To function properly, derailleurs need to have clean, smooth-running cables. As with replacing a chain, replacing cables is a maintenance operation, not a repair operation: Do not wait until cables break to replace them. Replace any cables that have broken strands, kinks, or fraying between the shifter and the derailleur. You should also replace housings if they are bent, mashed, or just plain gritty, or if the color clashes with your bike (this is really important!).
Campagnolo users should note that the Ergopower cable hook (i.e., the countersunk hole into which the cable head seats) is too small to fit a Shimano or other cable head—just another of those maddening parts incompatibilities. Be sure to buy a new Campagnolo-compatible shift cable before you begin work.
Install the cable from shifter to derailleur
- Disconnect the cable at the derailleur and snip off the cable-end cap (if installed).
- Click the shift paddle with repeated small pushes until it won’t let out any more cable.
- Push the cable until the cable head emerges from the cable hole at the base of the lever on the inboard side. Push the cable head out far enough to grab it. Pull out the old cable and recycle it.
- Push the new cable in through the hole and up through the lever body until the cable emerges from the housing entry hole at the upper base of the lever body on the outboard side.
- Route the cable to the derailleur. With external cables, guide the cable through each housing segment (making sure each housing segment has a ferrule on the end) and cable guide and cable stop.
Attach the cable to the derailleur
- Shift the rear shifter to the highest gear setting (smallest cog).
- If installed, put the chain on the smallest cog. The rear derailleur should be aligned under that cog.
- Route the cable through each of the frame cable stops and housing segments and the barrel adjuster on the back of the derailleur until you reach the derailleur’s cable-fixing bolt.
- On a SRAM rear derailleur, make sure that the cable wraps in the groove around the curved cable guide behind the fixing bolt and then routes above the bolt, to the side away from the jockey wheels. On SRAM X-Horizon derailleurs, the cable also goes through a little tunnel at the bottom of the curved cable guide on the back of the derailleur.
- Pull the cable taut and into its groove under the cable-fixing bolt. On all but SRAM rear derailleurs, the cable usually goes on the side of the bolt toward the jockey wheels. If you’re unsure about which side of the bolt the cable goes on, remove the bolt and look for a cable groove on the derailleur or the underside of the bolt’s washer.
- Tighten the bolt. On most derailleurs this requires a 4mm or 5mm hex key.
- Clip the cable 1–2cm past the bolt and crimp on a cap to prevent fraying.
Cutting cable housing to length
- Use a special cutter made for the purpose. Park Tool, Pedro’s, Shimano, SRAM, and Jagwire make good ones. Standard wire cutters (i.e., “side cutters”) will not cleanly cut index-shift housing.
- Cut the housing to the same lengths as the pieces you are replacing. If you have no old housings for comparison, cut the new pieces so that they curve smoothly. When you turn the handlebar, the housing should not pull from its cable stop. Allow enough length for the rear derailleur to swing backward and forward freely.
- Open each end of the Teflon liner that has been smashed shut by the cutter. Use a nail or toothpick.
- Place a ferrule over each housing end. The end at the lever probably will not need one; it depends on the lever, and most will accept only housing without a ferrule on it.
- After threading the cable into the housing, clip the cable 1–2cm past the bolt and crimp a cap onto it to prevent fraying.
Adapted from Zinn & the Art of Road Bike Maintenance, 5th edition, by Lennard Zinn, with permission of VeloPress.