Current Projects chat and miscellany

The chain that is currently on the bike could handle 48x20 with the rear wheel all the way forward in the dropouts, and 48x14 with it nearer the back - will 51x18 and 51x15 fit? Is there a way of working that out?

Yes, add up the numbers and divide the difference by 8

48+20 = 68
51+18 = 69
=> move wheel forward ⅛" i.e. probably won't work

48+14=62
51+15=66
=> move wheel forward ½"

will work, you're already using a difference of ¾"

or

48+20 = 68
51+15=66
=> move wheel backward ¼"

This calculation is not absolutely precise, but it's near enough for normal combinations of gearing and chainstay length. The amount of wear in your chain and the non-circularity of your chainring will make more difference than the difference between ⅛" and the true change in axis distance.