Wheel colliders slingshotting forward

I have a vehicle rig with four wheel colliders controlling the movement. For most of the development process, it seemed to be a pretty capable solution. The torque of the wheels is rather high with an equally high friction setting in order to prevent slipping on the difficult terrain in the scene.

However, as of recently, I’ve noticed some very odd behavior. What was previously a terrain with sloping hills, I’ve started adding more cliffs and sharp edges. I should mention that my terrain is a mesh imported and using a mesh collider with smooth sphere collisions turned on. What I’ve started noticing is a big burst of power forward when the vehicle goes over edges. The vehicle has four wheels that each can turn and apply movement, so not every wheel needs to be touching the ground for the vehicle to continue forward. However, these big bursts of speed (which sometimes send the vehicle flipping through the air) only occur if I go over a high point in the ground. What I’m worried about is the front wheels are lifting off the ground and the back wheels are suddenly pushing the vehicle with considerably less friction, causing the car to propel forward. I would have assumed that only 2 wheels on the ground = half the power but also half the friction, but I’m wondering if this isnt the case after all.

If anyone has run into this issue or can think of a reason for this behavior, I’d be really appreciative of the help!

If anyone has encountered this problem with wheel colliders, I haven’t fixed it but I’ve alleviated the problem. I basically made sure that every edge on my terrain was below a 40 degree angle (with some manual work in Maya). It seems the stronger an angle in a mesh, the worse the slingshotting occurs. Like I said, I haven’t fixed it, but luckily my rover isn’t flipping off into the skies when it hits certain bumps.

The wheel collider is actually implemented using a raycast that measures the distance to ground. Then, suspension forces are applied in proportion to this distance and drive forces are applied at the detected contact point. Since the “wheel” is effectively just a point, it can react very suddenly when it encounters an edge. Wheel colliders aren’t ideal for a bumpy track but as you’ve found, you can improve matters by helping the collider to react more gradually. Smoothing the track out is one option but you may also try using more than one collider at each wheel location, each with a weaker suspension spring. Putting a slippery capsule collider near each wheel can also help reduce the jerky reaction from the wheel collider.

It is even possible to implement wheels by actually rotating round colliders against the ground (say using a joint) and letting the friction move the vehicle. This tends to have a much higher CPU overhead than a wheel collider, however, and has problems of its own.