rec.autos.simulators

by **Ruud van Ga** » Wed, 25 Jul 2001 06:14:49

...

I use an upper torque limit, and a lower one. For the upper limit I
use a curve, since that's so characteristic for an engine. For the
lower limit, it seems a mostly linear relationship to RPM.
So:

throttleInput=0..1
maxTorque=curve(rpm)
minTorque=-engineBrakingCoeff*rpm
engineTorque=throttleInput*(maxTorque-minTorque)+minTorque

That's all folks. ;-)

Ruud van Gaal, GPL Rank +53.25
Pencil art : http://www.racesimcentral.net/
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by **Doug Millike** » Wed, 25 Jul 2001 13:21:48

> > 1)what is the formula for rolling resistance?
> This paper discusses models used in research and the
> limitation of various models as well:

> "Modelling of Rolling Resistance Test Data,"
> Parmeet S. Grover, SAE980251. SAE, 1998.

> You could probably still download this paper online for
> a small fee from www.sae.org.

Or SAE might make you buy SP-1338 (collection of tire/wheel papers from
1998). Just took a look, several other interesting papers in there.

-- Doug Milliken

by **Rafe McAulif** » Wed, 25 Jul 2001 19:37:18

Since you've had heaps of answers on rolling resistance...

With any object, there is a coefficient of friction for sliding, and
one for static, when you are gripping. The friction force is pretty
simple, if you know the force from the tyre to the ground, it is
simply coeff x reaction force = lateral force. Once the lateral force
is exceeded, the coeff will change from the static one to the sliding
one.

So the amount of grip available drops once you spin the tyres, but
this will heat the tyres at the same time. So the "wheel spinning
grip" will rise with temp. to a point, then drop. Also the pressure
will rise, reducing the contact patch

What coefficients to use? Depends on tyres, heat, road surface, etc.
You'll need to get a data book on experimental values to be accurate.
Normal tyres are around 0.3-0.4 generally. I know that F1 cars develop
a coeff of around 3-3.5, mainly due to the grippy *** and the huge
downforce at speed. Without downforce, it's impossible to go higher
than 1.0

Rafe Mc

by **Gregor Vebl** » Wed, 25 Jul 2001 19:42:23

Actually, that was indeed on of the 'rules' that were thought to be
quite true by physicists, but current tyre compounds proved the rule to
be false.

-Gregr

> Without downforce, it's impossible to go higher
> than 1.0

> Rafe Mc

by **Ruud van Ga** » Wed, 25 Jul 2001 21:06:18

>With any object, there is a coefficient of friction for sliding, and
>one for static, when you are gripping. The friction force is pretty
>simple, if you know the force from the tyre to the ground, it is
>simply coeff x reaction force = lateral force. Once the lateral force
>is exceeded, the coeff will change from the static one to the sliding
>one.

This is very much oversimplification for tires and I would advise
against your regular static vs. sliding friction model. The tire is a
flexing piece of ***, and preferably has a slow transition from
sticking to sliding.
The 'coeff' you speak of can be taken from Pacejka curves, but that's
hardly trivial when you start with a sim. Transitions from static to
sliding coefficients are not like shoving around a cupboard.

It seems both raising and lowering pressure away from the optimal
point reduces grip btw. Wasn't somewhere like 155kPa appropriate for a
sports car tire?
Anybody now the relationship of tire heating vs. slip ratio for
example? Don't know whether it would be just linear or quadratic, but
it seems to me a simple temp system could consist of:

heat (in Joules for example) += slipRatio*timeStep*heatCoefficient
heat += slipAngle*timeStep*heatCoefficient(Lat)
heat -= disippationCoefficient*(tireTemp-airTemp)*timeStep

And derive pressure and things like that from that.

Actually, your own car probably has tires doing just over 1.0, in the
area of 1.1 somewhat. See Genta's book; a normal tire these days can
do about 1.1, the Ferrari tires are ~1.7 (the b2 Pacejka constant),
and I would assume therefore the F1 cars today will do about 2.0-2.4.

The friction coefficient probably doesn't get this high, but thanks to
downforce, you get more upforce, normally referred to as load, and as
F_long=someCoefficient*normalForce (normalForce=load), the added
downforce leads to more available longitudinal force. That's raising
the load, not the coefficient.

As Gregor pointed out, this is not true. Perhaps tire hysterisis or
effects like that (*** gripping into the surface), but it can
easily outdo it's normal force. Extreme example; a pole of concrete
stucked into the ground. You surely have to apply more lateral force
to get it to move (if you succeed at all), than the force dragging it
down (gravity). Something sticky like that happens with *** I
think, on a small scale.
But there I may be totally wrong, but still, friction coefficients are
generally 1.1-2.2 from road car to F1.

Ruud van Gaal, GPL Rank +53.25
Pencil art : http://www.racesimcentral.net/
Free car sim : http://www.racesimcentral.net/

by **J. Todd Wass** » Thu, 26 Jul 2001 07:22:52

Righto. The (false) belief that friction coefficient can't exceed 1 led many
drag racers to believe that no one could ever go quicker than about 9 seconds
at 200mph in the quarter mile without aero downforce. Today, modern top
fuelers are running well under 5 seconds at 320+? mph. Friction coefficients
on these tires exceed 4. This friction coefficient was actually documented in
a physics/chemical handbook (you know those huge, heavy books full of nothing
but numbers?) I saw a few years ago.

Anyway, a common myth taught in early physics classes everywhere :-)

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://www.racesimcentral.net/

by **J. Todd Wass** » Thu, 26 Jul 2001 07:24:32

One more thing, the sticky-thing is probably part of the reason, but one
probable cause of this is the *** elements interlock with the road surface
to some degree, like velcro on a tiny scale.

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://www.racesimcentral.net/