Car Physics: getting controlled oversteer

a few weeks ago, i posted a question my tire model since i was not happy with the handling of my car. i was pointed to the pacekja formulae, which i have now implemented in my engine. however, it turns out that my made-up tire-model wasn't that bad, because i'm still having the same problems ;-( at higher speeds my car will always strongly understeer, no matter how much i push the throttle. if i simulate a locking differential, or pull the handbrake in curves i can make the car oversteer, however it will be impossible to get it back to control. instead it will spin around its vertical axis at least 360 degrees, more at higher speeds. i was having the same problems with my old tire model, so i assume some other part of the simulation is causing the problems. thanks in advance for any suggestions, jonas

jonas echterhoff <[email hidden]> wrote in message news:310820011554584609%[email hidden]... > at higher speeds my car will always strongly understeer, no matterhow > much i push the throttle. if i simulate a locking differential, orpull > the handbrake in curves i can make the car oversteer, however itwill > be impossible to get it back to control. instead it will spin around > its vertical axis at least 360 degrees, more at higher speeds. i was > having the same problems with my old tire model, so i assume someother > part of the simulation is causing the problems. What's the weight distribution, and how do the front/rear tyre parameters compare? Jonny

On Fri, 31 Aug 2001 15:54:58 +0200, jonas echterhoff <[email hidden]> wrote:

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>a few weeks ago, i posted a question my tire model since i was not >happy with the handling of my car. i was pointed to the pacekja >formulae, which i have now implemented in my engine. however, it turns >out that my made-up tire-model wasn't that bad, because i'm still >having the same problems ;-( > >at higher speeds my car will always strongly understeer, no matter how >much i push the throttle. if i simulate a locking differential, or pull >the handbrake in curves i can make the car oversteer, however it will >be impossible to get it back to control.

Many things can make that happen. In my case, I have a similar problem; a loss of grip when the car finally lets go (oversteers). A possible resolution might be to flatten the Pacejka curve (the height of the grip once it passed the peak) so the tires don't lose so much grip. In my case another thing *may* happen; I use tire rates to get loads (so if the wheel is 1cm 'in' the track, you get 0.01m*tirerate = ... Newtons. With a small stepsize, the forces this generates may be a bit too much for the poor Euler integrator and the tire will quickly bounce up and down the track, making for at least half the grip you'd normally have. What was it again? I have tire rates upto 200,000 N/m (F1 tires seem to be near that range), so this gives a natural frequency of ? That multiplied by 10, given Matt Jessick's experiences, should be enough for a Euler integrator. It's now at 250Hz most of the time in my case, lol, probably not even close to what is needed at those tires. Sticking in an inner loop that ups the frequency when doing the tires may help in this case. I haven't done so yet, so I can't tell from experience yet. Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

Ruud van Gaal wrote:

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> What was it again? I have tire rates upto 200,000 N/m (F1 tires seem > to be near that range), so this gives a natural frequency of ? That > multiplied by 10, given Matt Jessick's experiences, should be enough > for a Euler integrator. It's now at 250Hz most of the time in my case, > lol, probably not even close to what is needed at those tires. > Sticking in an inner loop that ups the frequency when doing the tires > may help in this case. I haven't done so yet, so I can't tell from > experience yet.

If you are using a first order Euler integrator E.g. pos += vel * dt then I'd use more than 30 to 100 times the dominant frequency. You can also cause major differences in the frequency response for this simple integrator by exactly how you arrange the two equations of a second order set. E.g.: vel += acc * dt pos += vel * dt may be noticibly more stable than the opposite order. Analyzing the stbility of a digital filter like this puts you right into digital control theory and can best be done with digital design methods and tools like MatLab, etc. (If you are a student, grab student versions of this kind of software while you can. It retails for $5000 or more... ;) With a tire spring, you can estimate the "wheel hop" mode frequency from the books. It will probably be pretty high. Above 10 up to 20 Hz or more. Make sure you do your stabiliy testing with sharp impacts like steps (curbs, for example) because step inputs have signal in the high frequency range that can interact with your wheel hop, etc., frequencies. -- Matthew V. Jessick Motorsims [email hidden] http://www.moto1.net Vehicle Dynamics Engineer (972)910-8866 Ext.125, Fax: (972)910-8216

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>at higher speeds my car will always strongly understeer, no matter how >much i push the throttle. if i simulate a locking differential, or pull >the handbrake in curves i can make the car oversteer, however it will >be impossible to get it back to control. instead it will spin around >its vertical axis at least 360 degrees, more at higher speeds. i was >having the same problems with my old tire model, so i assume some other >part of the simulation is causing the problems. >

At ras here, some of us have talked about two (or more, depending on how you break them down) methods for handling tire forces with slip ratio and slip angle. I don't use Pacejka yet, but instead use a very simplified curve, but I think the results would still be similar. Basically: 1. Using slip ratio, calculate longitudinal force. Then, using slip angle, calculate lateral force. If the combined force is larger than the tire can handle at its current load, trim the lateral force using the Pythagorean theorem. (c^2 = a^2 + b^2 for lurkers, basic high school stuff, nothing fancy) This means that the longitudinal force stays exactly as it's calculated from the slip ratio and the lateral force *only* is modified. This is how I do it, and the car can be oversteered through high speed corners with opposite lock and recovered with no problem (provided the yaw axis polar moment of inertia is big enough, otherwise it happens too fast to control). The longitudinal force itself is only trimmed if it's too large by itself. With this method, a spinning tire can develop no lateral force, which I don't believe is as close to reality as you might get with the next method. Brian Beckman's, Ruud vanGaal's, and Gregor Veble's methods are different, however (I think, correct me if I'm wrong, guys) They do something like this: 2. Using slip ratio and slip angle, find the combined force vector. Then, if it's outside the friction circle, trim *both* the lateral and longitudinal forces to fit, rather than just trimming the lateral force by itself, leaving the longitudinal force untouched. Actually, I think they're trimming the slip vector itself first somehow before doing this, so my description isn't exactly right. I still haven't done much with tire models, so am low-man on the totem pole here on that subject :-) Anyway, I have a feeling that this is a tire model thing (even though you've switched methods), so if you can lay out some details on how you're handling limit-force behavior, someone might be able to point something out. You've mentioned that with a locking differential you can get oversteer. I assume that otherwise you're using a fully open differential and the inside wheel spins, preventing oversteer, right? If this is the case, try lowering the center of mass a little at a time and see if you get somewhat controllable oversteer at some point (or widen the track width.) This might not help too much though, as you're saying that you're having trouble recovering the spin. If you'll describe your limiting-force technique, maybe there's a clue there. It's also entirely possible that the car setup itself is the culprit. Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

On Fri, 31 Aug 2001 20:13:46 GMT, mjessick-Motorsims <[email hidden]> wrote: >If you are using a first order Euler integrator E.g. >pos += vel * dt > >then I'd use more than 30 to 100 times the dominant frequency. Geez, a lot. But thanks. >E.g.: > >vel += acc * dt >pos += vel * dt > >may be noticibly more stable than the opposite order. Yes, hm, never figured out whether one would be better than the other. I'll try swapping them, see what it does. >With a tire spring, you can estimate the "wheel hop" >mode frequency from the books. It will probably be pretty high. >Above 10 up to 20 Hz or more. 20Hz is ok; that'd mean about 2000Hz tops to integrate, that's reasonably ok. Thanks for the info, Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

On 31 Aug 2001 22:36:25 GMT, [email hidden] (J. Todd Wasson) wrote:

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> Brian Beckman's, Ruud vanGaal's, and Gregor Veble's methods are different, >however (I think, correct me if I'm wrong, guys) They do something like this: > > 2. Using slip ratio and slip angle, find the combined force vector. Then, >if it's outside the friction circle, trim *both* the lateral and longitudinal >forces to fit, rather than just trimming the lateral force by itself, leaving >the longitudinal force untouched. > > Actually, I think they're trimming the slip vector itself first somehow >before doing this, so my description isn't exactly right.

'We' create a slip vector based on optimal slip; so you have a slipvector that is like: (currentSR/optimalSR,currentSA/optimalSA) Optionally using tan(SA) instead of SA itself, but it doesn't do a whole lot of difference I think. This vector is trimmed to unit size (1) and then the long/lat forces are combined in ratio to the slip vector; newLong=currentSR/optimalSR*pacejkaLong. Well, that's how I think it is in the code, anyway, lol. > It's also entirely possible that the car setup itself is the culprit. Oversteer and understeer are indeed THE parameters that change with every little bit you change about your sim, be it code or setup. Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

In article <[email hidden]>, J. Todd Wasson <[email hidden]> wrote:

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> >at higher speeds my car will always strongly understeer, no matter how > >much i push the throttle. if i simulate a locking differential, or pull > >the handbrake in curves i can make the car oversteer, however it will > >be impossible to get it back to control. instead it will spin around > >its vertical axis at least 360 degrees, more at higher speeds. i was > >having the same problems with my old tire model, so i assume some other > >part of the simulation is causing the problems. > > > > At ras here, some of us have talked about two (or more, depending on how you > break them down) methods for handling tire forces with slip ratio and slip > angle. I don't use Pacejka yet, but instead use a very simplified curve, but > I > think the results would still be similar. Basically:

<snip> > Anyway, I have a feeling that this is a tire model thing (even though you've > switched methods), so if you can lay out some details on how you're handling > limit-force behavior, someone might be able to point something out. > i use the exact algorithm outline in Beckmann's phors series. > You've mentioned that with a locking differential you can get oversteer. I > assume that otherwise you're using a fully open differential and the inside > wheel spins, preventing oversteer, right? yup.

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> If this is the case, try lowering > the center of mass a little at a time and see if you get somewhat controllable > oversteer at some point (or widen the track width.) This might not help too > much though, as you're saying that you're having trouble recovering the spin. > If you'll describe your limiting-force technique, maybe there's a clue there. > > > It's also entirely possible that the car setup itself is the culprit. >

i will try lowering the cm, however the most of the other car parameters are taken directly from real life, so they can't be that "wrong" ;-) this is a '66 dodge charger, btw. jonas

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> > If this is the case, try lowering > > the center of mass a little at a time and see if you get somewhat > > controllable > > oversteer at some point (or widen the track width.) This might not help too > > much though, as you're saying that you're having trouble recovering the > > spin. > > If you'll describe your limiting-force technique, maybe there's a clue > > there. > > > > > > It's also entirely possible that the car setup itself is the culprit. > > > i will try lowering the cm, however the most of the other car > parameters are taken directly from real life, so they can't be that > "wrong" ;-) this is a '66 dodge charger, btw. >

lowering the cm will make it easier to get oversteer, but still, the general attitude of the car is to understeer unless a lot of power is applied on the rear wheels, and once the rear breaks out, there's no way to recover. jonas

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>> i will try lowering the cm, however the most of the other car >> parameters are taken directly from real life, so they can't be that >> "wrong" ;-) this is a '66 dodge charger, btw. >> >lowering the cm will make it easier to get oversteer, but still, the >general attitude of the car is to understeer unless a lot of power is >applied on the rear wheels, and once the rear breaks out, there's no >way to recover.

I imported a similar dodge charger a few days ago myself :-) Great car! The yaw axis polar moment of inertia is probably the next thing you could mess with, but then again, if you've got real data already for that of course you'll leave it alone. I possible, you might try adjusting the Pacjeka values to flatten the curve after the peak. It seems that Ruud's sim (Racer) doesthe same thing your describing, once it breaks loose, you're gone. That was with version .46 though, haven't tried .47. My "curves" are actually constant slope lines that goes from 0 to max grip and then stay there. It's fairly easy to drive through corners with controlled oversteer, probably because the curve flattens this way. If that doesn't work, I don't really know what else to say. The only other possibility I can think of is to check for vertical force instability in the tires. I run my suspension forces directly to the ground so it's very stable. If you're tires are stiff springs, you might want to plot out the vertical forces at one of them while kicking the car sideways, along with the suspension force. If the tire vertical force is much more jumpy than the suspension force, that might cause the problem. The fix for that might be in better vertical dampening, or increased frequency (possibly greatly increased) for vertical tire movements. I'm running out of ideas here, so hopefully this works :-) My #1 bet now would be the Pacejka curve. Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

On Sun, 02 Sep 2001 12:21:45 +0200, jonas echterhoff <[email hidden]> wrote: >lowering the cm will make it easier to get oversteer, but still, the >general attitude of the car is to understeer unless a lot of power is >applied on the rear wheels, and once the rear breaks out, there's no >way to recover. Try looking in slomo at the lateral forces of the rear wheels when it breaks out. It's probably low. Then find out the big Why. :) Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

Ruud van Gaal wrote: >> >> then I'd use more than 30 to 100 times the dominant frequency. > > > Geez, a lot. But thanks. > Matthew just might be on the safe side with the above estimate :). From what I've seen, in oscillatory systems the stability is reached much sooner (say, 15-20 times the frequency), but if you want more accuracy, the above is probably to be used. Also, whether you are using a leapfrog or any other scheme for Euler might alter this stability number as well. But, in our sims, I don't think large accuracy is strictly necessary as we are dealing with mostly well damped modes here that just need to be surpressed and we are not looking at their actual effects as much. -Gregor

In article <310820011554584609%[email hidden]>, jonas echterhoff <[email hidden]> wrote:

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> a few weeks ago, i posted a question my tire model since i was not > happy with the handling of my car. i was pointed to the pacekja > formulae, which i have now implemented in my engine. however, it turns > out that my made-up tire-model wasn't that bad, because i'm still > having the same problems ;-( > > at higher speeds my car will always strongly understeer, no matter how > much i push the throttle. if i simulate a locking differential, or pull > the handbrake in curves i can make the car oversteer, however it will > be impossible to get it back to control. instead it will spin around > its vertical axis at least 360 degrees, more at higher speeds. i was > having the same problems with my old tire model, so i assume some other > part of the simulation is causing the problems. > > thanks in advance for any suggestions, > > jonas

i think i found out more bout this: yesterday i re-enabled some code to use a steering wheel in the sim. i was using keyboard input before, i know it sucks but under Mac OS X (the OS i'm using) at the moment i have some technical difficulties, not letting me use my sound library and input library at the same time, etc... - you know it's a new OS, many problems still have to be resolved. anyways: now that i'm using a steering wheel, the general attitude of the car to understeer is gone. i suspect that my simulated keyboard input was just turning the wheel to fast, resulting in understeer (this is realistic behavior, isn't it?). the car oversteers like hell, like one would except from an old dodge charger. thr problem of uncontrollability remains, however. but i have a guess as to what's causing it. my sim currently doesn't have a constant throttle input to keep the engine running, so when i'm not on the throttle (which i still can't control analogue, since i don't (yet) have a good, working steering wheel/pedals setup), there's a lot of brake force coming from the engine, which may very well be causing more oversteer when i'm off the throttle. i'll look into it, and thanks for the info you got me so far. jonas

Gregor Veble wrote:

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> Matthew just might be on the safe side with the above estimate :). From > what I've seen, in oscillatory systems the stability is reached much > sooner (say, 15-20 times the frequency), but if you want more accuracy, > the above is probably to be used. Also, whether you are using a leapfrog > or any other scheme for Euler might alter this stability number as well. > But, in our sims, I don't think large accuracy is strictly necessary as > we are dealing with mostly well damped modes here that just need to be > surpressed and we are not looking at their actual effects as much.

I agree that accuracy is definitely less important for this application than frequency response. Unfortunately, most writing on the subject only deals with accuracy because they assume that you won't be pushing the edge of stability (well, because they are concerned about accuracy ;) Keep this in mind while reading integrator comparisons. -- Matthew V. Jessick Motorsims [email hidden] http://www.moto1.net Vehicle Dynamics Engineer (972)910-8866 Ext.125, Fax: (972)910-8216

jonas echterhoff wrote: > lowering the cm will make it easier to get oversteer, but still, the > general attitude of the car is to understeer unless a lot of power is > applied on the rear wheels, and once the rear breaks out, there's no > way to recover. You might try programming an automatic countersteer, just to see whether you could recover if you were impossibly fast. I doubt that it really is "impossible" to recover, but you may well need to be overly fast in reacting. Another issue unrelated to the rear tires that can make spin recovery difficult is exactly how much front tire steer capability you give the user. In video games, without the seat-of-the-pants acceleration feedback, and because of graphical problems and others we are probably late detecting the rear end sliding out. This may mean you just need more front steer capability. If you don't detect the oversteer until 20 degrees, you may need 20 + peak slip angle degrees of front wheel steer to bring it back. If you only give the user say 20 degrees of steering it may well be impossible to recover... It may also be common to reduce the steering gain at higher speeds to combat the yaw damping reduction with velocity increase. This also has the possibility of reducing the front wheel steer capability below what is required to recover from an oversteer. Another problem that should show up as sims get more sophisticated is driver technique. Sometimes, chopping the throttle is the worst thing you can do in trying to recover a spin as it starts, because it reduces the weight on the rear tires. You have to trade off reducing the throttle induced oversteer (if that was how you got the rear end moving out) with reducing the rear weight transfer. This kind of thing is particularly frustrating for me to deal with on a daily basis, particularly early in a sim's development before all the needed feedback stuff is in place. There are people who will never agree that driving fast is any different than driving to the supermarket. And no amount of pointing at tire marks spinning to the inside off the exit of corners will convince them either. ;) My favorite quote along these lines is: "We don't need driver adjustable anti-roll bars because my Jeep Cherokee doesn't need them." -- Matthew V. Jessick Motorsims [email hidden] http://www.moto1.net Vehicle Dynamics Engineer (972)910-8866 Ext.125, Fax: (972)910-8216

>>With a tire spring, you can estimate the "wheel hop" >>mode frequency from the books. It will probably be pretty high. >>Above 10 up to 20 Hz or more. An err, ah, hmm, a, I mean a friend of mine says ;) to be very respectful of the wheel hop mode. The masses and stiffnesses, etc., in race cars are such that if you let it get numerically unstable it can actually bounce the car completely off the ground. This err, a, friend of mine, had a wheel hop instability last week due to high frequency being fed in from the engine rev limiting that had some very annoying results. These spring - mass - damper suspension systems are basically second order low pass filters. They have a resonance frequency and an amplification ratio that depends on the undamped natural frequency (spring stiffnesses) and the damping ratio. The higher the input frequency is above the filters cut off frequency, the more attenuation there is. (The disturbing input signal is filtered out.) The higher frequency inputs are attenuated in the high frequency modes as well as the low frequency modes because the cutoff frequency is much closer to the input frequency. If you allow a significant amount of power to enter them in the frequency range where they resonate, Bad Things Can Happen. If you put in a wheel hop mode, also be very careful about differentiating positions to get velocities. For example: suspensionRate = (suspensionPos - previousSuspensionPos)/deltaTime; is very dangerous. This differentiator is also a digital filter and it has the frequency response of _amplifying_ high frequency inputs rather than attenuating them. -- Matthew V. Jessick Motorsims [email hidden] http://www.moto1.net Vehicle Dynamics Engineer (972)910-8866 Ext.125, Fax: (972)910-8216

On Sun, 02 Sep 2001 19:16:26 +0200, Gregor Veble <[email hidden]> wrote:

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>Ruud van Gaal wrote: > >>> >>> then I'd use more than 30 to 100 times the dominant frequency. >> >> >> Geez, a lot. But thanks. >> > >Matthew just might be on the safe side with the above estimate :). From >what I've seen, in oscillatory systems the stability is reached much >sooner (say, 15-20 times the frequency), but if you want more accuracy, >the above is probably to be used. Also, whether you are using a leapfrog >or any other scheme for Euler might alter this stability number as well. >But, in our sims, I don't think large accuracy is strictly necessary as >we are dealing with mostly well damped modes here that just need to be >surpressed and we are not looking at their actual effects as much.

Right, if it doesn't explode the function, it's ok enough for me. ;-) Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

On Mon, 03 Sep 2001 18:24:28 GMT, mjessick-Motorsims <[email hidden]> wrote: >An err, ah, hmm, a, I mean a friend of mine says ;) >to be very respectful of the wheel hop mode. >The masses and stiffnesses, etc., in race cars are such >that if you let it get numerically unstable it can >actually bounce the car completely off the ground. Hey, what a recognizable thing. :) The karts and F1 cars had/have this when the frequency is too low. Even the normal cars when I give the suspension not enough room.

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>If you put in a wheel hop mode, also be very careful >about differentiating positions to get velocities. >For example: > > suspensionRate = > (suspensionPos - previousSuspensionPos)/deltaTime; > >is very dangerous.

I think I use that for the dampers. Should I try to use the math (I mean a derivative) more directly instead of this empirical way of determining suspension velocity? Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

On Mon, 03 Sep 2001 12:55:20 +0200, jonas echterhoff <[email hidden]> wrote: >anyways: now that i'm using a steering wheel, the general attitude of >the car to understeer is gone. i suspect that my simulated keyboard >input was just turning the wheel to fast, resulting in understeer (this >is realistic behavior, isn't it?). Yes, after the peak steering angle, the steering gets worse and you get understeer. I think without my wheel AND force feedback I can tell quite little about the real behavior that I see. However, as for analogue input; I use the mouse a lot. Takes some getting used to but it gives you analog throttle/brake (vertical) and steering. Not too bad. Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

In article <[email hidden]>, mjessick-Motorsims <[email hidden]> wrote: > You might try programming an automatic countersteer, > just to see whether you could recover if you were > impossibly fast. > I doubt that it really is "impossible" to recover, > but you may well need to be overly fast in reacting. > ok, as i meantioned in another post, i get better results by using a steering wheel, and i just tried adjusting the pacejka curve to a "flat" one (no loose of grip beyound optimal slip) to make things easier. it is possible to counter in oversteer situations (easier now with the modified pacekja curves), but not to recover to stability. even in a situation where a turn leads to a small drift of the rear, which normaly should be corrected by a small quick countersteer, the countersteering will lead to more oversteer into the other direction, requiring more countersteer, and after three turns of teh steering wheel, the car is completely out of control. jonas

> Yes, after the peak steering angle, the steering gets worse and you > get understeer. > I think without my wheel AND force feedback I can tell quite little > about the real behavior that I see. However, as for analogue input; I > use the mouse a lot. Takes some getting used to but it gives you > analog throttle/brake (vertical) and steering. Not too bad. uugh. i never got used to that setup. but i got my pedals to work now. i'm using a borrowed PSX wheel via PSX<->USB adapter, and i didn't know it had to be set up to use the pedals. should get a real USB wheel some day. jonas

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>ok, as i meantioned in another post, i get better results by using a >steering wheel, and i just tried adjusting the pacejka curve to a >"flat" one (no loose of grip beyound optimal slip) to make things >easier. >it is possible to counter in oversteer situations (easier now with the >modified pacekja curves), but not to recover to stability. even in a >situation where a turn leads to a small drift of the rear, which >normaly should be corrected by a small quick countersteer, the >countersteering will lead to more oversteer into the other direction, >requiring more countersteer, and after three turns of teh steering >wheel, the car is completely out of control. >

Hmm... A friend of mine and I did exactly that for real at about 120 mph in his '71 Firebird.... Maybe you've got it right after all? It should be difficult to recover, probably? Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

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>>If you put in a wheel hop mode, also be very careful >>about differentiating positions to get velocities. >>For example: >> >> suspensionRate = >> (suspensionPos - previousSuspensionPos)/deltaTime; >> >>is very dangerous. > >I think I use that for the dampers. Should I try to use the math (I >mean a derivative) more directly instead of this empirical way of >determining suspension velocity? >

Ruud, why not use vectors to calculate the relative velocity between the suspension mount and the appropriate ground triangle along the "suspension axis" instead? Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

> Hmm... A friend of mine and I did exactly that for real at about 120 mph in > his '71 Firebird.... Maybe you've got it right after all? It should be > difficult to recover, probably? > Todd Wasson sure this can happen in real life - i remeber once almost loosing control over an old VW Polo at about 40 mph this way - but it shouldn't be impossible to recover. at the moment, in my sim unless i drive real slow i never get past the first curve. jonas

Ruud van Gaal wrote:

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>>If you put in a wheel hop mode, also be very careful >>about differentiating positions to get velocities. >>For example: >> >> suspensionRate = >> (suspensionPos - previousSuspensionPos)/deltaTime; >> >>is very dangerous. >> > > I think I use that for the dampers. Should I try to use the math (I > mean a derivative) more directly instead of this empirical way of > determining suspension velocity?

It could allow a major frame rate improvement, because getting rid of the high frequencies could make the overall integration much more stable. -- Matthew V. Jessick Motorsims [email hidden] http://www.moto1.net Vehicle Dynamics Engineer (972)910-8866 Ext.125, Fax: (972)910-8216

jonas echterhoff <[email hidden]> wrote in message news:030920012352353285%[email hidden]... > it is possible to counter in oversteer situations (easier now withthe > modified pacekja curves), but not to recover to stability. even in a > situation where a turn leads to a small drift of the rear, which > normaly should be corrected by a small quick countersteer, the > countersteering will lead to more oversteer into the otherdirection, > requiring more countersteer, and after three turns of teh steering > wheel, the car is completely out of control. Once again, what's your weight distribution and how do the front and rear tyres compare? Ultimately, handling is *all* about the tyres. Jonny

> Once again, what's your weight distribution and how do the front and > rear tyres compare? Ultimately, handling is *all* about the tyres. > > Jonny weight distribution is about 55/45 to the front (i got that from actual car specs). front and rear tyres are equal. jonas

I left out a word ;) mjessick-Motorsims wrote: > The higher the input frequency is above > the filters cut off frequency, the more attenuation > there is. (The disturbing input signal is filtered out.) > The higher frequency inputs are NOT

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>attenuated in the > high frequency modes as well as the low frequency > modes because the cutoff frequency is much closer to > the input frequency. > > If you allow a significant amount of power to enter them > in the frequency range where they resonate, > Bad Things Can Happen.

-- Matthew V. Jessick Motorsims [email hidden] http://www.moto1.net Vehicle Dynamics Engineer (972)910-8866 Ext.125, Fax: (972)910-8216

jonas echterhoff <[email hidden]> wrote in message news:040920012016488268%[email hidden]... > > > Once again, what's your weight distribution and how do the frontand > > rear tyres compare? Ultimately, handling is *all* about thetyres. > > weight distribution is about 55/45 to the front (i got that fromactual > car specs). front and rear tyres are equal. In that case, the car's going to be an understeerer (certainly in the linear region) if you ignore tractive forces... which is not the problem! Probably is fairly representative, then, although I'd guess that major unknowns are things like roll centres and camber gradients; damping (it's often not really linear); and diff; though I'm not sure how much difference these would make. Jonny

hi, 1) we know that even if there is no spring/dampers and tires made of steel, there is always weight transfert and a load on the tires. So using spring/tire rate to compute load is not so good. I didn't know if my model is scientificly true but IMHO it's quite real. load = spring_damper_force + vertical_acelleration_at_the_wheel * weight_purcentage_at_this_wheel. * car_weight; load *= ( tire_touch_the_ground == TRUE ); the touchy things is to compute the acceleration at the wheel from body linera and angular speed and acceleration, but it's easly found on any physics/mecanics science web site. 1)when the tire don't touch the ground, load is null. 2)in normal condition, the load is spring/dampers force. 3)when the spring/dampers are compressed at the maxium, whe have spring_force at its maximum, damper_force at zero, and the acceleration of the car_body is directly transmit to the tire. Maybe it's not true but it's work well when the car have fast diving/rolling . I didn't really understand if when you talk about tire rate you talk about the spring effect of the tire or the spring's rate of the car or the all suspension forces : spring + dampers ? king regards, -- Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-42.98 Monster:-117.44 > > In my case another thing *may* happen; I use tire rates to get loads > (so if the wheel is 1cm 'in' the track, you get 0.01m*tirerate = ... > Newtons. With a small stepsize, the forces this generates may be a bit > too much for the poor Euler integrator and the tire will quickly > bounce up and down the track, making for at least half the grip you'd > normally have.

> > 1)when the tire don't touch the ground, load is null. > 2)in normal condition, the load is spring/dampers force. because the wheel is stopped by the ground and so don't have any vertical acceleration/speed. In my model the tire have no weight, only angular inertia. -- Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-42.98 Monster:-117.44

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>> Hmm... A friend of mine and I did exactly that for real at about 120 mph >in >> his '71 Firebird.... Maybe you've got it right after all? It should be >> difficult to recover, probably? >> Todd Wasson > >sure this can happen in real life - i remeber once almost loosing >control over an old VW Polo at about 40 mph this way - but it shouldn't >be impossible to recover. at the moment, in my sim unless i drive real >slow i never get past the first curve. >

You said you've got 55% load on the front tires. You're using Beckman's approach to Pacejka, so you have tire load sensitivity (degradation in mu with increasing load) included? If not, that might help a little. Otherwise, with so much load on the front end, perhaps you simply need less grippy tires on the front. As I mentioned earlier, my tire model is the worst of the ras bunch, but the car handles just great IMO. My camber changes don't effect forces yet and I've got no load sensitivity, but it seems easier to drive than what you've been describing regardless, so I doubt that's the problem you're having. With 55% load on the fronts and identical tires all the way around, you'll generate more lateral force at the front than the rear and should be getting lots of oversteer. Perhaps you might try lowering the grip at the front somewhat. It seems that most cars (not sure about a '66 Charger) have bigger tires at the rear than front anyway. If they had the same tires all the way around in real life, then it's possible the camber gradient would be to blame. I think most production cars (even high performance) get positive camber at the front outside wheel when cornering hard, which lowers front end grip in an attempt to keep the handling understeer. Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

Sebastien Tixier wrote: > > In my model the tire have no weight, only angular inertia. > Et tu, Sebastien ;)? -Gregor

???? maybe you try to do a joke in french, but your sentence is completly incomprehensible ... maybe,err, certainly because my english sentence was also incomprehensible ;o)))))) what did you want to say ? "And you sebastien ?" ?? if so the write sentence in french is ... "Et toi Sebastien ?" Gregor Veble wrote:

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> Sebastien Tixier wrote: > > > > In my model the tire have no weight, only angular inertia. > > > > Et tu, Sebastien ;)? > > -Gregor

-- Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-42.98 Monster:-117.44

"J. Todd Wasson" <[email hidden]> wrote in message news:[email hidden]... With 55% > load on the fronts and identical tires all the way around, you'll generatemore > lateral force at the front than the rear and should be getting lots of > oversteer. But more lateral force on the front than the rear doesn't necessarily mean that the torque produced by the front tyres will be greater than the torque produced at the rear. Converting force to torque involves a cross product of the force direction with the vector from the force position to the COM. Since the front tyre will be turned slightly, this will produce less torque than if it were aligned with the car's main axis.

Hi Sebastien, Nah, it was just a reference to 'Et tu Brutus', which I used as I am guilty of doing the same approximation (that guarantees good stability) myself. :) I wouldn't even dare to try typing French! ;) Greetings, -Gregor Sebastien Tixier wrote:

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> > ???? > > maybe you try to do a joke in french, but your sentence is completly > incomprehensible ... maybe,err, certainly because my english sentence > was also incomprehensible ;o)))))) > > what did you want to say ? "And you sebastien ?" ?? > > if so the write sentence in french is ... > > "Et toi Sebastien ?" >

well, i ve a little knowlege of Italien and i think the correct word is not "Et" but "i" that mean "And" in Italian. "I tu brutus" ... but maybe we had to continue ond rec.latin.qutation ;o))))) Gregor Veble wrote:

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> Hi Sebastien, > > Nah, it was just a reference to 'Et tu Brutus', which I used as I am > guilty of doing the same approximation (that guarantees good stability) > myself. :) > > I wouldn't even dare to try typing French! ;) > > Greetings, > > -Gregor > > Sebastien Tixier wrote: > > > > ???? > > > > maybe you try to do a joke in french, but your sentence is completly > > incomprehensible ... maybe,err, certainly because my english sentence > > was also incomprehensible ;o)))))) > > > > what did you want to say ? "And you sebastien ?" ?? > > > > if so the write sentence in french is ... > > > > "Et toi Sebastien ?" > >

-- Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-42.98 Monster:-117.44

Now you got me, I have absolutely no idea if what I've written is correct Latin, it was just a phrase I thought was appropriate :o)! Nevermind, nothing to see here, everybody, back to work ;). -Gregor Sebastien Tixier wrote:

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> > well, i ve a little knowlege of Italien and i think the correct word is not > "Et" but "i" that mean "And" in Italian. > > "I tu brutus" ... > > but maybe we had to continue ond rec.latin.qutation ;o))))) >

Sebastien Tixier <[email hidden]> writes: > well, i ve a little knowlege of Italien and i think the correct word is not > "Et" but "i" that mean "And" in Italian. > > "I tu brutus" ... > > but maybe we had to continue ond rec.latin.qutation ;o))))) I think the correct spelling is "Et tu, Brutu", actually. This is latin, not italian. -- -asbjxrn

Damn ! :o) So my first impression was the good one, i didn't understand anything ! What the meaning of "Et tu Brutus" ??? "Asbj�rn Bj�rnstad" wrote:

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> Sebastien Tixier <[email hidden]> writes: > > > well, i ve a little knowlege of Italien and i think the correct word is not > > "Et" but "i" that mean "And" in Italian. > > > > "I tu brutus" ... > > > > but maybe we had to continue ond rec.latin.qutation ;o))))) > > I think the correct spelling is "Et tu, Brutu", actually. This is latin, > not italian. > -- > -asbjxrn

-- Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-42.98 Monster:-117.44

Sebastien Tixier <[email hidden]> writes: > Damn ! :o) > > So my first impression was the good one, i didn't understand anything ! > > What the meaning of "Et tu Brutus" ??? "You too, Brutus" ? (Uttered by Caesar as he was stabbed to death according to Shakespeare) -- -asbjxrn

Aint this from a popeye flick? :) "Sebastien Tixier" wrote: | "I tu brutus" ... --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.276 / Virus Database: 145 - Release Date: 9/3/2001

On 04 Sep 2001 03:01:38 GMT, [email hidden] (J. Todd Wasson) wrote:

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>>>If you put in a wheel hop mode, also be very careful >>>about differentiating positions to get velocities. >>>For example: >>> >>> suspensionRate = >>> (suspensionPos - previousSuspensionPos)/deltaTime; >>> >>>is very dangerous. >> >>I think I use that for the dampers. Should I try to use the math (I >>mean a derivative) more directly instead of this empirical way of >>determining suspension velocity? >> > > Ruud, why not use vectors to calculate the relative velocity between the >suspension mount and the appropriate ground triangle along the "suspension >axis" instead?

Isn't that almost exactly the same? Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

" What the meaning of "Et tu Brutus" ???" And you too Brutus :) [paraphrased] "Sebastien Tixier" wrote: | Damn ! :o) | So my first impression was the good one, i didn't understand anything ! | What the meaning of "Et tu Brutus" ??? --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.276 / Virus Database: 145 - Release Date: 9/3/2001

oops I put the word "and" in there.. Damm I hope dont get banned from RAS!!?? haha. U show'off Asbj�rn U & U race to damm fast, last Sunday, at Rouen too :o)~ "Asbj�rn Bj�rnstad" wrote:

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| | "You too, Brutus" ? | (Uttered by Caesar as he was stabbed to death according to Shakespeare) | -- | -asbjxrn | Sebastien Tixier writes: | > Damn ! :o) | > | > So my first impression was the good one, i didn't understand anything ! | > | > What the meaning of "Et tu Brutus" ???

--- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.276 / Virus Database: 145 - Release Date: 9/3/2001

On Wed, 05 Sep 2001 13:16:13 +0200, Gregor Veble <[email hidden]> wrote: >Now you got me, I have absolutely no idea if what I've written is >correct Latin, it was just a phrase I thought was appropriate :o)! >Nevermind, nothing to see here, everybody, back to work ;). Hi guys, hey what's going on here? A thread in Latin? Let me just get and read the entire thread. Oh sorry! :) Back to work. Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

On 05 Sep 2001 07:06:20 GMT, [email hidden] (J. Todd Wasson) wrote: >Perhaps you might try lowering the grip at the front somewhat. It >seems that most cars (not sure about a '66 Charger) have bigger tires at the >rear than front anyway. I noticed at Spa that the F1 cars have equal tires in front and rear, unlike in Cart. How are they doing things there? (or is there a rule stating you must use equal tires in front and rear?) Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

exact.. i was totaly wrong , but i think the original sentence was "tu quoque fili" ;o) "Asbj�rn Bj�rnstad" wrote:

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> Sebastien Tixier <[email hidden]> writes: > > > Damn ! :o) > > > > So my first impression was the good one, i didn't understand anything ! > > > > What the meaning of "Et tu Brutus" ??? > > "You too, Brutus" ? > (Uttered by Caesar as he was stabbed to death according to Shakespeare) > -- > -asbjxrn

-- Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-42.98 Monster:-117.44

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> You said you've got 55% load on the front tires. You're using Beckman's > approach to Pacejka, so you have tire load sensitivity (degradation in mu with > increasing load) included? If not, that might help a little. Otherwise, with > so much load on the front end, perhaps you simply need less grippy tires on > the > front. >

anyway, i don't think that load distribution is relevant here, since i've tried just about anything from 100/0 to 0/100, however the problem (uncontrollability in oversteer situations) remains. it's not that i don't like general oversteer/understeer attitude of my engine. if i implement an FWD car it gives a lot of understeer just like IRL. the problem is that once the rear brakes out i never manage to regain control. i have not implemented roll centers (thought they would result from the other parameters, but read that they are necessary), so understanding and implementing them is the next suggestion i will try. jonas

Ruud van Gaal <[email hidden]> wrote in message news:[email hidden]... > I noticed at Spa that the F1 cars have equal tires in front andrear, > unlike in Cart. How are they doing things there? (or is there a rule > stating you must use equal tires in front and rear?) They don't, AFAIK. A couple of years ago, Tom Walkinshaw tried rear tyres all round at the Hungaroring until Dunlop banned it on "safety grounds"; he said afterwards that what he'd /really/ wanted to do was use front tyres all round at Hockenheim! It's probably just the perspective but I'm pretty sure the rears are wider. Jonny

If you don't model tire-load sensitivity (decreasing coeff. of friction with increasing load), none of your chassis adjustments will work properly, and a front-heavy car will be oversteer instead of understeer in neutral throttle conditions... when I was starting my own model I didn't do any fancy roll-center stuff at first, and even then the car would behave correctly in the case you describe, so I would guess you may have a bug in your existing stuff somewhere--writing good telemetry tools and printing out intermediate values is crucial for debugging physics code, as things can really seem to work even when terms are the wrong sign, unit conversion problem (if you live in the US like me and get quantities in slug-feet-squared and other such nonsense), or other things like that. Usually once you start working on roll centers, you will discover that you already had an implicit roll center already, but it's probably always exactly at ground level, which shouldn't be a bad place to start. Things to check off the top of my head would include your longitudinal force combining--try setting your slip ratio to zero and coast into a turn to simplify the situation, also check to make sure the "wrong end" of your tire curve checks out correctly, with no sine(x) = x approximations starting to bite you as becomes large, also check that you are correctly factoring in the angular velocity of the car body at the wheel point, that you aren't getting overlarge integration errors, esp from wheel hop or wheel rotation chatter, etc, etc. It can be very hard to get a good chassis setup even for a sim you wrote yourself--it's about as hard as setting up a real car, which takes years of experience, but you should be able to throw on a stiff front anti-roll bar on your vehicle and at least get it to plow like a pig into corners... keep on trying, the first time you can make your own car that feels right is an amazing experience! -Dave P. "jonas echterhoff" <[email hidden]> wrote in message news:050920011907488927%[email hidden]... > > > You said you've got 55% load on the front tires. You're usingBeckman's > > approach to Pacejka, so you have tire load sensitivity (degradation inmu with > > increasing load) included? If not, that might help a little.Otherwise, with > > so much load on the front end, perhaps you simply need less grippy tireson

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> > the > > front. > > > anyway, i don't think that load distribution is relevant here, since > i've tried just about anything from 100/0 to 0/100, however the problem > (uncontrollability in oversteer situations) remains. > it's not that i don't like general oversteer/understeer attitude of my > engine. if i implement an FWD car it gives a lot of understeer just > like IRL. the problem is that once the rear brakes out i never manage > to regain control. > i have not implemented roll centers (thought they would result from the > other parameters, but read that they are necessary), so understanding > and implementing them is the next suggestion i will try. > > jonas

> If you don't model tire-load sensitivity (decreasing coeff. of friction with > increasing load), none of your chassis adjustments will work properly, and > a front-heavy car will be oversteer instead of understeer in neutral > throttle conditions... i just read some info on this, and it seems that tire-load sensitivity and camber angles do seem to have an important impact on dynamics (i didn't know about that before, nor did i understand, why a front heavy car should be understeer in first place). the problem is that the pacejka formulae i use seem to take these things into consideration, however the pacejka constants i have (taken from Brian Beckmann's articles) zero out the effect. they don't seem to be suitable for the car i'm modelling anyways - at maximum efficiency the Beckmann tires project 1.7 times their normal Force. i though that in school physics they tought that coefficients of friction above 1 are not possible. if i use the unmodified numbers from Beckmann my car is actually likely to flip over in curves due to the high lateral forces. unfortunatly these seem to be the only pacejka numbers one can find on the net. any suggestions on where to get better numbers? jonas

Indeed, this is one of the most popular simple, sound and unfortunately wrong arguments taught in physics. There is absolutely no reason why coefficients of friction above 1 shouldn't be possible, and indeed racing tyres can go even over 2 for small loads. Most other popular and wrong theories taught in physics are probably found in fluid dynamics, especially with respect to mishandling the Bernoulli's equation, probably the most abused physics formula ever :). -Gregor jonas echterhoff wrote: > > i though that in school physics > they tought that coefficients of friction above 1 are not possible. >

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>>>I think I use that for the dampers. Should I try to use the math (I >>>mean a derivative) more directly instead of this empirical way of >>>determining suspension velocity? >>> >> >> Ruud, why not use vectors to calculate the relative velocity between the >>suspension mount and the appropriate ground triangle along the "suspension >>axis" instead? > >Isn't that almost exactly the same? > >

Hmmm.. Ummm.. Well... I dunno?? Is it? lol If it is, I sure went to a lot of extra trouble for nothing! I think it might be the same except you and I would be off by one time step probably, as you're taking the last time step's velocity and I'm taking the current one... Right? Guess I was just so proud of myself for coming up with a vector method all on my own :-) Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

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> With 55% >> load on the fronts and identical tires all the way around, you'll generate >more >> lateral force at the front than the rear and should be getting lots of >> oversteer. > >But more lateral force on the front than the rear doesn't necessarily mean >that the torque produced by the front tyres will be greater than the torque >produced at the rear. Converting force to torque involves a cross product of >the force direction with the vector from the force position to the COM. >Since the front tyre will be turned slightly, this will produce less torque >than if it were aligned with the car's main axis. > > >

Yeah, you're right. Guess I thought (wrongly) too quickly on that one. In that case, the load sensitivity issue would be the only factor, right? Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

>anyway, i don't think that load distribution is relevant here, since >i've tried just about anything from 100/0 to 0/100, however the problem >(uncontrollability in oversteer situations) remains. Yeah, you're probably right. As Mike Stanley pointed out, the yaw torque cancels out my point. Maybe load sensitivity isn't the problem either. Guess I can't help much there, sorry :-) > the problem is that once the rear brakes out i never manage >to regain control. >i have not implemented roll centers (thought they would result from the >other parameters, but read that they are necessary), so understanding >and implementing them is the next suggestion i will try. > I don't think that will fix it. Mostly, putting in roll centers at this point would effect transient behavior and total roll angle and that's about it. Once the car breaks away, it'll most likely still have the same problem. The only thing left I can think of to try is using less grip at the front. Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

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>i just read some info on this, and it seems that tire-load sensitivity >and camber angles do seem to have an important impact on dynamics (i >didn't know about that before, nor did i understand, why a front heavy >car should be understeer in first place). the problem is that the >pacejka formulae i use seem to take these things into consideration, >however the pacejka constants i have (taken from Brian Beckmann's >articles) zero out the effect. they don't seem to be suitable for the >car i'm modelling anyways - at maximum efficiency the Beckmann tires >project 1.7 times their normal Force. i though that in school physics >they tought that coefficients of friction above 1 are not possible. if >i use the unmodified numbers from Beckmann my car is actually likely to >flip over in curves due to the high lateral forces. >unfortunatly these seem to be the only pacejka numbers one can find on >the net. any suggestions on where to get better numbers? >

As Gregor said, friction coefficient can go well over one. Top fuel dragster tires have peak grip coefficients of over 4. If they can't go over 1, dragsters would never run better than about 9 seconds at 200 mph (they run 4.7-8 at 330+ these days, I think.) One of the big things about tires I think is that at very low load (nothing but tire and rim), even a skinny street tire may have a 1.7 coefficient, but it drops a little with every kg you add. Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

[email hidden] (Asbj�rn Bj�rnstad) writes:

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> Sebastien Tixier <[email hidden]> writes: > > > well, i ve a little knowlege of Italien and i think the correct word is not > > "Et" but "i" that mean "And" in Italian. > > > > "I tu brutus" ... > > > > but maybe we had to continue ond rec.latin.qutation ;o))))) > > I think the correct spelling is "Et tu, Brutu", actually. This is latin, > not italian.

I thought it was "Brute"... -- Gunnar #31 SUCKS#015 Tupperware MC#002 DoD#0x1B DoDRT#003 DoD:CT#4,8 Kibo�: 2 gnus don't kill people. gnus-summary-lower-score kills people.

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>a few weeks ago, i posted a question my tire model since i was not >happy with the handling of my car. i was pointed to the pacekja >formulae, which i have now implemented in my engine. however, it turns >out that my made-up tire-model wasn't that bad, because i'm still >having the same problems ;-( > >at higher speeds my car will always strongly understeer, no matter how >much i push the throttle. if i simulate a locking differential, or pull >the handbrake in curves i can make the car oversteer, however it will >be impossible to get it back to control. instead it will spin around >its vertical axis at least 360 degrees, more at higher speeds. i was >having the same problems with my old tire model, so i assume some other >part of the simulation is causing the problems. > >thanks in advance for any suggestions, > >jonas

today i accidently i tried something interesting: i link wheel rotational velocity to the road velocity (allowing only a limited difference), thus allowing only a limited amount of slip. this lets me go through curves with a constant drift angle, without requiring much user input (counter-steering or throttle regulation), giving a very arcardy feel. i mention this because a few weeks ago there was a thread on how to create convining arcarde modes. this is something one might want to try out. of course there would have to be many things to take care of (donuts, oversteer not caused by engine power). unfortunatly this gets me nowhere closer to a solution to my original problem. once i begin counter-steering (to get a wider curve radius) the car turns around into the other direction and gets out of control ;-( jonas

Are you sure this is a problem? Typically this is what would happen in a real car, where the after the opposite lock is performed the rotation in the other direction can be even more severe if you don't anticipate for it. I know, it happened to me once :). -Gregor jonas echterhoff wrote: > > unfortunatly this gets me nowhere closer to a solution to my original problem. once i begin counter-steering (to get a wider curve radius) the car turns around into the other direction and gets out of control ;-( > > jonas

J. Todd Wasson <[email hidden]> wrote in message news:[email hidden]... > I don't think that will fix it. Mostly, putting in roll centersat this > point would effect transient behavior and total roll angle andthat's about it. > Once the car breaks away, it'll most likely still have the sameproblem. It might affect roll couple distribution, though - a high enough rear RC would be equivalent to locking up the rear ARB... Jonny

if it's not a problem, then i must be a *really* bad driver, as with the current setup, i never get pass the first curve, unless i drive slow enough that the car doesn't skid. haven't experienced this with other sims, so i guess it's more a problem with my sim than with my driving. i've been trying to solve this for weeks but i'm running out of ideas. i don't think its related to suspension/load transfer, since the problem persists if i set the cm at ground level and/or simply pass car->mass*g/4 for wheel normal forces. i also don't think that the tire model is the problem, since i've tried just about everything related to this (different combination methods, etc..). i also tried all kinds of different parameters for the cars cm and inertia parameters - what else is there to change? jonas

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> Are you sure this is a problem? Typically this is what would happen in a > real car, where the after the opposite lock is performed the rotation in > the other direction can be even more severe if you don't anticipate for > it. I know, it happened to me once :). > > -Gregor >

On Thu, 06 Sep 2001 12:05:13 +0200, jonas echterhoff <[email hidden]> wrote:

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> >> If you don't model tire-load sensitivity (decreasing coeff. of friction with >> increasing load), none of your chassis adjustments will work properly, and >> a front-heavy car will be oversteer instead of understeer in neutral >> throttle conditions... > >i just read some info on this, and it seems that tire-load sensitivity >and camber angles do seem to have an important impact on dynamics (i >didn't know about that before, nor did i understand, why a front heavy >car should be understeer in first place). the problem is that the >pacejka formulae i use seem to take these things into consideration, >however the pacejka constants i have (taken from Brian Beckmann's >articles) zero out the effect. they don't seem to be suitable for the >car i'm modelling anyways - at maximum efficiency the Beckmann tires >project 1.7 times their normal Force. i though that in school physics >they tought that coefficients of friction above 1 are not possible. if >i use the unmodified numbers from Beckmann my car is actually likely to >flip over in curves due to the high lateral forces.

The Beckman numbers are correct and were taken from the book by Giancarlo Genta (uhm, "Modeling and Computer simulation"?). Indeed, there's a few things missing from these Ferrari numbers, which IS load sensitivity. There are more examples in the book (5 actually) which DO include load sensitivity, although these are very small numbers (but ofcourse, the loads get quite high, in the thousands). And as for u>1, yeah well, like Gregor & Todd said. :) Even the tires on your own car will probably go higher than 1. Take a hook around a bar, pull that; it gets 'more than 1' because it grips, not slips. Same for materials like rubber, which grab hold of the road more than just sliding. For really slipping materials, I'd think the Flateral<=Fnormal would be correct, although I don't think such materials exist on this planet. ;) Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

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>> I don't think that will fix it. Mostly, putting in roll centers >at this >> point would effect transient behavior and total roll angle and >that's about it. >> Once the car breaks away, it'll most likely still have the same >problem. > >It might affect roll couple distribution, though - a high enough rear >RC would be >equivalent to locking up the rear ARB... > >Jonny

Good point. Perhaps if he ran a high roll center at the front it would keep his car more understeer? Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

It sounds like you can't complete a turn even under moderate lateral acceleration? Perform a test where you slowly increase the steer signal while maintaining a constant speed on level ground. Does the eventually car spin out? (Final Oversteer) Or does the lateral acceleration received reach a peak at some steer angle and then drop off at higher steer angles (Final Understeer). Presumably, you have a car that is final oversteer. If even in this very benign case, the car will spin out. If so, reduce the torque capability of the front end until this doesn't occur through changing the tire lateral capability. (Which can be done with friction coefficients and/or through load sensitivity - for example: changing Anti-Roll Bars to change the amount of lateral load transfer betwen the fronts and rears.) The other test you might do is a step steer test. Step the steer signal (not necessarily to full lateral G level) and watch the car respond in yaw and lateral acceleration. If it doesn't respond as you like (stably and damped) then you have either (or both) a static stability problem or a damping problem or ... Subtle changes in the tire properties can affect the results greatly. jonas echterhoff wrote:

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> if it's not a problem, then i must be a *really* bad driver, as with > the current setup, i never get pass the first curve, unless i drive > slow enough that the car doesn't skid. haven't experienced this with > other sims, so i guess it's more a problem with my sim than with my > driving. i've been trying to solve this for weeks but i'm running out > of ideas. i don't think its related to suspension/load transfer, since > the problem persists if i set the cm at ground level and/or simply pass > car->mass*g/4 for wheel normal forces. i also don't think that the tire > model is the problem, since i've tried just about everything related to > this (different combination methods, etc..). i also tried all kinds of > different parameters for the cars cm and inertia parameters - what else > is there to change? > > jonas > > > Are you sure this is a problem? Typically this is what would happen in a > > real car, where the after the opposite lock is performed the rotation in > > the other direction can be even more severe if you don't anticipate for > > it. I know, it happened to me once :). > > > > -Gregor > >

> It sounds like you can't complete a turn even under moderate > lateral acceleration? > hmm, not quite.. i can make car setups wich tend to understeer and with those i can get through the curves ok. the physics feel realistic to me until the moment i begin to counter-steer in oversteer situations. but at that point the car begins to spin into the opposite direction and is much harder to catch again, so that in the end i never manage to regain control. the problem might somehow by related to the inputs i pass to my car, but i can't really think of how.

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> Perform a test where you slowly increase the steer signal > while maintaining a constant speed on level ground. > Does the eventually car spin out? (Final Oversteer) > Or does the lateral acceleration received reach a peak > at some steer angle and then drop off at higher steer angles > (Final Understeer). >

this always depends on how fast i go, and how fast i increase the steering signal. i'd say i can create both setups. but with any setup i use i have the problem that once the car spins out for some reason it is lost. jonas

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> It sounds like you can't complete a turn even under moderate > lateral acceleration? > > Perform a test where you slowly increase the steer signal > while maintaining a constant speed on level ground. > Does the eventually car spin out? (Final Oversteer) > Or does the lateral acceleration received reach a peak > at some steer angle and then drop off at higher steer angles > (Final Understeer). > > Presumably, you have a car that is final oversteer. > If even in this very benign case, the car will spin out. > If so, reduce the torque capability of the front end until > this doesn't occur through changing the tire lateral capability. > (Which can be done with friction coefficients and/or through > load sensitivity - for example: changing Anti-Roll Bars to > change the amount of lateral load transfer betwen the fronts and rears.)

actually i just tried decreasing my cars front torque capability, and it is getting better. i'm not where i want to get yet, but i'll keep trying. i guess i was just overestimating the effect of counter-steering versus what can be done by regulating throttle (RWD vehicle) in my setups. thanks for the helpful messages everbody sent. jonas