Size Matters

 Bikes  Comments Off on Size Matters
Aug 202011
 

Question:
I need to replace my rear derailleur. What are the differences between the short/medium/long cage? Does the length of the cage matter that much?

Answer:
The easy answer is, if you have to ask, get a long cage. A long cage will work in every situation. I’m one of those “teach a guy to fish, so he quits asking me to give him fish” types, though, so let’s impart us some knowledge.

Understanding the differences in cage length means understanding both things a rear derailleur does. We all know a rear derailleur moves the chain in and out from one cog to another, but the other, equally important thing a derailleur does is move forward and back, taking up chain slack that varies as you change gears. Imagine your geared bike as a singlespeed with a bunch of gears in the back, but no rear derailleur. Suppose you had a chain that was the exact perfect length to wrap around your 32-tooth ring up front and your 34-tooth cog in the back without being too tight, or too saggy. Great. Now leave the chain on the same ring up front, but move it to the 11-tooth in the back. Saggy, isn’t it? If only there was a way have the same length chain adjust itself for smaller or larger gears. Luckily, a bunch of people in the 1800s had the same problem, and realized some sort of spring tension on the chain was the answer. The bigger the difference between the sizes of your gears, the more chain you need your derailleur to be able to take up. The ability of a rear derailleur to handle a range of chain lengths is referred to as the derailleur’s “chain wrap capacity.” You can calculate your chain wrap capacity by subtracting the number of teeth on your largest front chainring from the number of teeth on your smallest front chainring, doing the same with the gears on your rear cassette, then adding the two values. So necessary chain wrap capacity on a bike with a 44-tooth big ring, 22-tooth small ring, and 11-34-tooth cassette, would work out like this:

(44-22) + (34-11) = 45

The longer the cage of a rear derailleur, the more flappy chain it can pull tight, so the wider variance you can have between gears. Sometimes a manufacturer will tell you the chain wrap capacity of their rear derailleur, but the bottom line is you should always be sure you have enough chain to reach every gear combination—even the crossed-up ones you shouldn’t use, but might shift into accidentally. For most mountain triples, that amount of chain requires a long cage rear derailleur, and those work just fine.

So why do people try to use any shorter cages? In theory, a medium or short cage derailleur may shift just a fraction better because a shorter cage equals a stiffer cage, and they’re slightly less vulnerable to getting smote by rocks and the local flora, so experimentation is fine, provided you’re careful. In fact, most configurations will accept a medium cage rear derailleur, but only if chain length is set up carefully, and you error on the side of leaving more chain than you think you need. It also helps if you get rid of your biggest or smallest cog to lower that required chain wrap number. For most people, the added risk and configuration challenges aren’t worth the time, making the long cage the go-to option.

Sticker Shock

 Bikes  Comments Off on Sticker Shock
Aug 152011
 

Question:
I have acquired a used XT 770 crankset. The big ring is trashed and I am going into sticker shock at Shimano’s replacement prices. Anyone have luck with other brands i.e. Blackspire, FSA, Race Face for this particular model? I have a hard time throwing down 60-100 dollars for a chainring that is not going to last long.

Answer:
I sympathize with your plight. Shimano does seem to subscribe to the same OE replacement parts pricing as car dealerships and national defense contractors. The big ring on a 770 series crankset actually has a 104mm 4-bolt pattern, the same pattern used by most other brands, so other rings will match up, but be careful. Much in the same way you can’t imagine being married to a unicorn or living in a world where dogs can drive, Shimano can’t conceive of a world in which people buy FSA or Blackspire rings and bolt them to their XT cranksets. If you know the history of Shimano chainrings over the past few years, you’ll appreciate what a miracle it is that even the bolt pattern happens to match now.

So the good news is that nearly everyone makes an aftermarket 104mm bolt-pattern 4-bolt 44-tooth ring that will match up to the bolt hole pattern on your 770 crankset. The bad news is that it won’t work as well as the Shimano. This is partly because Shimano just makes excellent cranksets and chainrings, but mostly because all Shimano parts are designed specifically to work only with other Shimano parts. They don’t intentionally prevent other rings from bolting on, but they don’t test for them either. This means that, even if something else can technically bolt up, it probably won’t mesh all pro-like to the crankset’s spider–it will most likely be slightly too wide or too narrow, and some rings may even need sections filed down if they’re making contact with the Shimano spider or arm itself. Generally, a Shimano crankset with another company’s ring on it also won’t shift as well, because chains, chainrings and cranksets are one of the things Shimano still does extremely well.

So it comes down to you: if you’re the ultra anal-retentive type who demands perfection, you should shell out for the Shimano ring. If, on the other hand, you’re more the frugal rebel type who can smile while pressing a shifter paddle a bit harder to convince the chain to shift into a less expensive big ring, look for the most basic-looking 44t 4-bolt, 104mm bolt pattern ring you can find and get your rogue on.

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