What will the impact to chain length of switching the number of teeth on chain ring and/or cog? This question is important for single speeders like me who have to get it exactly right. I had always assumed that removing two teeth from either the cog or the chain ring would require removing a pair of links from the chain. However I just switched from 39x22 to 42x15 ( net down four teeth and knocked out four links from the chain only to discover that was twice as many as I should have done. 😡

Fortunately I had an extra chain to save the day but I struggled to think how removing four teeth would not shorten the chain by four ......

Eventually it occurred to me that the chain is only in contact with the ring/cog for 50% of the circumference so the addition/subtraction of a link only has 50% of the impact on the chain length.

Make sense? Alternate thoughts?

W