Force distribution problems

Hi,

i need some help with a small tank project.

I have a tank rolling on suspended wheels built in a blueprint with constraints.
Moving the tank with applying vector force to the tankbody works fine:

Now i want to move the tank by rotating the wheels.
Only the wheels touch the ground, but i get a confusing result:

The tank is very slowly moving in the opposite direction…
Help would be really appreciated, thanks in advance.