Quote from Shotglass :

btw concerning this ... is there any reserch on the frequency response of a tyre to find the minimum frequency for a tyre model to be correct ?

How correct? Nothing's perfect generally. Although you could say that if you used any model at all and it output the same forces at any given condition that a real tire did, it's correct regardless of the frequency you're running it at. I.e., 5 deg slip angle for tire X at 1000N load makes 600N. As long as your model outputs that same number, it's right, even if you calculated it only one time.

Generally the more you crank up the frequency the closer you get to the "right answer," so it's more a matter of how close is close enough.

well im thinking about tyre deformations ... to quote one of my professors "the world is a low pass" and this of course applies especially for rubber so naturally there is a threshhold frequency above of which theres no real point calculating

Can you think of a specific example or case? I'm not quite sure what you mean there. Do you mean something like a dynamic FEM type of situation? If so, I don't know. Haven't seen anything on it or given it much thought really.

well if you restrict your calculations to a certein update frequency what youre doing is sampling in the time domain
now rubber with its low pass characteristics cant deform at any speed which means that there will be a frequency above of which the error wont decrease much more

Not sure. I suppose that might depend on the model, but I've never read any mention of this in any tire literature.

maybe its just because nobody ever cared much about the frequency other than trying to reach a high one
but the model shouldnt really have any influence on this

If I may just go back to the tyre 3D modelling using displacement maps thing, AFAIK part of the reason why road tyres are more forgiving and regain grips quicker than slicks is because twisting several small patches of tyre is easier than twisting one big tyre patch, not just the fact that you have a smaller contact patch for the same total area of tyre. Is this correct? And how would a tyre model, such as yours, go about working around that if so?

Quote from jtw62074 :

Just saw one online that takes two hours to run a single second's worth of simulation for one tire on a 3.4Ghz workstation, so that sort of thing is a ways off yet.

hmm i wonder how much you could gain on that by using an fpga programmed to do as many elements a possible parallely

Quote from Shotglass :

hmm i wonder how much you could gain on that by using an fpga programmed to do as many elements a possible parallely

No idea. Still, you'd have to do it 28,800 times faster than that to run all four tires

Quote from Shotglass :

maybe its just because nobody ever cared much about the frequency other than trying to reach a high one
but the model shouldnt really have any influence on this

You might want to research tire modelling online a bit. I think you might find this is sort of irrelevant.

Quote from axus :

If I may just go back to the tyre 3D modelling using displacement maps thing, AFAIK part of the reason why road tyres are more forgiving and regain grips quicker than slicks is because twisting several small patches of tyre is easier than twisting one big tyre patch, not just the fact that you have a smaller contact patch for the same total area of tyre. Is this correct? And how would a tyre model, such as yours, go about working around that if so?

"Regain grip quicker?"

http://www.performancesimulations.com/files/tire1.JPG

Where do these tires (except on the wet) ever "lose grip" in the first place? They don't. This is a myth that just won't die. They just rise up to a point and stay there. And slicks look pretty much the same. The curves for every tire are just a bit different. Some rise faster to their peaks and roll off a bit more suddenly than others do, but this appears to have a lot more to do with the cord angles and construction of the tire itself than the tread. The presence of tread pretty much just lowers the cornering stiffness (so the peak comes in later), but you can compensate for that with the construction to put the peak pretty much wherever you want it. I.e., if you saw more graphs like this you'd have no way of knowing whether you were looking at slicks or treaded tires, other than with a slick the peak might be much higher.

A tire feels "forgiving" when the force curve levels off gradually over a large slip angle range like the bias example in the link above. Your street tires work this way because that's how the tire engineers designed the construction. Ease of control is one reason. This is influenced much more by cord angles/layout than anything else.

When people feel their tires "lose grip," they aren't really losing any grip at all. It's just that the rate at which the force is increasing with slip angle is leveling off. It's a bit like people thinking their cars actually speed up once they hit the grass. Nope. They've lost acceleration, but the acceleration hasn't reversed.

When you're looking at a side force/slip angle situation, the tread's not so much twisting as it is being pulled to one side.

Quote from Shotglass :

well if you restrict your calculations to a certein update frequency what youre doing is sampling in the time domain
now rubber with its low pass characteristics cant deform at any speed which means that there will be a frequency above of which the error wont decrease much more

What low pass characteristics are you talking about here? This seems like just a bunch of hand waving to me at this point.

Quote from jtw62074 :

"Regain grip quicker?"

http://www.performancesimulations.com/files/tire1.JPG

Where do these tires (except on the wet) ever "lose grip" in the first place? They don't. This is a myth that just won't die. They just rise up to a point and stay there. And slicks look pretty much the same. The curves for every tire are just a bit different. Some rise faster to their peaks and roll off a bit more suddenly than others do, but this appears to have a lot more to do with the cord angles and construction of the tire itself than the tread. The presence of tread pretty much just lowers the cornering stiffness (so the peak comes in later), but you can compensate for that with the construction to put the peak pretty much wherever you want it. I.e., if you saw more graphs like this you'd have no way of knowing whether you were looking at slicks or treaded tires, other than with a slick the peak might be much higher.

A tire feels "forgiving" when the force curve levels off gradually over a large slip angle range like the bias example in the link above. Your street tires work this way because that's how the tire engineers designed the construction. Ease of control is one reason. This is influenced much more by cord angles/layout than anything else.

When people feel their tires "lose grip," they aren't really losing any grip at all. It's just that the rate at which the force is increasing with slip angle is leveling off. It's a bit like people thinking their cars actually speed up once they hit the grass. Nope. They've lost acceleration, but the acceleration hasn't reversed.

When you're looking at a side force/slip angle situation, the tread's not so much twisting as it is being pulled to one side.

Thanks for clearing that up for me. But if road tyres and race tyres alike never actually loose grip, howcome people say "slicks are less forgiving" and such things, if you cannot tell which curve represents which type of tyre on those curves? Is it down to the setup required to make cars on slicks fast, being less forgiving?

Quote from jtw62074 :

No idea. Still, you'd have to do it 28,800 times faster than that to run all four tires

youd be amazed as to how fast you can do stuff in hardware

Quote from jtw62074 :

What low pass characteristics are you talking about here? This seems like just a bunch of hand waving to me at this point.

the damping you were talking about ... a piece of rubber cant possibly jump from being deformed fully in one direction to being deformed fully in the other one its characteristics will always create a smooth transition with an upper frequency depending on the type of rubber ... and to model that you need a sampling frequency depeneding on the upper frequency of your rubber

Quote from axus :

Thanks for clearing that up for me. But if road tyres and race tyres alike never actually loose grip, howcome people say "slicks are less forgiving" and such things, if you cannot tell which curve represents which type of tyre on those curves? Is it down to the setup required to make cars on slicks fast, being less forgiving?

Slicks are less forgiving because of the shape of the curves. Its all in how the transition takes place. In the same way low profile road tires are less forgiving that tires with a bigger sidewall. The more gradual the transition the easier it is to feel the change in state and so the easier it is to catch.

Quote from Woz :

Slicks are less forgiving because of the shape of the curves. Its all in how the transition takes place. In the same way low profile road tires are less forgiving that tires with a bigger sidewall. The more gradual the transition the easier it is to feel the change in state and so the easier it is to catch.

Right. Another thing too is that slicks generally, although not always, peak at lower slip angles than street tires do. It's quite a bit easier to toss a car around if the peaks come in at larger angles.

Quote from Shotglass :

the damping you were talking about ... a piece of rubber cant possibly jump from being deformed fully in one direction to being deformed fully in the other one its characteristics will always create a smooth transition with an upper frequency depending on the type of rubber ... and to model that you need a sampling frequency depeneding on the upper frequency of your rubber

These are all pretty vague generalizations without mention of anything very specific though. I.e., can you give an example of a specific calculation you have in mind?

If you're talking about calculating pressure for instance on either side of an asperity, and you're looking at a damping/hysteresis effect, well... Damping force is function of velocity. It's an instantaneous, analytical calculation. And again, if you're talking about moving a block of rubber step by step, the higher the sampling rate you use (smaller time step, more calcs per second), the closer you'll get to reality. I guess I still don't understand where this "threshold frequency" is that your referring to, at least not with any specific example. I've read nothing about this anywhere in any papers/books on tires or vehicle dynamics at all.

its really just one of the basic properties of discrete time
anything you model in discrete time is necessarily bandlimited
therefore what youre trying to model must also be physically bandlimited
and if im not mistaken the way your algorithm uses past values (which in is a form of interpolation) will be the equivalent of an interpolation filter you use to get results in continuous time

the overall effect of this is that the gains from using a higher frequency will level out at some point

What Todd said in simple word.
The more u turn, the more the tire must give forces to counter act the centrifugal force.

As mush as centrifugal force is smaller than the max force the tire can give, people will said they dont loose grip.

When the centrifugal force is bigger, the tire cant counteract the centrifugal force, the people think they "loose" grip.In this cas that is true, that the car lost something, because now the car is sliding

And it is easy to see that slick with lower slip angle peak force, and bigger max force, they feels lees forgiving, because the transisition beetween the car not sliding and the car sliding is very short .

Quote from MagicFr :

As mush as centrifugal force is smaller than the max force the tire can give, people will said they dont loose grip.

What's that? There is no such thing as centrifugal force.
Its just inertia, really.

Quote from jtw62074 :

Right. Another thing too is that slicks generally, although not always, peak at lower slip angles than street tires do. It's quite a bit easier to toss a car around if the peaks come in at larger angles.

Based on what I've read and heard from the Caterham and other light race car drivers, bias ply versus radial construction has a big effect on this, as well as just the design of the tire. From what I remember, there are bias ply slicks with much larger optimal slip angles than street oriented radials.

Again, based on what I've read, the smallest working slip angles are found on the Indy Racing League cars, it's about 2 degrees. I'm not sure if the old CART cars had similarly small working slip angles when they ran high speed ovals (Champ cars are almost all road / street courses, and don't run any high speed ovals at all anymore). Modern F1 race cars use tires that run at 3 or 4 degrees slip angle. The old bias ply tires of the 1980's ran with 12 degrees of slip angle. I'm not sure how much more the slip angles were for the 1967 F1 cars, but if GPL's model is close, it was pretty large and the cars had a significant yaw to them when cornering.

Also depending on the load (downforce) factor, there can be some loss of grip if beyond a peak slip angle, according to this web page (which also meantions the 2 degree slip angle for IRL cars).

http://www.smithees-racetech.com.au/ackerman.html

Another link to an article on slip angles versus race car classes:

http://www.autoracing1.com/Mar ... 4/0302NextGenNewTires.asp

Quote from axus :

What's that? There is no such thing as centrifugal force.
Its just inertia, really.

Dont play with word You know what i meant

Quote from jtw62074 :

Not sure what you mean there, Loopingz.. Higher frequency sampling is a way to increase accuracy rather than decrease it. One of my earlier tire models for VRC was running at 30,000 Hz at one point. That was way overkill though but it did run ok in real time. The current VRC tire model runs at about 600Hz IIRC.

Quote from Shotglass :

its really just one of the basic properties of discrete time
anything you model in discrete time is necessarily bandlimited
therefore what youre trying to model must also be physically bandlimited
and if im not mistaken the way your algorithm uses past values (which in is a form of interpolation) will be the equivalent of an interpolation filter you use to get results in continuous time

the overall effect of this is that the gains from using a higher frequency will level out at some point

Well I tryed to explain the same idea as shotglass.
It is all about the balance between computer error and model error (due to frequency).
I dont know (or remember) was it is the error for floating point. Let's imagine we are in 32 bit and error is about 1/2^30~1e-9. So each computation bring 1e-9 of error. If the system as a lot of computation to do and or a lot of degree of freedom, you can go easily to 100 calcul step. Now if you run it at 10000 hz your computeur error will be 1e-3, which is may be still acceptable but it is noticeable (and even really worse at 100khz).
The total error is computer error (which grow with hertz) + model error (which drop with hertz). The most accurate simulation is for computer error = model error. Adding frequency will be a loss of time and a loss of accuracy.
May be it is not in your books but it is the first lesson about engineering computing. If I have time at home I can drow you the typical curves (or find it in my silly fortran curses).
End of off topic, it was just a reminder that growing the frequency is good... just until a certain point !

That is true, but, in tire physics we dont work on very big numbers, means, floating point is often use with number small enough to have much of the 32 bits used for the decimal part. wich keep the error very small.
For exemple, on big numbers, for example, 60000 i already use 16bit for the integer part.. my physics was precise to 10e-3.
So if you keep number bellow 1024, you still have lots of precision.

Plus most of OS has double floating point capabilities, i'm talking about what i'm working on, the X360 that compute double floating point as fast as simple floating point

Maybe so, Loopingz. I'd like to see whatever you can dig up. However, I think those calculation errors are very small indeed compared to the tire model errors in probably every case ever done to date.

Anyway, for sure you can't tell one bit of difference in the handling whether the tires in VRC run at 600Hz or 30000Hz, other than the frame rate drops a bunch. Which is really more accurate? I couldn't tell ya and am not concerned with it. I've changed the frequency of VRC wildly during testing, not told anyone about the changes, and nobody ever noticed a difference, so I'm not too worried about it although it's probably a valid point, however minor in this case.