Re: Patch for wheel spin velocity traction

David Savinkoff <[email protected]> Mon, 2 Nov 2015 12:39:07 -0700 (MST)
Newsgroups gmane.games.torcs.general
Message-ID <1482017308.8633430.1446493147409.JavaMail.zimbra@mailid.telus.net> 
----- Bernhard Wymann <[email protected]> wrote:
> Hi David
> 
> Thank you, I will have a look into this for a later version (1.4.0), I 
> plan as well to study the acual SAE papers and fetching a recent edition 
> of Pacejka's book.
> 
> Kind regards
> 
> Bernhard
> 
> 
> On 06/16/2015 01:16 AM, David Savinkoff wrote:
> > Hi,
> > . . . . . . .

Hi Bernhard,

After some time I returned to the TORCS traction issue and spent
some time and thought on the matter.

Here is a new patch that supersedes the previous, and is to be
applied to the torcs-1.3.6 source code. Please see.

The traction formula is actually very straightforward.
Excellent performance and Nobody else has it either. Ha HA

Sincerely,
David Savinkoff

ps.

To others that compile TORCS:
You will like this one.

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wheel.cpp.diff (text/x-patch, 2.7 KB) 
--- torcs-1.3.6/src/modules/simu/simuv2/wheel.cpp	2013-08-29 06:01:32.000000000 -0700
+++ torcs-1.3.6/src/modules/simu/simuv2/wheel.cpp	2015-11-01 17:19:45.407804960 -0800
@@ -174,12 +174,12 @@
 {
 	tWheel *wheel = &(car->wheel[index]);
 	tdble axleFz = wheel->axleFz;
-	tdble vt, v, v2, wrl; // wheel related velocity
+	tdble vt, vn, v, v2, slip_magnitude, wrl; // wheel related velocity
 	tdble Fn, Ft;
 	tdble waz;
 	tdble CosA, SinA;
 	tdble s, sa, sx, sy; // slip vector
-	tdble stmp, F, Bx;
+	tdble F, Bx;
 	tdble mu;
 	wheel->state = 0;
 
@@ -215,6 +215,7 @@
 
 	// tangent velocity.
 	vt = wheel->bodyVel.x * CosA + wheel->bodyVel.y * SinA;
+	vn = wheel->bodyVel.y * CosA - wheel->bodyVel.x * SinA;
 	v2 = wheel->bodyVel.x * wheel->bodyVel.x + wheel->bodyVel.y * wheel->bodyVel.y;
 	v = sqrt(v2);
 
@@ -227,34 +228,31 @@
 	NORM_PI_PI(sa);
 
 	wrl = wheel->spinVel * wheel->radius;
+
 	if ((wheel->state & SIM_WH_ONAIR) != 0) {
 		sx = sy = 0.0f;
-	} else if (v < 0.000001f) {
-		sx = wrl;
-		sy = 0.0f;
 	} else {
-		sx = (vt - wrl) / fabs(vt);
-		sy = sin(sa);
+		// Normalize the friction circle to get normalized vector components (sx, sy) while
+		// braking (vt), accelerating (wrl), or wheel counter spin (vt - wrl) for any angle.
+		slip_magnitude = 0.000001f + sqrt( MAX(MAX((vn*vn+vt*vt), (vn*vn+wrl*wrl)), (vn*vn+(vt - wrl)*(vt - wrl))) );
+		sx = (vt - wrl) / slip_magnitude;
+		sy = vn / slip_magnitude;
 	}
 
-	Ft = 0.0f;
-	Fn = 0.0f;
 	s = sqrt(sx*sx+sy*sy);
 
 	{
 		// calculate _skid and _reaction for sound.
-		if (v < 2.0f) {
+		if ((v < 2.0f) || (zforce < 0.0002f)) {
 			car->carElt->_skid[index] = 0.0f;
 		} else {
-			car->carElt->_skid[index] =  MIN(1.0f, (s*zforce*0.0002f));
+			car->carElt->_skid[index] =  s;
 		}
 	}
 
-	stmp = MIN(s, 1.5f);
-	
 	// MAGIC FORMULA
-	Bx = wheel->mfB * stmp;
-	F = sin(wheel->mfC * atan(Bx * (1.0f - wheel->mfE) + wheel->mfE * atan(Bx))) * (1.0f + stmp * simSkidFactor[car->carElt->_skillLevel]);
+	Bx = wheel->mfB * s;
+	F = sin(wheel->mfC * atan(Bx * (1.0f - wheel->mfE) + wheel->mfE * atan(Bx))) * (1.0f + s * simSkidFactor[car->carElt->_skillLevel]);
 
 	// load sensitivity
 	mu = wheel->mu * (wheel->lfMin + (wheel->lfMax - wheel->lfMin) * exp(wheel->lfK * zforce / wheel->opLoad));
@@ -264,11 +262,10 @@
 	wheel->rollRes = zforce * wheel->trkPos.seg->surface->kRollRes;
     car->carElt->priv.wheel[index].rollRes = wheel->rollRes;
 
-	if (s > 0.000001f) {
-		// wheel axis based
-		Ft -= F * sx / s;
-		Fn -= F * sy / s;
-	}
+	// wheel axis based
+	s = -0.000001f - s; // make Ft, Fn and s negative with stable divide by zero avoidance.
+	Ft = F * sx/s;
+	Fn = F * sy/s;
 
 	RELAXATION2(Fn, wheel->preFn, 50.0f);
 	RELAXATION2(Ft, wheel->preFt, 50.0f);