rec.autos.simulators

If slip ratio is defined as:

     Va / Vf - 1

where Va = angular velocity of wheel under acceleration and Vf = angular
velocity of free rolling wheel, what is the value when Vf=0? That is,
in the first timestep that the wheel receives torque from the drivetrain,
what is the slip ratio considered to be? I have s feeling the answer to
this is obvious and it's my perception of the problem that's wrong.

Second, how do you pronounce "Pacejka"? God forbid it ever should come
up in conversation... ;)

Thanks for your help, including all of the stuff in the archives.
Great, great stuff.

Jason
379

  I think Pacejka is pronounced PasEEjka, but I don't know for sure.

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

PaJAYka is close to the way we were taught ;-)

Jonny

  Ahhh..  I stand corrected.   Thanks :0)  

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

Try getting some info on document SAE950311. A method by Barnard IIRC,
which is mentioned in other papers as well. In short, an extra
indirection is used to indicate slipratio, and it only changes when
the wheel starts rotating. It's delta is based on the relaxation
length of the tire. It also has some low-speed trouble, but less so
than applying a direct SR formula like above.

It is mentioned briefly in the thesis of Erik Lowndes. Links on my
page (at the 'links' page ;-)) to several of these. I think some
papers of Michael Sayers also talk about it. But take care, it's only
a couple of lines mostly. :)

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

Yes I understand that. But it seems that there must be some (short) period
of time where the axle is starting to spin but the wheel has no forward
velocity -- the *** "winding up" as it were -- until the tire starts
to slip and a forward force is generated. I guess I'm confused about how
the slip ratio gets from zero to not-zero.

I see from the archives that the Genta book seems to be the introduction
to this subject, so I have placed my order. I see that Ruud has also
mentioned a couple of SAE papers that I will try to check out.

Jason

I would suggest starting with #2. I think #3, despite it's easy
understanding, is problematic because I think the spring are too stiff
and you'll have trouble with the integrations. After you get #2 to
work, you can add #1 without too much hassle (extend it). There's some
pretty damping trouble you may have to go through there.

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

  Whoops, my AOL newsreader didn't show your reply here, sorry about that.
Like Ruud said, it's easiest to ignore this and just let the wheel accelerate
at the first time step.  Physically, what's really happening is, the wheel hub
rotates a little, causing the tire to wind up slightly before the car moves.
Right then, you've got a slip ratio, even if the tire wasn't really slipping
because there is a velocity, and theoretical free rolling velocity is still 0
(infinite slip ratio, or undefined slip ratio).  However, some portion of the
rear of the contact patch slips during this wind up (the load approaches 0 at
the rear of the contact patch, so any wind up in the tire at all will cause
some slippage).  With any slippage at all, the hub is rotating at a different
speed than theoretical free rolling, so there's suddenly a slip ratio as soon
as a torque is applied to the wheel.  

 In the case where the car is stationary, even without any slippage though, the
wheel hub gains some rotational velocity before the car moves, so there's still
a slip ratio the instant the wheel gets a torque applied to it.  It would be
infinite of course, since you're dividing by the car's velocity of 0.  In
reality, the hub can rotate a small amount while only generating a small force,
so slip ratio can be kind of a wierd concept when the car is stationary.  I
switch to a different model at low speed that treats the tire and hub as a
torsion spring so the problem doesn't surface, but method #1 that Ruud
mentioned is probably better:

  Although I haven't seen it working yet though.

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

...

With all the cars out there for Racer, I think the problem with
switching to a spring model is smoothly going from one method to the
other. Ofcourse you can calculate both and mix the results of both (so
it gradually becomes the slipratio calculation once the car gets some
velocity). Mind you, I think some commercial carsims do this, or at
least switch to another model at low speed.

In the end, low speed is not of the greatest importance in a *racing*
sim, where you're generally topping 200 km/h.

I'm still going to retry this one with the damping force that gets
negated at every timestep (not mentioned in the SAE950311 document
itself, but it IS in the Lowndes thesis).

It worked reasonably in my sim, EXCEPT at low speeds, lol. But as
Gregor mentioned, above a certain speed you can ignore relaxation
length anyway as it has a negligable influence then (he even stated a
formula for the exact velocity which I casually forgot ofcourse).

I think either the SA or SR approach is still used even in my sim.
Hehe, I don't even know anymore myself.

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

  Yes, that is a problem :0)  I need to really tweak my torsion "spring and
damper" rates to keep it smooth, and even then sometimes there's a noticeable
jerk and odd behavior... My current open differential model doesn't like it
when the car is moving slowly.  I switch to this at the lowest possible speed,
but would rather use the method you were after.  However, the torsion thing
does let me stop on hills though :-)

  > Ofcourse you can calculate both and mix the results of both (so

  Yikes, I don't know about getting my system to do that as it is now!  :0)
Very tough.  Wouldn't even know where to start.

  >I'm still going to retry this one with the damping force that gets

  Please let me know how it goes.

  Yeah, I remember that discussion, and forgot the formula too :0)

  >I think either the SA or SR approach is still used even in my sim.

  lol.  I know how that goes.
Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

The Barnard method will do that too; it will keep the SR balanced so
that the forces cancel eachother out, IIRC (a big IIRC! ;-)).

Hm, if both methods are in separate functions, it would be like:
NOW:
- if v<thresHold then CalcForcesSpringModel()
  else CalcForcesSR()
FUTURE:
- CalcForcesSpringModel()
  SaveForces(forceInfoStructure)
  CalcForcesSR()
  if v<thresHold then scale=v/thresHold
  else scale=1
  AverageForcesWith(forceInfoStructure,scale)

But I know; my system is as uncontrolled as well so I can't just plug
in an RK4 integrator or undo a physics step (as Baraff tends to do
when penetration occurs; btw I like the FastCar approach where
penetrations are resolved not by recalculating all forces with a
smaller timestep, but slightly translating all penetrating objects
along the collision normal so they don't penetrate anymore; not too
accurate but with small translations I don't think anyone'd see the
difference).

I will. It's not on the planning for anytime soon though.

Lol. I checked, and it turns out I took out the slipAngle relaxation
stuff (SAE950311), but I do use it longitudinally, SR. Without the
damping though, hm?! (damping is #ifdef'd out) Amazing.
So the SR route works, somehow. The formula to discard relaxation
length is:

  if u*timeStep/B > 0.5 then SR=<direct calculation>
  else <SAE950311 differential stuff>

Where u is the wheel spin velocity, and B is the longitudinal
relaxation length.

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

Guys, guys, keep those notes handy at all times :).

By the wheel spin velocity, you mean the translational velocity in the
longitudinal direction, right :). Indeed, the criterion above is what is
needed for the approach to work, otherwise you get an instability in
your calcs (in principle you can let the inequality to go to > 1, but
it's better to be on the safe side).

-Gregor

When is your book of 'All the things you always wanted to remember
about carsims but we're unable to cope with?' out? ;-)

Ah yes, that's what I meant! I saw 'wheelVel.x' and slammed down an
inaccurately converted phrase (I was so happy I had a word for the
wheel's rotational velocity along the X axis, well, Z axis in OpenGL,
X in SAE, so confusing). Sorry about raising some confusion there
perhaps. (is anybody still listening but us and Heisenberg to check
whether his compensators get invented?)

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

I've tried both of these techniques in my sim. The time reversal approach is
very accurate and looks correct, but can be quite expensive if you're
colliding with a dense patch of polygons, so I eventuually discarded this
one. I'm currently using the second approach, which is simple to implement
and not too expensive. However it's not without its problems. The main one
is that there's no guarentee that by pushing an object along the collision
normal, you're not pushing it inside other polygons. The result of this is
that you can either get an object still penetrating after the timestep, or
blipping between positions on alternate timesteps. You can avoid this by
trying up to (eg) 5 movements, and if it's still penetrating after this,
move it back to it's position at the beginning of the timestep, but this can
result in "sticky" cars. It really depends on the nature of the geometry
though, and as you pointed out, the size of your timestep.

I'm currently looking into an approach which is about halfway between these
two. It's very similar to the fastcar approach, except that I move an object
out of the geometry along the direction it came in (ie if a point is
penetrating, get the vector from its old position to its new position, and
move it out along this line). This won't take it back to the exact position
it entered (since it may be moving in an arc), but it should be pretty
accurate. You can then estimate the portion of the timestep elapsed until
collision (using the the ratio of the length of the vector just calculated
and its corrected position), give it a collision impulse, and move it along
its new path. Again there will be potential sticky problems with this, but
I'll see how it pans out.

Mike.

  Yeah, the one paper did that, I remember.  That's why I'd rather use it
instead of the torsion/ring type low speed model (once you figure it out, of
course ;0))

  >>  Yikes, I don't know about getting my system to do that as it is now!  :0)

  Ah, good idea :0)  I do the NOW part, but hadn't though of scaling and
averaging things that way.  Will give it a try sometime and let you know how it
works, thanks.

  You mentioned Baraff's stuff before.  I've got the papers but don't want to
look right now so I'll just ask you :0)  Does he undo the whole time step and
then keep readjusting it and recalculating the result until it's close enough?

  The FastCar approach sounds good too.  I've done absolutely 0 collision
detection yet, so...  Will just absorb for now :-)  I know I can't see any
problems with FastCar's collision detection.

  Will keep that in mind.  I need to get the diff code up to par, then I think
it's time to tackle the tire model, finally :0)

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com