Cables play a crucial role in shifting road bike gears. Shifting gears affects tension on the cable. Cable tension moves the derailleur, which in turn moves the chain to the chosen gear. The cable will stretch over time, making it necessary to adjust the cable on occasion. Eventually, even if it appears in otherwise good shape, the cable will need to be replaced. Bicycling.com recommends changing cables on your road bike each season.
Shift the gear shifter until its cable has the most slack. For the rear gears, this would be the equivalent of shifting into the smallest rear sprocket. For front gears, this would mean the small front chain ring. Shifting into this position will make it easier to access the head of the shifting cable, which is anchored in the shifter body.
Loosen the shifter cable anchor bolt, using a 5mm Allen key. Each cable is attached to its derailleur by an anchor bolt.
Cut the metal crimp from the end of the shifter cable and pull the cable from the derailleur.
Squeeze the brake lever on the shifter. Inside you’ll see the head of the shifter cable. Push the old cable through the eyelet and pull it from the shifter.
Thread a new cable into the shifter. The new cable will pass through the same hole from which the old cable emerged.
Push the cable all the way through until its head nestles in place inside the shifter. Grabbing the other end of the cable, route the cable through the cable guide beneath the bike. A rear shifter cable will next pass through the cable housing at the back of the bike.
Pull the cable through its anchor bolt on the derailleur. Holding the end taut with a pair of needle-nose pliers, tighten the anchor bolt over the cable.
Cut the cable an inch from the anchor bolt and, using your needle-nose pliers, squeeze a fresh metal crimp over the end. The crimp will keep the cable end from fraying.
Before beginning work, memorise how your old shifting cables are routed.