I'm building an electric skateboard. I am trying to calculate the bare minimum force and torque to move a 180 lb object on 4 polyurethane wheels from standstill for motor selection. I got lost in finding the coefficient of friction for polyurethane wheels and whether I am using the equation for rolling friction or the standard model used in schools of a big block with mass moving across a floor with no wheels. Things I know I have to take into account are gravity, normal force, total weight of the whole system, force on each wheel, coefficient of friction and wheel size to find force and torque. Wind resistance is negligible. Much help appreciated.