Getting contact force between two rigidbodies

I would like to know if there is a way of getting the contact force (aka normal force in physics) between two rigidbodies. Not just in a case of a collision, but also when a rigidbody stands still on top of another rigidbody. I know in pure Physx that information is acessible for use, but in unity I counldn't figure out a way to access it.

I'm needing it to correct the slip-based model for the wheel colliders the physx uses, because as I understand, the model takes the slip between the wheel and the road to calculate the friction force, but doesn't take in account the normal force between both. The result is that the friction can get the same values both on a horizontal floor, or on a ramp, and even on a wall. Just a slight contact can get full friction force.

So my idea to solve this was multiplying the stiffness factor by the normal force between the road and the wheels.

So that is the problem. Anyone has any idea? Or a diferent approach on solving this? What I need is a more realistic wheel behavior for my game, so the vehicle stop climbing mountains =P

Thanks in advance! =)