1. A friction drive system, comprising: a drive assembly comprising a motor and a contact surface capable of engaging with a tire of a wheeled vehicle; and a control unit comprising an automatic traction control system, wherein the automatic traction control system comprises a normal force adjustment mechanism capable of automatically increasing an amount of normal force between the contact surface and the tire after slippage is detected, when the friction drive system is mounted to the wheeled vehicle, and wherein slippage is detected by determining whether a speed of the motor has increased.