Can you solve these physics riddles? ft Simone Giertz - Part 1/3

The bike issue...

When examining (modern) bicycle pedals, you'll notice that they are plates (more or less) with an axle that has ballbearings. This allows the pedal to freely rotate. When you push down hard, e.g. put your weight on the pedal, you sometimes notice a grinding sound and feeling. This means that the insides of one or more of the ballbearings are being crushed.

Now imagine that these ballbearings are attached to something that isn't supposed to twist and turn, such as a crank. This is the part of the bicycle where forces from the pedal are transferred to the central axis of the bicycle, where the large chain wheel sits. This crank has to flex as little as possible to make sure your pedalling is efficient. This means it has to be fixed tightly to the central axis, as well as the pedals.

The most force exerted on the ballbearings, is when the pedal is on top, slightly in front of the central axis. Cyclists often put their entire weight on it, for faster acceleration. This also means that the ballbearings may start crunching and twisting, causing the force to be transferred to the mounting point of the pedal, i.e. the screwing part of the pedal in the crank.

Since we are pushing downward and forward against the pedal, the screw also moves in that direction, which means clockwise on the right pedal. The left pedal pushes in the same direction, but the crank is in the opposite direction, so the screw twists anticlockwise.

So if the screw on the left pedal was the same as on the right, pushing too hard on the pedal would cause it to loosen.

You can test this by using a plank and mounting two blocks (without ballbearings) on each side, at the end. Now rotate them, with the 'pedals' in fixed positions. You'll notice that one side will loosen up, while the other one tightens.