[ODE] Freely rotating wheel?

[Diana Harsh]

Hi, 

New to ode here. 

I'm trying to simulate a vehicle which has two driven wheels (left and 
right) balanced by a third, freely rotating, dolly wheel to the rear. The 
two driven wheels seem to work fine using hinge-2 joints. However I can't 
seem to set the dolly wheel to rotate freely. The simulation looks like it 
is dragging the wheel and this causes the trajectory to waffle all over the 
place, eventually fliping the vehicle. 

For the dolly wheel I have used a hinge-2 joint. I have set the dParamHiStop 
and dParamLoStop to infinity (so there is no limit to their DOF on the two 
axis). I think the problem may be with the dParamFMax value. If I set it to 
0, it seems to definitely drag the wheel. If I set it to a large value, it 
works but continues to upset the trajectory. 

Any suggestions to make it rotate freely? Is this even my problem? 

Thanks in advance,
James