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.

__________________
speedub.nate"

Admit It: You're Never Getting Over That Log in One Piece

(Note: this information is probably useful only to me, but this blog happens to be a convenient place to store it away for future reference...)

I run the big chainring on my mountain bike into a lot of stuff - downed logs, rocks, small rodents. Maybe it's time I bought a bashguard. Bashguards are sturdy pieces of plastic or metal designed to protect the chainrings on your mountain bike cranks. Normally, they replace the largest chainring, reducing the weight of the cranks and protecting the teeth of the smaller rings. Since I hardly ever use the largest ring on my mountain bike and routinely hop over a lot of logs, a bashguard might be a good investment. Some things to keep in mind:

• Bolt pattern: Cranksets use 4- or 5-bolt patterns (i.e. the chainrings are held to the crankset via 4 or 5 bolts). Only a bashguard with the same bolt pattern will fit on your crankset.
• Bolt Center Diameter (BCD): Distance from bolt to bolt on your crankset. As with the bolt pattern, the BCD of the bashguard must match that of the crankset.
• Tooth coverage: If you replace the largest ring with a bashguard, usually you buy a bashguard to fit the middle ring. Depending on the size of that ring (usually 32, 34, or 36 teeth), you'll pick your bashguard. Even if you're currently running a 34T middle ring, if you ever plan to put a bigger 36T ring on there, you'd better buy the larger bashguard so you don't have to buy another one later.

Of course, considering all the missing and bent teeth on my large chainring now, I already have a de facto bashguard, eh?

Indiana University's Little 500

I'm an unabashed fan of the 1979 film "Breaking Away." (Cali and I ate five boxes of Raisin Bran just for the box tops - so we could trade them in for a "free" copy of the movie!) Centered on the Little 500 bike race at Indiana University, it's a wonderful story about a young man, struggling with the transition from care-free teenage years to the real world. If you get a chance, rent it. In the meantime, enjoy this article by Jim Caple of ESPN about the race and the movie.