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Old 07-17-14, 11:09 AM   #1
rms13
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Short cage vs long cage RD?

I've read a few threads on this topic but I'm still confused. When do you need a long cage? My bike came with long cage and it's SRAM Apex compact 50/34 with 11-32 cassette. I've read that you only need long cage with triples but do I need long cage for my set up or could I go short if I upgrade down the road?

Someone was also telling me that short cage provides better shifting and reliability. Is that necessarily true?
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Old 07-17-14, 11:27 AM   #2
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Large Gear Ranges Require Long Cages. LGRRLC. remember that. LGRRLC. sort of like LSMFT (Lucky Strike Means Fine Tobacco), but different. i doubt if there is much difference in shifting quality these days. maybe 10-20 years ago, anyway, this is coming from a guy that's been riding singlespeeds exclusively for about 10 years.

or, i could give you the long version. (not mine BTW).

You've got a medium cage. Sram makes a short cage mountain derailleur in the X.0 line, but only goes as low as a medium in the X.9.

Quick answer: The medium cage will work, but you'll drop your chain if you accidentally shift to the small-small combo. Suspension *could* be a factor, depending on how much "chainstay growth" your frame experiences as your suspension cycles.

Long answer:

Derailleurs have a rated capacity. This is their ability to take up excess chain. After all, you need just about all of your chain to run in the big-big combo, whereas you have a bunch of extra links doing nothing when you run in your small-small combo.

Not that either of those cross-chain combos are normal to run in, but let me get to that in a minute.

Manufacturer stated derailleur capacities are as follows:
Shimano long = 45T; medium = 33T
SRAM long = 43T; medium = 37T; short = 30T

Speaking from experience, Shimano is a bit conservative in their capacity rating. I can only assume the same is true of SRAM (I'll get to that, too).

The easy capacity formula is to add your big ring & cog sizes, then subtract your small ring and cog sizes. It looks like this:

cap req'd (T) = (BIG ring - small ring) + (BIG cog - small cog)

...so for a typical 44-32-22 mountain crank & 11-34 cassette...

T = (44T - 22T) + (34T - 11T)
.. = (22T) + (23T)
.. = 45T


Using this simple forumla, you would need a derailleur with a 45T rated capacity to absorb all the possible extra links of a typical 27-speed drivetrain.

(I make the assumption SRAM stated capacity is conservative, since they list 43T as the long cage capacity -- 2T short of what is required by this forumla).

Where do shorter cage lengths come into play? Right here!

Even though the long cage will, in theory, take you down to the 22x11 gear combo and hold adequate chain tension, let's be logical: 22x11 is a combo you don't use!

Rather than use the generic formula, let's map out the capacity for each gear combination (based off of a Shimano cog pattern; SRAM will be slightly different):



44x34 starts off at zero because in that combo, all of the chain is being used up by the ring and cog, and the derailleur needs to take up none of it. As you shift through the cassette range (moving down the column), the amount of free chain increases as the cog size decreases.

Take a look at the useable gears, which I've outlined in green and yellow. Those fall near the stated capacity of the medium cage derailleurs. (I mentioned that Shimano's stated capacity is conservative, and in practice, I find their medium cage to be closer to 39T.)

For instance, in the middle ring (32) and the small cog (11), the table shows you've got to absorb 35T. This is near the stated capacity of either of the medium cage derailleurs. This gear combo remains useable, but you'd be better off shifting to your big ring for better chain tension.

You can also see that to use a SRAM short cage derailleur (30T capacity) on this drivetrain would leave you with two or three unusable gears while in the middle ring, and only about three useable gears from your granny ring. (Any number greater than 30T on the table would be near the limits of the short cage derailleur.)

Oops! Accidentally shifted into the unusable "red zone"? Nothing major: the derailleur cage folds back on itself, the chain droops, and you maybe drop the chain if you don't catch it in time.

In my opinion, it'd be stupid to size a chain any smaller than what is required to shift into big-big. If you accidentally force a shift into that combo, which is certainly possible when you're tired or "in the moment", you don't want to break anything. So chain length will be the same no matter what derailleur you choose.



Benefits of a shorter cage length?
- snappier shifts
- better chain tension
- less chain slap / greatly decreased drivetrain noise (!)
- better obstruction clearance / improved spoke clearance.
- slight weight loss -- but you gotta be a real weight weenie to appreciate this one.

Last edited by hueyhoolihan; 07-17-14 at 11:31 AM.
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Old 07-17-14, 11:57 AM   #3
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I thought that sram has the apex wifli rd to handle the 11-32 cassette. I don't think a short cage rd can handle the 11-32 cassette.
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Old 07-17-14, 12:18 PM   #4
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I thought that sram has the apex wifli rd to handle the 11-32 cassette. I don't think a short cage rd can handle the 11-32 cassette.
+1. Short answer, I you switch to a short cage RD with that configuration, you run the risk of damaging the RD when you shift to the largest front/rear gears -- accidently or not.
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Old 07-17-14, 12:29 PM   #5
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Its about chain slack take-up. (big+big)-(Small+small) = how much chain slack you get.

to use a short cage , make the cassette with a Much higher Low gear range, say an 11- 23t
and make the 2 chainrings much closer to each other in size like 46-34..

yea it wont have the same low gear , but you have to HTFU and work harder on the hills there after.

If you can climb the hill in a higher gear than your rivals you will reach the top earlier. keep the tempo Up.

its the smaller differences , that will make the shifting snappier

Last edited by fietsbob; 07-17-14 at 12:34 PM.
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Old 07-17-14, 01:26 PM   #6
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Thanks for the responses, especially huey as that was one of the most detailed responses I've ever gotten on the forums!

I'm sure fietsbob and others here remember the troubles I had trying to understand chain lengths when building up this bike a few months ago. Even though some of this stuff is simple concepts on the surface they still mystify me. Glad to have experts to teach me something new every day so I avoid my next catastrophe.
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Old 07-17-14, 01:56 PM   #7
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Here's a quick related video on the topic:
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Old 07-17-14, 02:08 PM   #8
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Originally Posted by rms13 View Post
Thanks for the responses, especially huey as that was one of the most detailed responses I've ever gotten on the forums!

I'm sure fietsbob and others here remember the troubles I had trying to understand chain lengths when building up this bike a few months ago. Even though some of this stuff is simple concepts on the surface they still mystify me. Glad to have experts to teach me something new every day so I avoid my next catastrophe.
thanks, but, as i noted in the response, after my quasi-humorous first salvo, that long winded explanation, was not mine. i googled it. and coincidentally it was from Bike Forums, a while ago.
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Old 07-17-14, 03:07 PM   #9
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The key difference (as stated above) is chain take up capacity. That has to exceed the sum of the differences in the largest and smallest sprockets front and rear (12-30 with 26-36-46 rings = 30-12 + 46-26 = 18+20 = 38t total).

There's some fudge of a tooth or few if you measure the chain very carefully (wrapping big/big+1" = minimum chain length).

The advantage of short cage derailleurs are lower weight, higher ground clearance, and slightly higher chain tension(in most cases).

Actual shift performance is identical in Sram derailleurs where the cage is mounted on center with the upper pulley, and slightly different in Shimano or Campy derailleurs where the cage is mounted between the pulleys. For this reason Campy and Shimano are more sensitive to chain length.

That's the basics in a nutshell.
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