rec.autos.simulators

I've been reading your car physics threads with great interest.

I'm writing a game at the moment and I'm experiencing a problem:

When I try to model a free-rolling wheel, I am getting a negative slip ratio
as the car accelerates off (I'm using SAE 950311). Of course the result is
that the wheel tries to brake the car - hence my wheel never rotates.

I'm sure I'm missing the road-reaction forces, but I'm not quite sure how to
calculate them. Any ideas?

Thanks for you help...

Al

Hi Al

Another Sim Programmer, we are multiplying (unite brothers and we shall take
the world!)....ahem :)

Ok, this is how I do it, I am quite prepared to be corrected - please do!

Calculate the linear speed of the wheel from the velocity of the car (both
linear and angular)

i.e. WheelVelocity = CarVelocity + CarAngularVelocity x
WheelPositionAtContactPatch

where all the variables a VECTORS (evil i know :)).  and "x" is a cross
product.

Once you have the linear speed of the tyre at the contact patch you can
calculate the angular speed of the wheel using V = rw, where r = wheel
radius and V is linear speed (WheelVelocity.x) and w is a scalar angular
velocity.

This gives you a way to calculate the rotational position of the wheel,
while at the same time you can get the force produced by the inevitable -ve
slip.  This force applied to the car and then you can forget about
accelerating the free wheels - as they will be rolling at the same speed as
the car.

Sorry if that is all a little illegible, I am not feeling very literate
today ;)

All the best,
Ash
http://www.siroccoracing.com

Thanks for your swift response Ash!

It's certainly an education :-)

I think I see where you're coming from...

I can see how this would work for a completely free-rolling wheel, but I
need to be able to adjust power ratios front and rear.
(My 'definition' of free-rolling wheel is where I set all the power to one
axle)
So in the case of 20% power to the front axle, there must be a force from
the road also increasing the speed of the wheel as it's being 'pushed' from
behind - or by the road. So I really think I'll need to apply a torque or
force to the wheel from the road?

This has really got me wondering...

Cheers,

Al

Hi Al

Yep, your right - that approach wouldnt work.  Ok, I am going to have to try
and think harder :)

Ok, I am kind of thinking aloud here, when calculating the angular
acceleration of the rear wheels I use the road reaction force of the wheel
from the previous timestep.  But in your case (and in mine if I actually
calculated the angular velocity of the front tyres) the tyres arent
accelerated by the force exerted by that tyre directly.  Perhaps we need to
use the acceleration of the car to get the combined force of the road.

My head hurts - I'll try to come up with something a little more intelligent
later on :P

Gregor where are you? :)

Ash

Hi Ash

How are you calculating and defining 'road reaction force'? I think this is
key. Also, I think this would be limited somehow (by the drivetrain) on
driven wheels/axles.

Al

The road reaction force is simply the force produced by the tyre in the
previous timestep (but in the opposite direction as its a reaction).

I am not sure this will work for "free rolling" wheels, hence I was
suggesting using the acceleration of the car to determine the force that was
used to accelerate the car in the previous timestep.  This force would need
to be "distributed" around the wheels somehow.

Has anyone any comments/suggestions?

I am sure I saw a post by Gregor Veble on this very subject not that long
ago.  perhaps a google groups search will yield something.  He isnt in today
or I would ask him.

Thanks for the help
Ash

Note that the torque at the wheel because of the engine then is:
T=T_engine*gearing

Then:
- Apply Fx to body
- Apply (F_engine-Fx) as rotational acceleration for the wheel
spinning. (note that T=F*r, so from Fx you can make torque by
Tx=Fx*wheelRadius and then you get T_engine-Tx, so T=I*w => w=angular
acceleration = (T_engine-T_x)/I_wheel).

Even if this would in the first few timesteps make the wheel stop,
that shouldn't matter, since then SR gets down to 0, making Fx move to
0 (check that; with low speed & Pacejka you may want to damp Fx down
to 0 as well). In that case, F_engine-Fx ~ F_engine, and the wheel
should start rotating. In bits, the body should accelerate, jittering
a bit around the wheel's origin at first.

Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art  : http://www.marketgraph.nl/gallery/

I think I may have misunderstood, but off the top of my head, for the case
of 0% power to a wheel, wouldn't the wheel rotate backwards. (Beacuse I'd
simply get -Fx)

Hang on, I'm getting a negative SR , so I the sim is probably generating a
negative Fx on this wheel. -Fx would make this positive, which means that
the wheel may start rolling forward. Hmmm :)

I'll give that a try...

Thanks

Al

from there on, jittering starts with the forces, but in average the
body will move forward, and the wheel will spin the right way (on
average!).
It might help to do one velocity-reversal hack here; if the wheel spin
velocity changes sign, reset it to 0 for that step. Although probably
you shouldn't need that with SAE950311.

Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art  : http://www.marketgraph.nl/gallery/

I've just tried your idea and the results seem to be about right

(I'm losing grip on undriven wheels at certain points, but this may be
something to do with relaxation length)

I shall be testing it this evening :)

Can I confirm that the Fx in (Fengine - Fx) is the force from the tyre/road,
and not the force from the car body?

Thanks again

Al

Or load.

Fx = Pacejka, which is the road reaction force. All forces are
couples, but this is the one that the road exerts on the tire, which
makes the car move forward. More Fx=more grip.

Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art  : http://www.marketgraph.nl/gallery/