Hi there.
I'm experimenting with the PhysicsVehicleNode atm and experienced some serious problems.
The node is supposed to be a cart, and so far it's wokring fine. I can accelerate, steer, brake and so on. But as soon as i accelerated for two seconds or so and want to go into a curve it falls over…
So i searched the forum but found nothing which satisfied my needs.
I tried to lower the center of mass as it is described in the bullet wiki but it didn't work either.
So i got here to beg for your advice ;D
Is there any way to make the vehicle stable? I can't need a vehicle which falls over if you try to steer…
Some information to my vehicle (just copied out my code… It's a "cart creator" 
):
private static float comp=0.3f;
private static float damp=0.5f;
private static float mass=10f;
private static float maxTravel=10; // "maxSuspensionTravel"
private static float frictionSlip=1000000; // As described in the bulletwiki… should make the vehicle act like a cart
private static float stiffness=150f;
private static float rollInfluence1=0f; //Wheel1
private static float rollInfluence2=0f; //Wheel2
private static float rollInfluence3=0f; //Wheel3
private static float rollInfluence4=0f; //Wheel4
private static float wheelRadius=0.25f;
