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Trento Corsa
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Discussion Starter · #1 ·
I have a new SRAM X9 long cage derailleur, and another X9 that I now realize is "not long". Both are brand new, but the short one is ~16mm shorter.

What is the proper application for long versus short cage derailleur? Will the short one work with a 11/34 cassette? Does it matter if the bike has a rear suspension or is a hardtail?

Is my shorter X9 a "short" cage or "medium" cage? Thanks //kct
 

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Derailleurless
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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 usable, 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.
 

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Trento Corsa
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Discussion Starter · #4 ·
Thanks to Speedub.Nate for an outstanding explanation! Comprehensive and well written.
//kct
 

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Nate!

I'm sitting here thinking of the same choice in XTR derailleurs. Amazingly clear, concise and helpful.
 

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i like dirt.
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How would I calculate it if I run a single 38 in the front and a 11x32 in the rear? What size derailleur would I need? I'm interested in buying a Shimano Saint for my Specialized P.2 but I'm having trouble deciding what size I should get.
 

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Derailleurless
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That's an easy one... go as short as you can possibly go.

See, it's not the *size* of the front chainring that factors in, but the *range* between the biggest ring and the smallest ring.

With an 11-32 in the rear, you've got a 21T spread, well within the capacity range of Shimano's short cage derailleur.

(The capacity requirement for the front rings is Zero on your setup because it doesn't change at all.)
 

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Old man on a bike
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One of the most impressive responses I've seen on these boards. Great work speedub.nate!
 

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one more question...

Great explanation..but whats the diff between "rapid rise" and "non rapid rise"? thanx in advance....

'
 

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Derailleurless
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"Rapid Rise" is Shimano's trademarked name for what is otherwise known as a low-normal derailleur. This means spring tension pulls the chain in/up the cassette to the lowest gear, and cable action pulls it out/down to the high (small) cog.

The reverse of this is high normal, where the thumb press / cable pull drags the chain up the cassette towards the lowest (biggest) cog.
 

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thanx!

great info..I love these forums!
 

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Riding free's the mind
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med cage

I'm finding my SRAM XO med cage works great with 11-34 cog. In fact it can accommodate the extreme large-large gear combo, but as we all know, it's not recommended with full-susp bikes (chain growth upon compression). The shifts are definitely snappy, quick and precise- partly due to the shorter cage, and mostly to SRAM's shift system.
 

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Derailleurless
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HTail said:
I'm finding my SRAM XO med cage works great with 11-34 cog. In fact it can accommodate the extreme large-large gear combo, but as we all know, it's not recommended with full-susp bikes (chain growth upon compression). The shifts are definitely snappy, quick and precise- partly due to the shorter cage, and mostly to SRAM's shift system.
That, and my lasting impression during my first ride after switching to a medium cage on my 1x9 (32/11-34) was how remarkably quite my bike had become.
 

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HTail said:
The shifts are definitely snappy, quick and precise- partly due to the shorter cage, and mostly to SRAM's shift system.
Oh, so that's why my new SRAM X9 (long cage) doesn't feel as snappy as my old XTR medium cage. Also more chain slap and noise. I wondered if the medium cage X9 would handle the 34-11 rear cog with a normal triple chain ring... and apparently it will.

Thanks.
 

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Riding free's the mind
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Again, med cage should be fine as long as you're conscious about your gear combos and follow the chain length guide measuring- large chainring to large cog + 2 links.
 

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Another big thanks to Speedub.Nate!!!

Its been nearly 7 months, and his outstanding tutoral still goes VERY much appreciated:thumbsup:
 
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