Are you trying to model the friction between a rolling wheel and a surface? Try using the Spatial Contact Force block for nearly rigid wheels rolling on flat or convex surfaces, or the Magic Formula Force and Torque block for tire modeling.
If you want to see some examples for how to extract this quantity using the Transform Sensor block, look at the Simscape Multibody Contact Force Library. As those blocks model contact forces and friction, they need the relative velocity at the contact point. For shapes that have some degree of symmetry, that calculation can be done explicitly in Simulink.
--Steve
