Why is there no friction at the top and bottom of loops? A beginner's question: Why does friction vanish at the top and bottom of loops? If friction works in the opposite direction of motion, then surely there must be some element of friction that is in the same direction as the normal force at the top of the loop? Or does friction have to always lie on the plane on which the motion occurs?
Edit in attempt to improve the question: Given a motorcycle travelling in a looping, then when it's at the right-most point of the looping there are three forces acting upon it - its weight due to gravity, which points downwards, friction, which is exactly opposite in direction and equal in magnitude to its weight, and centripetal force, which coincides exactly with the normal force. My understanding is that, as the motorcycle moves from the right-most point to the very top of the looping, the friction gradually decreases (because the part of the weight vector it needs to compensate gets smaller) until, at the very top, there is only the centripetal force acting on it, which is now the sum of the normal force and its weight. What happens with the friction at this point?
 A: For any object A moving along on another object B the direction of the friction force acting on A is opposite to the direction of relative motion or attempted motion of A relative to B. This direction must lie in the plane of contact between A and B. If B exerts a friction force on A then A exerts an equal and opposite friction force on B.
In the case of a motorcycle the engine of the motorcycle is trying to turn the bottom of its drive wheel backwards. Friction exerted by the ground on the motorcycle opposes this motion, so it acts in the forwards direction on the motorcycle. This friction force is what drives the motorcycle forwards. The motorcycle exerts an equal and opposite backwards friction force on the ground. This is the force that drives gravel backwards if the motorcycle runs over loose ground.
The same thing happens with a walker. If a hamster runs on the ground it is the friction force exerted by the ground on the hamster that drives the hamster forwards. If a hamster runs in its wheel it is the friction force exerted by the hamster on the wheel that drives the wheel backwards.
So the friction force exerted by the loop on the motorcycle will always act forwards, in the direction of motion of the motorcycle, and will only disappear if the normal force exerted by the loop on the motorcycle becomes zero or if the motorcycle stops accelerating tangentially i.e. if speed of the bottom of its drive wheel relative to the ground or loop is zero.
