This applies to bicycles, motorcycles, chainsaws, and
anything else where power is delivered through a chain
drive. A new chain, running on a new sprocket, ideally,
makes equal contact with all the teeth that are engaged. As
the chain wears, the pitch increases, so the load is carried
more by the teeth that are just coming into mesh. Of course,
the sprocket wears also, so the load tends to be properly
carried by all the teeth. If you now replace just the chain,
the pitch will be shorter than the sprocket pitch, and all
the load will be carried by the teeth that are just going
out of mesh. The wear on the chain and teeth will be rapid.
Since the sprocket is already worn, any metal that it loses
will make the problem worse, not better, so the problem gets
worse. The new chain won't last very long.
In practice, chains seem to wear out more rapidly than
sprockets, so you can get away with changing the chain
more often than the sprocket. On chainsaws, the ratio is
about three to one. I don't know what it is on bicycles.
On chainsaws, the best practice is to rotate three chains
in service, so they all wear out together, and then
replace all three AND the sprocket. I have never heard of
anyone doing this on a bicycle, but, theoretically, it
sounds like a plan.
Comments?