Prevent tire from rotating on Z?

I'm trying to setup a tire as my character using Unity's "Ball User Control" and "Ball" scripts from standard assets. It's all working great with a mesh collider not a sphere, but im trying to not let it fall over on it's Z rotation. I have it frozen on the Z in the rigidbody constraints but eventually if I try to turn it, it will fall over or if it collides with something.

How can I prevent it from falling over?

Thanks so much in advance!