>> Thanks for your response Jeff!
>>> You're assuming each wheel get's equal torque?
>> In the absence of a differential (see my original post) this must
>> happen mustn't it?
> If you're simulating a go-kart with a solid rear axle, then you can
> just treat the whole axle as a single rotating "thing" with its own
> effective inertia including whatever gearing you wanted with the other
> shafts and so forth that are attached to it. Then assign both tires
> to this same angular velocity and feed it to your tire model for the
> next iteration. For the front tires of course you'd just use their
> own inertias and have independent angular velocities as you're
> undoubtedly already doing.
> Things are nice and simple when there aren't any diffs in the mix. :-)
>>> When the reaction torque is less than the drive torque, the tires
>>> are spinning and accelerating. The net torque, drive - reaction,
>>> divided by the angular inertia of the tires, drivetrain, and
>>> engine determines the rate of angular acceleration. Normally
>>> the drive torque falls off at partial throttle once the engine
>>> reaches some rate of rotation (rpm), or the engine hits the
>>> rev-limiter.
>> I have the drive torque falling off according to a pretty standard
>> curve so that part is fine. When I try this though, what I seem to
>> find is that when accelerating the reaction torque very quickly
>> exceeds the drive torque, so the kart drives off backwards. If say
>> after one timestep the engine has accelerated the wheels to produce a
>> 0.01 slip ratio which on my curve comes out at about 0.1 Fx/Fn, so
>> with say 400N load on the wheel this is a 40N longitudinal force per
>> wheel. If I use this to generate the reaction torque (ie multiply by
>> wheel radius) this seems to exceed the drive torque pretty easily. My
>> engine data are based on some torque curves for a go-kart engine I
>> found on a manufacturer website so I would have thought they were
>> reasonable. Are my timesteps simply too large? I'm running it at 30Hz
>> at the moment (there's no suspension so I thought this would do for
>> now).
> Oh yes, 30Hz is way too large of a time step for a car simulation,
> especially a kart drivetrain. I run my simulations at 250 or 333Hz.
> With higher frequencies you'll still have oscillations, but they get
> smaller with increasing frequency. I run my drivetrain and tire
> simulation at a higher frequency (just an inner loop of a few cycles
> depending on what works well, and only if it's really needed). If I
> recall correctly, that SAE paper you mentioned covers the relaxation
> approach. If you implement that or something similar and tune it
> well, it should reduce the oscillations.
> Several simulators seem quite happy with 100Hz so long as you run the
> drivetrain in a loop at a higher frequency, mine included, although it
> seems to feel nicer at 250+. For the drivetrain you might try
> 300-500Hz or so.
>>> From what I understand, Pacejka doesn't deal well with static to
>>> dynamic or dynamic to static friction situations, as these require
>>> seperate curves (once slipping, the reduced reaction force on the
>>> tires from slipping has to be lowered further still before the
>>> transition from dynamic to static friction occurs). The other
>>> issue with Pacejka is slow speeds, or spinning the tires forwards
>>> when stopped or moving backwards, or simply stopped on a banked
>>> or angled (hill) section of track.
>> I'm not using Pacejka, but I have a curve relating slip ratio to
>> longitudinal force which is shaped similarly to Pacejka (fairly
>> standard I think, rises more or less linearly to a peak at 0.1 slip,
>> then falls off with increasing slip). I have some low speed handling
>> based on SAE950311 in my slip ratio calculation, but the issue seems
>> to be that the slip ratio very quickly reaches a value approaching my
>> optimal slip ratio and therefore a large longitudinal force is
>> generated, which generates a large reaction torque on the wheels
>> making them spin backwards, which generates an even larger
>> longitudinal force backwards. I've obviously got something wrong but
>> I'm not sure what it is :/
>> Thanks,
>> James
> Matt Jessick used to call this the "bang bang" problem. Wait until
> you try to get your clutch to lock :-)
> Anyway, just crank up the frequency and it will get better. Starting
> from rest will require a relaxation approach of some kind. A quick
> and easy way to get around this is to switch to a different slip ratio
> calculation at low speed. One based on the velocity difference
> instead of the ratio. Then, at some low speed, you switch to the
> correct slip ratio calculation. Better yet is to do both calculations
> and then use a weighted average so it transitions somewhat smoothly.
> It's not perfect, but if you play with it enough most people won't
> ever notice the model switching unless they're really looking for it.
> I wrote the physics engine for Virtual RC Racing. You think kart
> tires are hard to deal with? Try inertias of almost 0... :-P
> test
