More on Rear Derailers - Shimano Names, Compatibility

Also from a thread on MTBR.com:

"Most rear derailleurs are made the way they have always been, in what Shimano now calls a 'top-normal' configuration. What this means is that if you disconnect the cable, the derailleur returns to the smallest cog, which is the top gear - it's 'normal' state is the top gear, hence 'top-normal.' They wish that we would all buy 'low-normal' derailleurs, which is exactly the opposite - the normal state is the biggest cog, or the lowest gear. So now we have two different models, which in the LX world are M580 (low-normal) and M581 (top-normal).

Then there's cage length, which is confusing enough to the majority of people that it is a sticky at the top of this forum. Basically longer cages allow you to run bigger gear spreads witout fear of cross-chaining in exchange for slower shifting and higher weight. Medium cage derailleurs (the only truly short cages are on road bikes) are far less forgiving of cross-chaining but are lighter and shift quicker. The majority of people in this world use long-cage derailleurs and are thrilled with them. So Shimano makes it's nicer derailleurs in long cage (SGS) and medium cage (GS) flavors for those who want the choice.

So, to end this mini-novel, an LX derailleur comes in 4 flavors:
M580 GS Low-Normal medium cage
M580 SGS Low-Normal long cage
M581 GS Top-Normal medium cage
M581 SGS Top-Normal Long cage. This is the one you should probably get, and in years past this was the only choice you would have ever had."

Also, on SRAM-Shimano compatibility:

• All Shimano shifters are compatible with SRAM front derailers; all SRAM shifters are compatible with Shimano front derailers.
• SRAM X.x shifters require SRAM X.x rear derailers.
• However, SRAM Attack and Rocket shifters are compatible with Shimano rear derailers.

Rear Derailers - Short Cage vs. Long Cage

I found this explanation on MTBR.com to be very informative!

"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.

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