Old 11-11-10, 09:18 AM
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well biked
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Originally Posted by pjcampbell
Largest Cog26teethChain Wrap Capacity27teeth
Not sure what chain wrap capacity is vs largest cog.
Here's some helpful info: largest cog compatibility is pretty straightforward, it's the largest cog the manufacturer feels confident you'll be able to use with the derailleur without the upper pulley bumping into the largest cog on the cassette. Keep in mind, it's not a hard and fast number. There can be a variation of several teeth based on where, exactly, your derailleur hanger puts your derailleur relative to the rear axle. Longer hangers will allow a larger cog, for example. It really does vary from frame to frame, so the only real way to know if a certain combo will work is to try it.

Chain wrap capacity is a completely different issue. Keep in mind, a rear derailleur has two main functions: move the chain from cog to cog, and keep the chain tensioned properly. The chain tension function works because of the spring action of the cage, which allows the derailleur to keep steady tension on the chain even as the chain moves from cog to cog or from chainring to chainring up front. In other words, it "wraps" the excess chain, and a derailleur can only wrap so much; this is known as it's chain wrap capacity. The longer a derailleur's cage, the more chain it can "wrap." A drivetrain has a maximum chainwrap requirement, and to insure that the chain will remain tensioned properly in all possible gear combos, you need a derailleur that has enough chain wrap capacity to meet the maximum chainwrap requirement of the drivetrain.

A simple formula is used to determine a drivetrain's max. chain wrap requirement: Largest chainring minus smallest chainring plus largest cog minus smallest cog equals max. chainwrap requirement. Example: 53-39+28-11=31. So a drivetrain with a 53/39 crankset combined with a 11 x 28 cassette would need a derailleur with a chainwrap capacity of 31 teeth. This assumes the chain is sized so that it is only as long as necessary to safely cover the big ring/largest cog combo.

ALWAYS size your chain at least long enough to cover the big/big combo; the results can be catastrophic if it will not and you shift to this combo. On the other end, smallest chainring/ smallest cog, a little slack in the chain is not nearly as big a concern; of course you shouldn't be in this combo anyway, so if you're going to use a deraiilleur with not quite enough chainwrap capacity for your drivetrain, make sure the chain's long enough to cover big/big and try to avoid small chainring/smallest cog or two.

Last edited by well biked; 11-11-10 at 09:21 AM.
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