I know that the physics community generally hates this kind of question, though I don't know why. Like Richard Feynman said, you can tell an expert by his reluctance to answer honest but naive questions. Okay, I understand that a wheel and axel can be viewed as a lever system with the fulcrum at the axel. Also as a lever arm with the fulcrum at the point the wheels touch the ground. So, if a force is applied at the axel, parallel to the ground, the lever is turned and the system moves forward, a new point on the wheel continually making contact with the ground. Also, if a torque is applied to an axel which is fixed to the wheels (doesn't turn independently) the system moves forward. This is how a car is propelled, even though the torque comes from within the system. In terms of forces, how does this twisting motion cause this movement? I know the ground reaction force must be the cause, but bear with me. Imagine you have a solid rod with a wheel on each end, to which you are attached so that any force you apply is an internal force (in a seat or something, to simulate a car). You wrap your hand around and twist the rod. The only way the system can move forward, as a lever with fulcrum at the ground, is if the net force is perpendicular to the rotation axis and to the radial axis, and in the forward direction. How does this twisting effect produce such a force? Very confusing to me, these details, since the ground reaction force is right at the fulcrum. Any thought experiments which might help me?