In a nut shell....could someone recommend which I should get for an 8 spd XT drivetrain. I'm looking at replacing my Deore (although no complaints with the shifting) to an XT or XTR.

Thanks in advance.

There are trade offs to both. We'll skip the calc of chainwrap capacity unless you repost that you want to understand the math.

Med cage. Pro: Less fragile material dangling in harm's way. (envision a dog with saggy balls leaping a fence) Con: less leverage to take up slack


Long cage Pro: longer arm provices greater slack pulling leverage. Con: one inch closer to rocks, roots, sticks, brush snags.

Most riders will not notice the difference riding.

(This is definitely not a black and white answer, although my personal choice would be a medium cage.)

Think of the derailleur cage as a storeage drawer for your excess chain.

Sometimes you need to use all your chain (i.e. accidental shift to the big-big gear combo) so the derailleur doesn't need to "store" any of it.

But most often (i.e. middle ring, middle of cassette) you have a handful of links that need to be wrapped up. The derailleur cage stores these excess links by wrapping them up as the cage folds over on itself.

Obviously a longer cage has greater capacity in order to store more excess chain.

The downsides to the longer cage are a few:
* Increased chain slap (longer cage has less "authority" over chain),
* decreased obstruction & spoke clearance,
* greater slop (or at least the potential), and, for the the ultimate in weightweeniedom,
* a few extra grams (cage weight only; chain length remains the same no matter what cage length is used).

As far as I'm concerned, the reduced chain slap the shorter cage provides is the most notable of the above reasons. Don't read too much in to that list of negatives — long cage derailleurs work very well and on used on the vast majority of bikes.

<i>So how do you know what you need?</i>

Shimano lists a capacity figure for their various cage lengths. The current XT GS (medium cage) is listed at 33T, the SGS (long cage ) at 45T.

Determining your capacity needs can be as easy as calculating the difference between your big-big combo and your small-small combo. With a conventional 44/32/22 crankset and 11/34 cassette, it would go like this:

Total Capacity = (BigRing - SmallRing) + (BigCog - Small Cog)
= (44T - 22T) + (34T - 11T)
= (22T) + (23T)
= 45T

The 45T answer the formula comes up with looks like a SGS derailleur with 45T capacity is a perfect match. <b>But who ever shifts into small-small on purpose?</b> More often than not, if you're in the granny ring, you don't shift past the mid point of the cassette — the 20T cog.

<i>So taking a second look at the drivetrain:</i>

44T (Big) Ring & 34T Cog = Zero (no excess chain — derailleur cage sticking straight ahead)

22T (Granny) Ring & 34T Cog = 22T excess chain OK (44T - 22T = 22T — 22T)
22T (Granny) Ring & 30T Cog = 26T excess chain OK (34T - 30T = 4T — +26T)
22T (Granny) Ring & 26T Cog = 30T excess chain OK (30T - 26T = 4T — +30T)
22T (Granny) Ring & 23T Cog = 33T excess chain OK (33T - 30T = 3T — +33T)
22T (Granny) Ring & 20T Cog = 36T excess chain: Over Shimano's 33T stated capacity.

Good thing is, Shimano is very conservative in their stated capacity. In my experience a GS cage will hold tension up to 36 or 37T. At around 39 or 40T the cage begins to "fold over" on itself, to where the chain is doubling up and rubbing against itself on its trip to and from the tension pulley.

The other good thing is that if you do happen to accidentally shift into one of the smaller cogs while in the granny ring, the worst that happens is you lose chain tension and possibly your chain falls off if you don't catch it in time. In all reality, it's nothing to be bothered about.

So my suggestion is medium (GS) cage if you can deal with the granny gear shifting limits and want the benefit of reduced chain slap. Otherwise, if you don't want to be bothered by any of it, you won't go wrong with a long cage.

That's what I'm talking about. Thanks for the "skinny" and the AR versions. I can appreciate both and will decide on the Med cage.

(This is definitely not a black and white answer, although my personal choice would be a medium cage.)

Think of the derailleur cage as a storeage drawer for your excess chain.

Sometimes you need to use all your chain (i.e. accidental shift to the big-big gear combo) so the derailleur doesn't need to "store" any of it.

But most often (i.e. middle ring, middle of cassette) you have a handful of links that need to be wrapped up. The derailleur cage stores these excess links by wrapping them up as the cage folds over on itself.

Obviously a longer cage has greater capacity in order to store more excess chain.

The downsides to the longer cage are a few:
* Increased chain slap (longer cage has less "authority" over chain),
* decreased obstruction & spoke clearance,
* greater slop (or at least the potential), and, for the the ultimate in weightweeniedom,
* a few extra grams (cage weight only; chain length remains the same no matter what cage length is used).

As far as I'm concerned, the reduced chain slap the shorter cage provides is the most notable of the above reasons. Don't read too much in to that list of negatives — long cage derailleurs work very well and on used on the vast majority of bikes.

<i>So how do you know what you need?</i>

Shimano lists a capacity figure for their various cage lengths. The current XT GS (medium cage) is listed at 33T, the SGS (long cage ) at 45T.

Determining your capacity needs can be as easy as calculating the difference between your big-big combo and your small-small combo. With a conventional 44/32/22 crankset and 11/34 cassette, it would go like this:

Total Capacity = (BigRing - SmallRing) + (BigCog - Small Cog)
= (44T - 22T) + (34T - 11T)
= (22T) + (23T)
= 45T

The 45T answer the formula comes up with looks like a SGS derailleur with 45T capacity is a perfect match. <b>But who ever shifts into small-small on purpose?</b> More often than not, if you're in the granny ring, you don't shift past the mid point of the cassette — the 20T cog.

<i>So taking a second look at the drivetrain:</i>

44T (Big) Ring & 34T Cog = Zero (no excess chain — derailleur cage sticking straight ahead)

22T (Granny) Ring & 34T Cog = 22T excess chain OK (44T - 22T = 22T — 22T)
22T (Granny) Ring & 30T Cog = 26T excess chain OK (34T - 30T = 4T — +26T)
22T (Granny) Ring & 26T Cog = 30T excess chain OK (30T - 26T = 4T — +30T)
22T (Granny) Ring & 23T Cog = 33T excess chain OK (33T - 30T = 3T — +33T)
22T (Granny) Ring & 20T Cog = 36T excess chain: Over Shimano's 33T stated capacity.

Good thing is, Shimano is very conservative in their stated capacity. In my experience a GS cage will hold tension up to 36 or 37T. At around 39 or 40T the cage begins to "fold over" on itself, to where the chain is doubling up and rubbing against itself on its trip to and from the tension pulley.

The other good thing is that if you do happen to accidentally shift into one of the smaller cogs while in the granny ring, the worst that happens is you lose chain tension and possibly your chain falls off if you don't catch it in time. In all reality, it's nothing to be bothered about.

So my suggestion is medium (GS) cage if you can deal with the granny gear shifting limits and want the benefit of reduced chain slap. Otherwise, if you don't want to be bothered by any of it, you won't go wrong with a long cage.