A Shimano 105 is a bicycle gear set designed for a road bike. The gear system incorporates two cables, one that operates the brake and one that controls the derailleur. The derailleur is the mechanism that moves the chain to a given sprocket. The derailleur cable is also known as a gear lever cable. Generally, gear lever cables are replaced as part of a yearly tune-up or if the cable is frayed or damaged at any point.
Take the bike for a short ride and shift into the gear that gives the gear lever cable maximum slack. For the front gear, maximum cable slack corresponds with the lowest gear. For the rear gear, maximum cable slack equals the largest gear.
Loosen the bolt on the derailleur that secures the shifter cable with a 5mm (13/64 inch) hex spanner. The shifter cable is attached at one end to the Shimano 105 gear lever and at the other end to the derailleur.
Cut the metal cap, called the "ferrule," away from the end of the cable, using cable cutters. Grab the cable where it enters the derailleur and pull it free.
Squeeze the gear lever on the Shimano 105 gear set to expose the head of the cable. The cable head is located on the side of the top of the gear lever.
Grab a portion of the cable and push it forward, forcing the head of the cable out of the Shimano 105 gear lever. Grab the head of the cable and pull the cable free.
Thread the replacement cable through the hole in the Shimano 105 gear set. This is the same hole from which the old cable emerged. The cable has two ends, with a metal fitting at one end, which is designed to anchor the cable inside the mechanism.
Pull the cable taught until the metal fitting is fully anchored.
Pass the cable along the bicycle. The cable follows the exact route of the old cable, passing through the cable housing and cable stops before arriving at the derailleur.
Thread the cable beneath the derailleur bolt and pull the cable tight.
Tighten the derailleur bolt onto the cable, using the 5 mm (13/64 inch) hex spanner. The cable will be excessively long and will need to be cut.
Cut the cable just past the derailleur bolt, using the cable cutters. Leave enough cable, approximately 5 to 7.5 cm (2 to 3 inches), to allow for future adjustments of the cable.
Press a new ferrule onto the tip of the cable. Squeeze the ferrule onto the cable using pliers.