rec.autos.simulators

I've finally managed to get my weight transfer fixed for anti-pitch
and anti-roll (or high roll centers) I think. Hehe, yes a basic
requirement actually, but well. :)

And ofcourse, a new problem crept in; the car could become unstable,
it jitters left/right quite ***ly at times, as if something like
this happens (at low speed, at higher speeds things are better):
- a small lateral force spike occurs
- the car gets a torque, weight transfer and weight shifts to the
other side
- the a larger lateral force is generated in return
- larger torque to the other side

In other words, a kind of blowup which doesn't settle down by itself.

I didn't use the SAE950311 lateral relaxation length approach (since I
had trouble with that), but to find the problem I reintroduced it.

Small summary of lateral relaxation length to fight slipangle
jittering at low speed: instead of calculating slipangle (SA) directly
from wheel velocities, you make it a state variable. You vary it
according to the wheel spinning velocity:

tan(SA)dt - u/b*tan(SA) = v/b

where u=wheel longitudinal speed, v=lateral speed, b=relaxation length
(say 0.91).

Ok, so I tested it again by throwing the car sideways. The car stops,
then swings BACK again! Like a spring. Here's what I think happens:
- v is large, the car goes laterally with great speed
- u is 0, it doesn't move forward
- from the above formula, it reduces to tan(SA)dt=v/b. This means
tan(SA) will constantly go up. This is ok, since SA then goes
progressively to 180 degrees (as tan(SA) approaches infinite).
- Fy (lateral tire forces) are generated to stop the car going
sideways, assume Fy>0 for this direction.
- The car stops., SA is still 180 and forces are still generated, so
the car starts going the other way.
- v begins small, so tan(SA) doesn't go down that quickly at the
start, and Fy stays well >0.
- only as the car builds speed into the other direction, v becomes
higher and gains the 'power' to reduce tan(SA). In the mean time, the
car has gained seriously notable speed. In fact, if the car was thrown
sideways for 5 meters, it will 'bounce' back just about the same 5
meters.

I think Petri once, a long time ago, suggested capping tan(SA). I
believe that was 20 or something which worked for him. But given the
above, I fail to see how tan(SA) could become 0 quickly so that SA
becomes 0, and the lateral forces cease.
In other words, I don't see how the car would be able to stop without
bouncing back more than just a little bit (which is to be expected
with this differential SA approach).

A lowspeed hack like reducing tan(SA) more quickly if u/v are both
close to 0 seems like a paradox, since SAE950311 was designed to work
for low-speed situations. ;-)

Anybody with any experiences with this?

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

Can't help with your question but just wondering - did you have camber
enabled whilst carrying out these tests and if so how good is the pacejka
you're using?

Hi Ruud,

I think that Alex Smith had this exact same problem (the spring effect).  I
think he got it resolved in the end.  Mine doesnt seem to have the same
effect, or perhaps it just isnt noticable, or perhaps my relax lengths are
completely wrong. You have made me paranoid now :D

Have a quick slide in my sim and see if you see the same effect, if not, I
may be doing something right for a change and i'll have another look at the
code and see if I can help

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

...

BTW Gregor Veble once stated (for longitudinal relaxation lengths)
that you could skip the relaxation (or perhaps SHOULD) when:

  u*dt/B > 0.5

(where B=longitudinal relaxation length, dt=timestep).
I don't know the reasoning behind this, but would something like this
need to be applied to the lateral direction as well, perhaps?

Thanks,

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

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

Please, if you could. I have some debug variables so that I can ignore
all user input (for repeatability), and have initial car velocity
variables so I can throw the car out laterally quite precisely.

If it does work at your place, can you figure out what tan(SA) is
doing and why it returns to 0 so fast when the car gets to a stop.

To me, it sounds logical to say that whatever v/b ADDS to tan(SA) it
must also take away (subtract) later on when the car stops. In other
words, this takes as much time, hence the car wants to go back
completely to its original position.

Perhaps you do some more low-speed checks to reduce Fx/Fy when
standing still? (that still doesn't explain why SA would return back
to 0).

Thanks! This is an ancient problem here for some time now, and I must
get it fixed. Notice I left out the damping near lowspeed currently;
are you using that? (if so, where do you get the cornering stiffness
from?).

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

I was using 0.1 for both lateral and longitudnal.  I just tried upping the
longitudnal to 0.8 and it seemed to work well, with maybe just a little
spring action.  I remember the paper (or maybe gregor) suggesting that the
lateral should be around 10 times lower that the longitudnal.

I'll try and dig the code that I am using out tonight, although there is a
bit of a family emergency in progress at the minute so I might not have a
chance :(

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

Hm, I thought it was the other way round (in the paper); longitudinal
being 10x stiffer. Since laterally the tire walls are much less stiff
than around the circumference.

Ok thanks, good luck with the emergency; hope it isn't too serious.
If you (at some point) mail me a source code extract (if you want to
disclose it) that might be helpful enough as well.

Thanks,

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

Just tried it the other way round.....horrible springing motion.  It
eventually comes to a stop, but *** indeed.

I have the same problem :/.

Gregor is back from holiday, hopefully he can shed some light!

All the best,
Ash

Hi Ruud, that was one of the things I tried... a long time ago, indeed! I've
been concentrating more on eye candy lately - yes I know, shame on me :-) In
my defense, I *have* also implemented double a-arm and macpherson suspension
geometries, viscous limited slip diffs (front, rear and center), a pretty
good autoclutch, and some other stuff. :-)

Quite frankly, I don't even remember how anymore, but I got it working
without the capping - it's exactly like in the SAE paper now. There's a
little bit of wibbly-wobbly behavior if you're going at a very low speed and
make sudden drastic steering movements, but not that much more than would be
expected given that the tires are supposed to be springy (it looks like a
few centimeters movement, part of which is of course body roll too). It
could probably use a little bit of damping, but I'm not getting 5-meter
rebounds from sliding for sure!

One thing I did do was switch to treating fully locked, non-rolling tires as
springy contact patches that use a straightforward Coulomb friction model.
Maybe that would help you as well, at least in some situations, if you're
not doing so already? It also ensures you can't steer the car with fully
locked wheels. Intuitively, it would also seem to match reality closer than
using Pacejka in that situation.

Petri Blomqvist

Um, disregard my last post, Ruud. :-( I did a little testing and revisited
my code, and looks like I hadn't quite made it work after all. I tested full
sideways slides without applying any brakes, and I do occasionally get a
rebound. It's not quite 5 meters - maybe 1 meter or so max - and it does
dampen pretty quickly, but obviously even that is totally unacceptable. From
looking at the code, I have at some point also tried a hack of using a
minimum of 7.5 m/s for the forward velocity of the tire in the equation.
That eliminates the huge lurches and wobbles at low speeds, but has the
unfortunate side effect of making  the car crawl sideways on banked surfaces
at those same speeds.

Back to ye olde drawing board...

Petri Blomqvist

There might be something fundamentally wrong with the formula when
u~0. At that point, tan(SA)dt=v/b, so no way this is going to work.
With u well above 0, I'd bet this model works far better.
Where's my pencil? ;-)

I don't do that, but I think the friction circle combination method
takes care of most of that. Steering with locked wheels doesn't work
in any case. :)

Thanks for responding, and perhaps someone knows a solution/hack to
get around the sideways problem...

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

One possibility might be to use this model even when the wheel is rolling
slowly... will have to try that. Naturally, it's very likely to create some
other problems. :-)

Petri Blomqvist

Probably. ;-)

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