The above equation, where resistance is inversely proportional to radius r seems to be based on the discredited "Coulomb's law" (Neither Coulomb's inverse square law nor Coulomb's law of friction). See dependence on diameter. Equating this equation with the force per the rolling resistance coefficient, and solving for b , gives b = C r r r . Therefore, if a source gives rolling resistance coefficient ( C r r ) as a dimensionless coefficient, it can be converted to b , having units of length, by multiplying C r r by wheel radius r .