Manic Mechanic – With derailleurs and bottom brackets, size does matter

Deus ex machina: Chris Currie

In need of technical assistance? Send your questions to [email protected]. Please include "Manic Mechanic" in the subject line.

Size Matters

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?

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 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. 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 shorter cages? In theory, a medium or short cage derailleur may shift 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 err 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.

“Standards”

There are too many damn “bottom bracket standards!” I can’t keep them straight and I don’t really understand why we need them! Can you set me straight on the difference between BB30 and Press Fit 30, if there is a difference?

Agreed. The word “standard” isn’t necessarily the most accurate way to describe current bottom bracket or headset systems. While the smoke is clearing, never assume any two systems are compatible, let alone synonymous.

A “BB30,” which uses circlips to retain bearings inside the frame’s bottom bracket shell, is not the same as a “Press Fit BB30,” which presses into the frame in a way similar to conventional headset cups. The goal in both cases is the same: bigger diameters equal stiffer structures, which equals more power.

High end road bike manufacturer BH has just developed a BB30 variation that allows extremely light frame designs, and we can probably expect still more systems to appear in the marketplace, because the basic idea behind oversized bottom brackets is sound: bike frames and crankset spindles need the largest surface area possible to ensure maximum power transfer, and the bigger the diameter of the bottom bracket spindle holding the crank arms together and larger the bearings, the more power you can squeeze from every watt being exerted. Plus, you get a stronger and more durable system.

Basically, we’re just using bigger bottom bracket spindles these days, which caused us to find larger bearings, which in turn caused us to need a bigger diameter hole in our frames. Old designs enlarged the spindle, but not the frame, which meant tiny bearings that wore out even faster, but by enlarging that whole bottom part of the frame, we can now fit enormous bearings inside that frame shell, where they’re better protected from the elements and lighter, because neither the cups nor the frame require threads.

Tucking large bearings inside the bottom bracket shell of the frame also allow the frame to be wider in that key area, which makes for a much stiffer overall frame. This is the same thing we saw happening when the industry began to move from smaller diameter steel frame tubing to oversized aluminum tubing. Increasingly lighter materials require larger and larger diameters. That’s what we’re seeing in the current bottom bracket “standards.”