rec.autos.simulators

Hi ,

I'm currently solving the problem of the car moving when it's stopped
because the slip angle is not good when the wheel are not rolling.      

My first intuition was to use an usual friction force, F = . Load.
Because the forces involved in the tires are friction forces.

Fm = maximal force
d = dynamic coef
n = static coef
L = load
v =  linear wheel speed

so the equation is

If v == 0
        Fm = - Sign(v) * s * Load
else
        Fm = -Sign(v) * d * Load

The difficulty is that you compute a MAXIMUM force that CAN'T reverse
motion.

And because of how i made the dynamic engine and collision detection
and resolution , i cant compute the real force needed. I would need to
resolve an equation in realtime for all the contacts ( 4 wheels in the
easiest case ). However it's for a realtime game on PS2, so CPU is
not infinite here ;o)
I try with something proportional to wheel load anc wheel acceleration
but it didn't seem to be good :o)

Another solution should be to compute the amout of force to that would
make the speed null but it's not right with 4 wheel acting on the car
because each wheel act on the other wheel by the car chassis.

So i heard about wheel spring to handle a car stoped on a banked
surface. We resolve the problem for the longitudinal.
But i still have problem with lateral force.

So how i'm searching for archive about wheel spring...

Anyway, this is how i understood it  ;o) :

At low speed, i need to simulate a wheel as spring.
Spring forces are F = k .dl.

So

 Can i use lateral wheel speed as delta length or do i need to store a
wheel information when entering in low speed behaviour to compute
the delta length ? What information i need ? wheel position doesn't
seems good ;o)

any help will ne apreciate for wheel spring or to compute effeciently
the frictional force

thanks in advance

Sebastien TIXIER - Game Developer
Dynamics and Car Physics
http://www.racesimcentral.net/
GPLRank Normal:-44.24   Monster:-124.44

Hi Sebastien,

check the other ongoing thread (Pacejka probs) to
hear about the possible solution. The best solution that I have seen so
far is the relaxation length approach, I believe Ruud put a link to the
proper SAE paper in a post in the other thread.

But to give you a general idea what it is about; as you say, the problem
with slip angle/ratio is that you need to divide by the value of
longitudinal velocity when calculating them. The way around this problem
is that you don't consider slip angle and ratio as direct functions of the
wheel velocities, but as state variables that change with time. Let me
just put the eqs. here:

d(Slip_lat)/dt = 1/L * (V_lat - |V| Slip_lat)

d(Slip_long)/dt= 1/L * (V_long - Omega*R - |V| Slip_long)

where L is the relaxation length (of the order of a fraction of wheel
radius R), V_lat/V_long corresponding velocity components, |V| the
absolute value of the velocity with which you would normally divide when
calculating the slip angle/ratio (depends a bit on your definition), and
Slip_lat and Slip_long the (function of) slip angle and slip ratio,
respectively, again depending on what definition you use.

To see why these equations are good, consider the stationary situation
where the left side is equal to 0. Then you will get

Slip_lat = V_lat/|V|

Slip_long= (V_long - Omega*R)/|V|

which are just the usual definitions for the slip angle/ratio or functions
of those. From these equations you may also see how you need to modify the
original equations should your definitions of slip angle/ratio or the
functions of those that you might use be different.

When the values do change with time you can see that the dynamic equations
do not have any divisions (well, apart from 1/L ;) ) in them and are thus
well behaved at all speeds. It's also possible to show that they allow the
car to stop well on hills as they behave sort of like a spring at small
velocities. It's best to try that and see.

Oh, the forces are calculated from the dynamic slip angle/ratio just as
before.

I hope this helps (and maybe someone else as well :) ).

Greetings,

-Gregor

P.S.: Got a PS2 last weekend. When can I expect to see the final results
:)?

On Tue, 9 Apr 2002 11:06:29 +0200, Gregor Veble

[SNIP]
Very interesting :o)

Meanwhile, i correct the problem by adding a force
F = -k * lat_speed * Load , at very low speed ( < .5 ms-1 ).
It works good, with k = 5.0 for our range of application.
As we have to finish the game in some* weeks, i dont have time to
try more elegant solution :o)
Anyway i'll definitvly implement this in the new car physics for
the next game, i also have to implement the Jacobian matrices
and Lagrange stuff for the resting contacts , real friction forces ,
rigd body joint constraint for REAL suspensions system ( Mc Pherson
suspensions, etc... )

V-Rally 3, on PS2, out on June 2002 in Europe, dont know for US.

* : i cant tell when we finish the game

www.vrally.com

Believe me, there a real Car physics on this game , however we have to
add driving help because you just cant drive a car with a PS2 pad :o(

BTW,have you try GT3 with ALL helps off ? you need to explorer options
a bit but, it's far better with all helps off.

regards,

Sebastien TIXIER - Game Developer
Dynamics and Car Physics
http://www.eden-studios.fr
GPLRank Normal:-44.24   Monster:-124.44

Hi Sebastien!

This solution is practically the same in essence as the one Dave mentions
he used for Viper. I believe he says he limits the absolute
value of velocity with which you divide to get the slip
angle/ratio. Mathematically what you propose is quite equivalent, only
the parameters are given in a different way. The only artifact is that the
car will slide slightly when stopped on a hill, but I think you can live
with that (not that much stopping in rallying ;) ).

Sounds like an amazing, do keep us updated!

I have the Logitech FF GT wheel, can you add a last minute cheat in there
to enable us to disable this help? (pretty please :) ?)

I believe I turned off everything I could, and the result is pretty
amazing. They (deliberately) left the inertias of the cars pretty high to
make them easier to handle, but the physics engine itself seems to be
rather good, certainly it does what is asked of it as it doesn't ever
spoil the illusion. It's indeed a very fine racing game and a driving sim
(did I get definitions correctly ;)?)

Good luck with the game, I'll be sure to get it when it's out!

-Gregor

On Tue, 9 Apr 2002 14:29:02 +0200, Gregor Veble

Well, not so much , because in fact in my implementation i made
the coef factor dependant with the ground banking ;o)
I also plan to add a force to counter act the amout of force the
gravity add that makes the car move perpendicular to ground normal.
( this sentences sounds incomprehesible to my french hear :o( )

i will ! my evening readings are mathematicals for the moment.
I have to revise all my algebrical studies :o)

The cheat already exists .. oups.. i whish an infogrammes marketing
guys is not listening at the moment ;o)

Plus some more cheats :o)  ( hehehe ! )

I'll tell when the game will be released.

BTW, we manage FF in the game. And officialy support the blue PS2
logiteck FF.

Some of us went to the GDC, and the presentation of GT3 car physics
where pretty amazing !!! I try to get a summuray and post it here
if there's no confidential stuff.
Too bad the colision engine and collision responses sucks , IMO.

Thanks !

Sebastien TIXIER - Game Developer
Dynamics and Car Physics
http://www.eden-studios.fr
GPLRank Normal:-44.24   Monster:-124.44

On Tue, 9 Apr 2002 11:06:29 +0200, Gregor Veble

...

Are you sure it shouldn't be:
d(tan(Slip_lat))/dt = 1/L * (V_lat - |V| tan(Slip_lat)) ?

This is what is used in SAE950311 I think (a paper highly recommended
to just buy from sae.org).

BTW L_lat and L_long differ; L_long is estimated as about 1/10th of
L_lat (in the paper anyway ;-) ).

For a real implementation, I'd get the paper; there are some quirks
when the wheel velocity's sign changes (those pesky sign reversals are
a source of trouble in every spot in a sim).

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

Hi Ruud!

It doesn't really matter (I didn't say what Slip_lat was anyway, did I
;) ), as the core behaviour doesn't change. I personally don't like to use
tan anyway as it tends to somewhat strange behaviour at angles close to 90
degrees, and also it is much harder to reproduce proper sliding behaviour
when wheels are locked and sideways with it. Still, engineers love it and
most data is given in those units. But since sin x is approx tan x for
small x where stuff is most important anyway, it's really irrelevant what
one uses.

Sure that's true, but that's the material for lesson 2 :).

Ah, the sign of the velocity; of course that is terrible, even more so
since it's also very much dependent on how you define your force vs. slip
curves. Better to keep it out of the discussion for now (let's keep the
lion sleeping just a tad longer ;) )

-Gregor