Wheelcolliders falling through ground

Hello,

I need some help with wheel colliders. I have made a car model in blender and imported it to unity. There I have attached four wheel colliders, one for each wheel, everyone in a single empty game object parented to the main car object. I have aligned them so the sit around the edge of the wheels.

I have a terrain also made in blender that the car rides on that has a mesh collider. If I only drive along the surface the wheel colliders work but if the car is dropped from some height or is trying to turn to rapidly, some or all wheels go through the mesh collider. It seems to be a problem not only with meshcolliders since I have a unity terrain object also in my scene and it acts the same way there.

I've tried to work around the problem using a second collider in a sibling object to every wheel collider, of type spherecollider that uses a frictionless physics material however these colliders dont react nicely with the kinks in the meshcollider suddenly just stopping and such.

The only suggestion that I can come up with regarding whats happening is that the verts in the mesh collider is spread out, my ground mesh is quite low poly, so that the wheel collider has trouble finding it.

If anyone has any thoughts I'd appreciate it!