rec.autos.simulators

  WHY CAN'T IT JUST WORK??!!!!!

  Thanks, I feel a little better now.....
Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://www.racesimcentral.net/

Ash

|
|   WHY CAN'T IT JUST WORK??!!!!!
|
|
|   Thanks, I feel a little better now.....
| Todd Wasson
| ---
| Performance Simulations
| Drag Racing and Top Speed Prediction
| Software
| http://PerformanceSimulations.Com

-Gregor

I'm sorry, I feel your pain. Been there, done that, ate my damned T-shirt
out of frustration, but you're on your own on this one. It's so far out of
anything I've ever dealt with that I can't even comment.

Hey Doug, got your ears on? Poor Todd's about to explode and needs emergency
tire physics treatment.

KFG

Just because tires know what to do,
  doesn't mean that we know how they do it...

The tire modeler's version of:
  Just because you are paranoid,
    doesn't mean that they are not watching you.

I don't think that I can say much at this point - things are so
implementation dependent.

-- Doug

                Milliken Research Associates Inc.

|
| Just because tires know what to do,
|   doesn't mean that we know how they do it...
|
| The tire modeler's version of:
|   Just because you are paranoid,
|     doesn't mean that they are not watching you.
|
| I don't think that I can say much at this point - things are so
| implementation dependent.
|
| -- Doug

|                 Milliken Research Associates Inc.
|
|
|

|
| >




| > >
| > >   WHY CAN'T IT JUST WORK??!!!!!
| > >
| > >
| > >   Thanks, I feel a little better now.....
| > > Todd Wasson
| > > ---
| > > Performance Simulations
| > > Drag Racing and Top Speed Prediction
| > > Software
| > > http://PerformanceSimulations.Com
| >
| > Todd, dear Todd, and your thesis review board has been waiting patiently
in
| > the other room, but now they're starting to get itchy.
| >
| > I'm sorry, I feel your pain. Been there, done that, ate my damned
T-shirt
| > out of frustration, but you're on your own on this one. It's so far out
of
| > anything I've ever dealt with that I can't even comment.
| >
| > Hey Doug, got your ears on? Poor Todd's about to explode and needs
emergency
| > tire physics treatment.
| >
| > KFG
| >
| >
| >
| >
|

  Nope, I'm just trying to get the very, very, very simple basic equation at
the beginning of the SAE paper we just read to work without even including it
in the model yet!  The differential equations?   Good grief....  lol

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

What paper is this - # and title?

SAE 9503111
"Tire modeling for low-speed and high-speed calculations"

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

How are you implementing them? I use SAE950311 but have been a lot of
problems as well, to the point where I'm now doing collision detection
and AI to get my mind on 'simpler' things. ;-)

Strange to see how a slipratio can stay beyond 20 for some time after
some firm braking. And that with a relaxation length of 0.7m. :(

If only Genta had given me the units of the Pacejka longitudinal tire
stiffness, than I could do some searching...

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

That's the one. :)
Here's what I do. It's been modified here & there, now & then to check
for the ugly damping swinging at low speed. Better to start out at
some speed (both world velocity and spin speed), since the diff
equations work out better.

Hope it helps a bit. And don't forget to multiple Fx (road reaction
force) by sgn(u)! Funny stuff like that. In itself, it works pretty
ok, UNTIL you get at low speed, OFCOURSE! ;-)

/***********************
* Calculate Slip Angle *
***********************/
void RWheel::CalcSlipAngle()
// Based on the wheel world velocity and heading, calculate
// the angle from the wheel heading to the wheel's actual velocity
// Contrary to SAE, our Y-axis points up, and slipAngle>0 means
// in our case that the you're in a right turn (basically)
// 31-03-01: SAE950311 implementation (using relaxation lengths
// and a differential equation for the slip angle) for low-speed etc.
{
  DVector3 velWheelWC;

  if(!IsOnSurface())
  {
//qdbg("CalcSA; not on surface\n");
    // Not touching the surface, no need to do slip angle
    slipAngle=0;
    // Tire springs back to 0 instantly (should be slowly actually)
    tanSlipAngle=0;
    velWheelTC.SetToZero();
    velWheelCC.SetToZero();
    return;
  }

  // Get velocity of contact patch wrt the track (=world)
  DVector3 cpPos,*p;
  p=susp->GetPosition();
  cpPos.x=p->x;
  cpPos.y=p->y+susp->GetLength()+radius;
  cpPos.z=p->z;               // Kingpin effects are ignored
  car->GetBody()->CalcBodyVelForBodyPos(&cpPos,&velWheelCC);
  car->ConvertCarToWorldOrientation(&velWheelCC,&velWheelWC);
  ConvertCarToTireOrientation(&velWheelCC,&velWheelTC);
qdbg("velWheelTC.z=%f, rotZ=%f\n",velWheelTC.z,rotationV.x*radius);

  // Derive change in tan(slipAngle) using SAE950311
  // u=longitudinal velocity, v=lateral velocity
  rfloat u,b,v,delta;
  u=velWheelTC.z;
  v=velWheelTC.x;
  b=relaxationLengthLat;
  delta=(v/b)-(fabs(u)/b)*tanSlipAngle;

  // Integrate
  tanSlipAngle+=delta*RMGR->time->span;
  // Derive new slip angle from state variable 'tanSlipAngle'
  slipAngle=atanf(tanSlipAngle);

/***********************
* Calculate Slip Ratio *
***********************/
void RWheel::CalcSlipRatio(DVector3 *velWheelTC)
// Calculate current slip ratio.
// Assumes that CalcSlipAngle() has been called (for velWheel?C to be
valid)
// 31-03-01: SAE950311 implementation (using relaxation lengths
// and a differential equation for the slip ratio) for low-speed etc.
{
  rfloat vGround,vFreeRolling;

//velWheelTC->DbgPrint("velWheelTC in CalcSR");
  if(!IsOnSurface())
  {
//qdbg("CalcSR; not on surface\n");
    // Not touching the surface, no slip ratio
    slipRatio=0;
    // Tire vibrations stop
    differentialSlipRatio=0;
    return;
  }

  // SAE950311 algorithm
  rfloat u,delta,B;
  // Calculate new u (actual velocity of wheel)
  u=velWheelTC->z;
  B=relaxationLengthLong;
  if((lastU<0&&u>=0)||(lastU>=0&&u<0))
  {
    // 'u' changed sign, reverse slip
    differentialSlipRatio=-differentialSlipRatio;
  }
  lastU=u;
  // Detect sign of 'u' (real velocity of wheel)
  if(u<0)signU=-1.0f; else signU=1.0f;
  // Eq. 26

//delta=(fabs(u)-rotationV.x*radius*signU)/B-(fabs(u)/B)*differentialSlipRatio;
  if(u<0)
  {
    // Eq. 25
    delta=(-u+rotationV.x*radius)/B+(u/B)*differentialSlipRatio;
  } else
  {
    // Eq. 20
    delta=(u-rotationV.x*radius)/B-(u/B)*differentialSlipRatio;
  }
  // Integrate
  differentialSlipRatio+=delta*RMGR->time->span;
  if(differentialSlipRatio>2)differentialSlipRatio=2;
  else if(differentialSlipRatio<-100)differentialSlipRatio=-100;
  // Set slip ratio for the rest of the sim
  slipRatio=differentialSlipRatio;

 qdbg("CSR; u=%f, R=%.2f, w=%f, B=%f, delta=%f, DSR=%f, SR=%f\n",
 u,radius,rotationV.x,B,delta,differentialSlipRatio,slipRatio);

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

-Gregor

I know we discussed this before, but anyway. Does the slip ratio not
relax soon after you release the brakes? I believe that in that case the
force on the tires should rotate them and return the slip ratios to
normal values very rapidly.

Also, does this happen only when the car ends up sliding sideways, or
also when braking in a straight line?

-Gregor

Certainly the Abstract makes the paper seem like "the right stuff".

Because it's from Iowa State, there is a good chance that the work
was connected with the National Advanced Driver Simulator (NADS),
which is located nearby...a www search should turn up info on this
huge government funded project (last I heard it was over US$40M).

<political comment>
Personally, I think that fraction of my taxes (maybe $0.50??) could
have been spent on something better than the NADS...
</political comment>

-- Doug