Chainwheel

This video shows how a bicycle chain engages a chain wheel. First, a worn chain. The bicycle's rear wheel is being held against turning freely, so the engagement is The chain's links have lengthened with wear, and so they start high, where the distance between chain wheel teeth conforms to their length. This chain is relatively dry, un-lubricated. The chain rollers press against the forward faces of the teeth and descend into the gaps between teeth toward the bottom of the chain wheel. Here's the same chain, lubricated. There is no substantial difference in how it engages the chain wheel. Now, a new chain. You can't see light under the teeth of this chain, but we can do a little experiment to show where the tension is being held. Pinching the links and pulling them away from the chain wheel, they are tight at the top, but after a quarter turn they are loose in the gaps, and then finally pulling against the opposite faces of the teeth. More details at SheldonBrown. com.