A Symposium on Shift Cables

A frayed, partially broken shift cable

A while back, I heard a participant in an indoor cycling class talking about shifting problems with her road bike. I could tell she was a distance rider with plenty of endurance rides under her belt, as opposed to someone who rides a few miles on the Monon Trail on holiday weekends. It was also pretty clear, though, that she had limited knowledge of how her machine works. I say that because her problem was a frayed shift cable, but she had no idea why it caused her problem!

My first road bike (a 1964 Garlatti) had shift levers on the down tubes: you could follow the entire length of the cables running from the levers to front and rear derailleur. Not so my current road bikes (a 1994 Trek 1400 and a 2015 Fuji Altamira): they both have Shimano STI® integrated shifters, with the end of the shift cable concealed inside the shifter beneath the rubber “hood.” Cables, in case you didn't know, aren't solid wires: they’re made of multiple strands of stainless steel braided together, which allows them to be flexible while minimizing stretch.

Each time you shift your bike, that cable bends slightly with the lever's rotation. After hundreds of these tiny movements, even steel strands become fatigued and break (see image): it's the same thing as repeatedly bending a steel paper clip. One or two broken strands will not affect shifting, but as more and more strands break, the cable becomes “stretcher.” A stretchy cable doesn't move the derailleur the expected distance, so your shifting becomes sloppy and the chain flops around on the cluster. No amount of fiddling with limit screws or barrel adjusters (you do know what those are, right?) will correct the problem: you have to replace the cable, preferably before it breaks completely.

Shift Cables

A deteriorated cable viewed in position in the shifter

There are three points at which shift cables are stressed enough to break: within the shifter housing (see image), at other end of the cable where it’s held taut by the pinch nut, and at the bottom of the frame where it wraps around the bottom bracket. I once had a cable for the front derailleur break at the bottom bracket, but have replaced several cables, both front and rear, that broke at the shifter. It’s fairly easy to check the end of the cable: it has a “knob” on the end to hold it in place, and usually breaks right at the knob. Peel back the rubber cover on the hood and squeeze the brake lever to get a look at the insides of the shifter: if you see a frayed cable, now’s the time to replace it before it breaks.

Cable Housings Old-timers like me will remember that in the days before indexed shifters, cables went around naked. Now they run through plastic-looking tubes from the shifter to a point somewhere on the down tube, and the rear derailleur cable has an additional short length of housing, too. These are the cable housings, and they are essential to the operation of indexed shifters. That's because they create an opposing force to the pull on the cable as you shift. Housings deteriorate through time, too, though I have only needed to replace my set once. Part of your regular maintenance schedule should be inspecting the housings for cracks or, in come cases, rust. If a housing comes apart, it won't allow the cable to move its derailleur.

Replacing a Shift Cable

Replacing deteriorated shift cable(s), and housings if necessary, is a task a competent DIY mechanic can perform. You’ll need new cables, of course, and a cable cutter tool, especially if you’re also replacing he housing. Regular wire cutters aren’t sharp enough to cut the cable and will crush the housing, though some say that bolt cutters work for cables. If you do replace housings, the new housing must be the same length as the one it replaces. New ferrules (the “caps” on the end of the housing) are also in order and are typically included with the housing.

The internet is lousy with videos and posts on how to change cables, so I won’t bother trying to tell you how (I use Zinn and the Art of Road Bike Maintenance, myself). What I want you to take away from this post is 1) that poor shifting may be a symptom of deteriorated cables and 2) it really isn’t that hard to replace the things.

Brake Cables

The cables for caliper brakes operate on the same principle as shift cables: there’s a braided stainless steel cable within a plastic housing of a specified length. These cables are heavier than shift cables, and the two are not interchangeable. Replacing them is similar to replacing shift cables.

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