Chain Length
 

I'm replacing my 12-23 cassette with a more touring friendly 11-32. How do figure the correct chain length?

To find the minimum chain length, loop the chain around the big chainring and 32T cog -- bypassing the rear derailleur -- and add 1". The whole system may be happier with a slightly longer chain, so if you have a reusable master link, you might start out with a chain that is a couple links longer than the above formula and see how the rear derailleur responds when you're in the small-small combination. (If the chain is too long, the lower derailleur pulley will swing up all the way, and parts will start bouncing and rubbing on each other -- not good.)

Here is a chain length calculator to get you started: http://www.machinehead-software.co.u...engthcalc.html

Awesome. Thanks!

This is the first cassette swap I've done and just realized that I've never had to figure this out before. I've always had an old chain to base the new one off.

There are 3 ways.

The shortest chain method is to loop the chain around the big front/big rear, and add 1". This has zero fudge room, and represents the absolute minimum safe length you an use.

If the RD has enough capacity you can use a longer chain, measured by the small/small combination and shortened until the RD cage can take up all the slack.

If there's enough capacity, Shimano suggests a length between these, based on the angle of the cage in a certain combination.

I prefer to use a longer chain, so I have the capacity to cut out a link in case I ever have to. But I always start by confirming that it's long enough via the big/big method.


One other note -- if you have a road bike with a 12-23 cassette originally, you'll almost definitely need a longer chain, but you may also not have enough take up capacity if it has a short cage RD. Here you have a choice. You can buy a new long cage RD, or if it's only slightly under capacity, can use the shortest chain possible, and avoid the small/smaller combinations (rarely used anyway) where the RD can't take up the slack.

Besides the "chain take-up" capacity you will also need to consider the "largest rear cog" capacity of the derailleur you have. Some road type derailleurs will only accomodate a 28- or 30-tooth large cog; the jockey wheel may not have enough clearance from the 32-tooth cassette if this is the case. This capacity will specified be in the manufacturer's tech documents. Depending on the bike this capacity may be exceeded by a few teeth but you will need to try it and see.