Shortening chain

[GOHughes]

I have just ordered a 39T chain ring to lower the gearing on my Brompton. it currently has the standard 50T chain ring. How many links do I have to remove?

[Brucey]

you can work this out fairly easily. There is an 11T difference in chainring size and the chain wraps around about half the chainwheel. Therefore the chain needs to be (11/2) x 1/2" shorter, i.e. 2.75". Your choices are actually 2.5" or 3" shorter, because the chain has a 1/2" pitch. Since the former is only possible with a half-link involved, it pretty well has to be 3" shorter if you are using 3/32" derailleur chain.

cheers