dtseng
Well-Known Member
(1) The Right-Hand Rule: use your four fingers to indicate the direction of rotation, then the thumb is pointing to the direction of advancement. Most of the screws we use have right-hand threads. In bicycles, there are two left-hand threads: right-side of BB shell and left pedal.
(2) The rotational quantities are directional (in engineering terms, it is called vector). Take a bicycle wheel, hold both ends of the axle with your hands and make it rotate in the forward direction. Apply the right-hand rule (you can do the same using left-hand rule). The rotational momentum A is perpendicular to the wheel plane as shown in the diagram. Now push the left end of the axle down and right end up. The torque B is pointing to the rear. The combined quantity is C, that is, the torque B makes the wheel to turn left. If you pull the left end of the axle and push the right end (i.e., make the wheel turn left), the torque would make the wheel to fall to the right side. Therefore, when you make left turn, you have to lean the bike to the left to counter the centrifugal force and the left-turn torque causing the bike to fall to the right side.
(2) The rotational quantities are directional (in engineering terms, it is called vector). Take a bicycle wheel, hold both ends of the axle with your hands and make it rotate in the forward direction. Apply the right-hand rule (you can do the same using left-hand rule). The rotational momentum A is perpendicular to the wheel plane as shown in the diagram. Now push the left end of the axle down and right end up. The torque B is pointing to the rear. The combined quantity is C, that is, the torque B makes the wheel to turn left. If you pull the left end of the axle and push the right end (i.e., make the wheel turn left), the torque would make the wheel to fall to the right side. Therefore, when you make left turn, you have to lean the bike to the left to counter the centrifugal force and the left-turn torque causing the bike to fall to the right side.
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