[ODE] Tank simulation how?

[Csáki Péter]

Hello ODE people,
 
I'm using ode for my vehicle physics but now I need help from you.
I was thinking that a series of wheels would produce the needed effect (they are not rendered in 3d). I wanted to set the wheel's speed differently for both sides. For example the left wheels rotate forward and the right wheels rotate backward. I thought it would rotate the vehicle. But it is not working as I expected.
 
I have done a search but I couldn't find any solutions.
 
So does anyone have an idea how to do it? As you see it doesn't have to be an accurate simulation as I want to keep things fast (on cpu at least).
 
Note that my english is not too good - sorry for that.
 
Thanks,
Csakip