rec.autos.simulators

i guess that something is worng about my calculation of the lateral, or
the way of combing these with the longitudinal forces. i basically
calculate lateral forces as the dot-product of the wheels velocity, and
the vector orthogonal to the wheel. i multiply this with a constant
factor and mass carried by the wheel.

then i sum my lateral force and the longitudinal force, and clamp the
result to the maximal force the wheel can transfer to the road.

somewhere on the list i also read about another way of combining these
forces, where the longitudinal is clamped to the maximal force, and
then the lateral force is applied as 'what is left in the traction
budget' using the pythagorean theorem. i also tried this, and it would
greatly raise the impact of the longitudinal force over the lateral
force, letting me do much tighter donuts, and making the car react much
more to the throttle in curves, and thus, unfortunatly, increasing my
problem of unhandable oversteering.

does anyone see something obviously wrong with my approach? or is there
value i might not have considered yet?

thanks for any advice..

jonas

ps. is there a good reference (preferably online) for this sort of
questions?

First of all you may want to look into Pacejka magic formulae for
calculating the tyre forces. This seems to be the industry standard method
for simulating vehicles. This also has the advantage that you don't have to
cap the forces to the friction circle, since this is part of the model (if
you used the combined version of the formula). The only online description
I've found of this is Brian Beckman's series
http://www.esbconsult.com.au/ogden/locost/phors/phors.htm

I understand that G. Genta's book and Race Car Vehicle Dynamics by Milkenen
and Milkenen also cover this. (I'm sure someone on this group can give you
complete references for these).

As for your current problem, this is one I've encountered myself and I'm
still ironing out the problems. It seems to be caused by the relative forces
applied in longitudinal and lateral directions on a tyre. If you apply too
much longitudinal force, you'll have less grip to play around with at the
rear causing oversteer (assuming rear wheel drive). Try
playing around with the force you're applying in this direction and see how
that affects things.

Good luck,

Mike

no, this is not the problem in my case (i have implemented a kind of
traction control to avoid this) - it also happens with FWD cars or when
i'm giving zero throttle.

jonas

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/

"... upper and lower bounds for maximum grip... "

-Gregor

Brian's method is the simplest method that satisfies some basic
requirements (linearity at small slips, max force reached for both
components at combined slip simultaneously, upper and lower bounds for
maximum slip within those for independent ones etc.), but that's just
one way of doing it, and real life data may vary from this a bit. But if
actual combined slip data is unknown, Brian's method is as
good as any, and probably the simplest possible one at that.

Genta's method probably takes more combined slip info into account, and
if you have that info it's probably a good idea to use it. If you don't
however, there's no reason to switch to it.

Cheers,

-Gregor

It's not from the Genta book, it's from Pacejka and eh, Bisselink or
something. Added some r1-r6 constants. I guess the list of parameters
is endless. :)

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/

as a late professor of ours used to say, with enough fitting parameters
you can even describe an elephant :).

Cheers,

-Gregor

-Gregor

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/

There may have been some good responses already but I've not seen any (my
news server is not getting all the posts)...

Anyway, if an oversteer car is driven above the "critical speed", then it
is "fixed-control un-stable".  In this case, there is no such thing as
"stable oversteer"!  In other words, it will take some control action
(wiggling of the steering wheel) to get the car to follow a set path.  

The more oversteer is it, the quicker will be the divergence(spinout) -- so
you may have just chosen a setup that is too extreme for you to control
with the limited driver feedback of a sim (compared to the
seat-of-the-pants feel in a real car).

<shameless plug>
This is covered in great detail in Chap 5 of "Race Car Vehicle Dynamics",
order directly from the bookstore at <http://www.sae.org>.
</shameless plug>

--Doug Milliken
  Milliken Research Associates Inc.