1) The collider should have no more than 40/60 triangles.
2) A material called “GL” must be assigned to the collider inside the editor. This is a special material specifically made for meshes that are not rendered. Meshes with this material are used only for collisions.
3) The collider must not extend with any vertex, under the floor of the car.
4) The collider must have no holes. The mesh must be closed. Once the collision mesh is done, simply save the fbx from the editor like a kn5 file, using name “collider.kn5”. The file must be placed in the same folder of the car LODs.