long or medum cage rear DR?
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This is from the Phattire site http://www.phattire.com/phattire1/srx9espremob.html:pipspeak said:
"Pay attention to chain wrap capacity when selecting cage length. Chain wrap is a chain length measurement denoting what the derailleur can accept to shift properly. The formula is:
(large chain ring - small chain ring) + (large cassette cog - small cassette cog) = chain wrap. Let's say you run 22/32/44 rings and a 11-32 XT cassette. Chainwrap is:
(44 - 22) = 22 (32 - 11) = 21
22 + 21 = 43 A SGS or long cage is required for optimal shifting."
The short cages are on road bikes.
Having said that, I have been using a medium cage with the same ring and cassette sizes for about 6 months and have had no probelms, and greatly reduced the noise from the derailleur slapping on the frame. But this is with an XT derailluer and I understand that SRAMs don't have this problem.
Wombat
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The numbers Wombat are technically correct and often used to demonstrate chain capacity as it relates to derailleurs and cage length, but the forumla is simple and assumes you'll be using a small/small cross-chain combo with some regularity.pipspeak said:
If you look at it from the approach that from the middle ring you'll use your entire cassette, and from your granny ring you'll only access the four or five biggest cogs on your cassette, you'll see that running a medium cage derailleur is fine -- the shorter cage still has enough excess "chain capacity" to handle these normal gear combinations.
What you do have to account for is that should you accidentally shift to small/small, the chain will droop due to lack of tension, and eventually derail.
Your chain length is sized exactly the same as if you were running a long cage, so you'll always have enough chain to shift into big-big (again, assuming you accidentally shift there) with no worries of breaking anything (an often cited misconception of shorter cages).
But as Wombat points out, the benefit of a medium cage is reduced chain slap. As you come bounding down the hill, excess chain bouncing around has less authority over the derailleur's tension spring (the derailleur cage, after all, is just a lever arm with a spring at the end). This is the case regardless of brand.
You'll also get slightly increased obstruction clearance, which gives a tiny bit of margin as you're weaving through a rock garden or thick singletrack, and if you do smack the derailleur and bend it, there is reduced chance of it entering your wheel and eating your spokes. You might notice slightly crisper shifts as well.
So if you can deal with the occasional dropped chain should you accidentally shift into small-small, I'd suggest you try the shorter cage length. To me, the reduced chain slap is very real and worth it. I've run the shorter cage with conventional drivetrains in the past and it was absolutely no hassle.
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