Contact Force Solidworks Motion

I am trying to determine the contact forces between the wheels and the floor, in a four-wheeler.

I can run the simulation in solidworks motion. I select "Results and Plots" and choose "Contact Force" between wheel and floor. However the given results does not correspond to the ones I calculate theoretically.

Why does this happen? What type of calcuation does the solidworks?

I'm using version 2016.

Thanks in advance

SolidworksSimulation