Chad’s Bicycle Chain

Chad Purviance needs to know which size and length of chain he needs for his 21 speed mountain bike, the length depends on a few different things which Chad hasn’t told us, but we can still give him some clues to answer his problem

Question: How long should a chain be and what size for a 21 speed mountain bike.

Thanks for your question Chad, the chain size will be a standard size 3/32 of an inch, (but you do need to ask for one that is for 7 speed) which is for derailleur gears, the other size (bigger) is only used for track cycling or for BMX as these are for a single gear. A derailleur chain is narrower and more flexible as it has to bend to go from the extremes of gear changes, for instance from the small inner chain ring to the small outer sprocket at the rear and the other way of the large outer chain ring to the large inner sprocket. These gears are the extremes and twist the chain, but the chain needs to occasionally do this and has to be flexible enough.

As to the length, there are a few different ways of doing this: -

· The first way is to put the chain round the small chain ring at the front and round the small sprocket at the back and through the gear derailleur (like normal), you want the spring in the derailleur to take up the slack up on the chain and then check that the chain is long enough to go round the big front chain ring and the big rear sprocket.

· The second way is to put the chain round the large chain ring at the front and the large sprocket at the back, without putting it through the derailleur gear mechanism, one link longer than it takes for the two ends to meet and that should be right for most gear combinations.

· The third method is to thread the chain through the gear (as normal) putting it on the small sprocket and the small chain ring, with it like this there should not be a gap between the top of the chain and the top of the lower sprocket, this is not as complicated as it sounds and is not that different from the first way, this is the Campagnolo method.

I hope this has helped you Chad? You can always let us know how you got on?