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Sorry but I thought that yo did have an understanding of the term. Capacity has two meanings WRT rear ders.
First is the cog diameter (tooth count) which the der pulley wheels can get under. So a 28T max capacity, in this case, means that with the der hanger of the spec that the der brand chooses
the upper cage pulley wheel will clear the underside of that 28T cog.
The wrap capacity is the amount of chain length (in teeth) that the der/cage design (independent on the hanger) can play out and take in so that when the chain is in the big/big combo there is enough chain to fit but not so much so when in the small/small there is chain droop. So an 11-28T cassette has 17 tooth of range and a 34/50 crank ring pair has a 16 tooth for a total chain wrap capacity of 33 teeth.
Chain droop is when the der cage can't fully take in the chain so the chain hangs down, or droops. A der cage tension spring that is broken/weak could do this with an otherwise properly speced out system. Or a chain that is too long could do this. Usually seen when in the small/small combo. Andy.
Last edited by Andrew R Stewart; 02-08-16 at 01:27 PM.
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