Friction problem with increased gravity (demo_buggy)

Hi, 

I'm stuck with a new problem: my simulation runs fine as long as gravity is set
to 0.1. When I increase this value, my vehicle slips as soon as it encounters
even very tiny obstacles.
I checked the same on the ODE example demo_buggy and the same problem occurs.
With gravity at 0.1 the buggy easily drives over the ground_box obstacle while,
as soon as gravity is increased to 9.81, it gets stuck against the obstacle and
the driven front wheel experiences 100% slip (contact mu is dInfinity, friction
model is dContactApprox1).
How can I increase the friction to get enough traction on the wheel to move over
the obstacle?

Thanks,
Thomas