter is statically and dynamically is irrelevant, unimportant, and arbitrary.

[Jersey Tom]

Ehh.. don't think I'd go QUITE that far.

The old kinematic roll center concept is way overblown by some, certainly. There is definitely still insightful knowledge gained by evaluating the changes in vertical from lateral load, particularly on an SPMM. The "roll center" just summarizes those effects.

It's not the most accurate term.

[RideRate]

But it doesn't summarize them accurately. Therein lies a massive problem with the concept. There are more accurate methods of summarizing that are just as or even more simple. Any reason you tell me you use a rollcenter is going to be the exact reason why I don't.

[Jersey Tom]

If I put a car on an SPMM and measure the force-based roll centers directly, I'd think that would be pretty accurate

The question is if the kinematic centers match up with that at all. I haven't seen data either way.

[RideRate]

Of course it doesn't. Or else Bill Mitchell would have never needed to craft the force-based approach. On top of that, a force-based approach is more correct, but the particular method out there currently still suffers from similar ills as the kinematic concept. Ah, well.

[Jersey Tom]

Which approach do you prefer for predicting steady state and dynamic lateral load transfer?

[RideRate]

Neither.. That's my point. Why would you even need a rollcenter to predict lateral load transfer? You don't.

[ubrben]

Neither.. That's my point. Why would you even need a rollcenter to predict lateral load transfer? You don't.

How would you apportion the geometric load transfer then? A full dynamic model, e.g. ADAMS is pretty hardcore. I believe OptimumG have a steady-state program coming that will use force based roll centres to apportion load transfer.

Good rule of thumb is that the force based roll centre is going to be higher and biased laterally towards the outside tyre than the kinematic centre.

Biggest problem with force based roll centres is that you need a tyre model to simulate them accurately.

Ben

[Jersey Tom]

Going through a corner, I'm interested in how load transfer is split among the front and rear axles, and the relative rates at which they build (as a function of time).

As the concept is generally taught (ex. RCVD), the apportioning of geometric load transfer is approximated by the roll axis height, and roll axis inclination.

So, as Ben points out, if you completely throw out roll centers... how do you account for it? Assuming you want to use something OTHER than the ultimate full-out ADAMS model.

[RideRate]

How would you apportion the geometric load transfer then? A full dynamic model, e.g. ADAMS is pretty hardcore. I believe OptimumG have a steady-state program coming that will use force based roll centres to apportion load transfer.

Good rule of thumb is that the force based roll centre is going to be higher and biased laterally towards the outside tyre than the kinematic centre.

Biggest problem with force based roll centres is that you need a tyre model to simulate them accurately.

Ben

For ss geometric load transfer, your fastest and most accurate simplification is instant centers. Yes, this is kind of what the rollcenter is trying to do, but if you try to APPLY the geometric forces at the rollcenter you've botched the calc. It's the rollcenter's concepts that cause the most problems. This method neglects tie/toe rod, but that's ok. You can account for it easily if you want. It also neglects some other things, but I'm not getting into that as I'm sure very few have ever worried with accounting for them. If you want to go further, you can solve the 6x6 matrix. Further, you can use the sims out there, or build your own.

OptimumG is coming out with something. I've had quick chats with Claude about some of these concepts and how his methodology and my methodology don't agree. He claims our methodologies actually do agree, but I'm not yet convinced.

Tire models are NOT the biggest problem with force based roll centers. The fact that it still attempts to find a "center of roll" for the chassis is the problem. I don't care where this "force based" rollcenter ends up either. Tires are complex beings, but on the detailed scale. For figuring what's happening with your car you only need the broad concept of what a tire does which is simple. Detailed tire data is a bit over-rated in its usefulness for all but the more advanced race cars and teams.

[RideRate]

Going through a corner, I'm interested in how load transfer is split among the front and rear axles, and the relative rates at which they build (as a function of time).

As the concept is generally taught (ex. RCVD), the apportioning of geometric load transfer is approximated by the roll axis height, and roll axis inclination.

So, as Ben points out, if you completely throw out roll centers... how do you account for it? Assuming you want to use something OTHER than the ultimate full-out ADAMS model.

Even worse, roll axes. That's an ill-founded concept applied twice and connected with an imaginary line. I mean, really??

If you want to know how lateral load transfer is split front to rear all you need are roll rates. Suspension geometry doesn't play into this . If you want to know the rate at which they build you also need to know your roll inertia. You can calculate it how you see fit (CAD model, testing, calculation), but I can tell you your roll center (kinematic for force-based) doesn't help you calculate it.

For your geometric stuff use instant centers, but think about how you apply them. Quick and proper analysis is THAT simple. Rollcenter height is the most irrelevant thing ever. It tells you NOTHING. I don't care how it's generally taught, if the logic is flawed. RCVD is a great book, but be careful to remember it's dated and far from perfect. Further analysis to complete your dynamics needs to couple geometric and spring forces, but it still uses all these same concepts. It's not hard to produce this info and the best part is it is accurate and based on proven dynamic concepts.

My argument is always THINK about what you're doing before subscribing to stuff that can't explain things down to the most fundamental and intuitive level. Using the rollcenter leads people to design suspensions that work, sure, but not by the proper understanding or analysis and it keeps you in this design "window" that you shouldn't be stuck in. You can achieve your goals with more precision by forgetting about rollcenters and worrying about dynamics. If you worry about a rollcenter's location statically and dynamically you limit yourself by millions of solutions to your problem that are likely much much better.

[Jersey Tom]

If you want to know how lateral load transfer is split front to rear all you need are roll rates. Suspension geometry doesn't play into this . If you want to know the rate at which they build you also need to know your roll inertia. You can calculate it how you see fit (CAD model, testing, calculation), but I can tell you your roll center (kinematic for force-based) doesn't help you calculate it.

For your geometric stuff use instant centers, but think about how you apply them. Quick and proper analysis is THAT simple. Rollcenter height is the most irrelevant thing ever. It tells you NOTHING. I don't care how it's generally taught, if the logic is flawed. RCVD is a great book, but be careful to remember it's dated and far from perfect.

I certainly don't agree with everything in RCVD, nor everything Claude goes on about, not Carroll Smith or anyone. I don't think anyone's got it ALL figured out... nor will anyone in the foreseeable future! Personally my command of dynamics and linkages is pretty weak. Tires are by far where I'm best.

Not sure that 'roll center' location is pointless. If I do a lateral force compliance test on an SPMM I can get the dFz/dFy slope at each corner. I can summarize both those rates at each axle with a (Y,Z) co-ordinate at their intersection ("roll center") assuming I know the track width as well. By using the front and rear axle co-ordinates together, I can summarize the dFz/dFy of all 4 corners simultaneously with a line ("roll axis"). I can do this at any combination of ride height and chassis roll displacement. That seems like it would be valuable.

What to DO with those relationships in a simulation, and how to resolve it into something useful... is admittedly beyond me. I'll admit I'm a bit blank on "just use instant centers" as well.

Mostly, there are quite a few interpretations on the whole roll center thing, anti's, geometric vs elastic WT, etc. You point this out between yourself and Claude. The only thing in common between all of them is I have never seen any real data to support which is most applicable, or most correct! In any event, I believe quasi steady-state simulation gets ya a long ways in designing a car, and in that case I just roll everything together into a global "TLLTD" variable where I'm not too concerned in the individual contributions. I'd like to nail dynamic as well though.

What is your background, by the way? Just curious.