[ODE] Car Sim Steering Causing Instability

[smythst]

OK, I've actually gotten that steering problem fixed, but I don't understand 
why. The way I was adding force to the car was by adding a torque to the two 
rear wheels which are each connected to the car chasis via a Hinge2 joint:

dBodyAddRelTorque(body[3],0,fx + rollingFriction,0);
dBodyAddRelTorque(body[4],0,fx + rollingFriction,0);

However I've only just noticed that test_buggy.cpp does this:

for(i = 0; i < 2; i++){

	dJointSetHinge2Param (joint[0],dParamVel2,fx +rollingFriction);
	dJointSetHinge2Param (joint[0],dParamFMax2,0.5);
}

So the later works for my sim. What I don't understand is why? What's wrong 
with adding a torque to the rear wheels? It's what an engine does in real 
life, and it worked for a straight line in my sim (and as I said earlier 
steering worked as long as you didn't turn more than 180 deg).

Anyone know the physics behind this?

Cheers.