rec.autos.simulators

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

Vehicle Dynamics Engineer  (972)910-8866 Ext.125, Fax: (972)910-8216

Ruud van Gaal
Free car sim  : http://www.racesimcentral.net/
Pencil art    : http://www.racesimcentral.net/

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/

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/

jonas

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

  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

  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

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

--
Matthew V. Jessick         Motorsims

Vehicle Dynamics Engineer  (972)910-8866 Ext.125, Fax: (972)910-8216

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

--
Matthew V. Jessick         Motorsims

Vehicle Dynamics Engineer  (972)910-8866 Ext.125, Fax: (972)910-8216

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

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