[ODE] Different Friction between Ground/Wheels when using Joint Motors or dBodyAddTorque?

[Michael.Rauh at Dornier.eads.net]

Hi, I have a comprehension problem requarding Torque and Friction and one
considering ODE ;) Hopefully, we can solve both at once?

I have a 4-wheeled car with spheres (mass m=10kg, radius r=0.5m) as wheels
connected to the chassis with Hinge-2 joints. The chassis has very small
mass (< 1kg) in respect to the wheels so we can neglect it for some
calculations.

Beneath the car a single sphere is lying on the ground, having the same
mass and dimensions as one of the wheels (could be the spare wheel ;)

surface.mu is set to 1.0, gravity is -9.8.

Now I accelerate the car by setting the joint motors on all 4 wheels to
dParamFMax2 = some torque T. I apply the same amount of torque to the
single sphere using dBodyAddTorque. I start using a small torque, look what
happens, and repeat everything with increasing torques.

The result is: the single sphere can stand a significantly higher torque
before it begins slipping than the car wheels. For the above dimensions,
the car wheels start slipping at approximately 50Nm, the single sphere
needs ~80N before slipping occurs.

How to explain that?

The maximum friction force for the above wheels is mu*m*g = 98N. The torque
where the car wheels began slipping (50Nm) would match this: F=T/r =
50Nm/0.5m = 100N. But what about the single sphere? Or is there something
wrong with my view on that problem? (Is the force exerted from the wheel on
the road really T/r?)

Please enligthen my mind.

Greets
Michael