Shifters control the derailleur movements on a bicycle, according to industry recognised mechanic Sheldon Brown. Shimano 105 shifters adjust the derailleur mechanisms moving the chain between gears on a road bicycle featuring drop handlebars. The right-hand shifters controls the rear derailleur, and the left-hand shifter controls the front derailleur. Shimano 105 shifters feature little in the way of repairable or adjustable components. Replace broken shifters or adjust derailleurs if shift mechanisms perform poorly. The most common shifter malfunction arises from dirty or corroded cables running between the shifter and the derailleur.
Loosen the hex head bolt clamping cable at the derailleur.
Remove the shifter if broken. Shifters incorporate hidden hex head bolts under rubber brake hoods, the hand grip of the shifter. Insert and turn the hex head wrench counterclockwise to loosen the bolt. Move to Step 6 if shifter needs adjustment only.
Loosen shifter bolt entirely and pull shifter with cable free from the bicycle. Replacement shifters typically come with new cables.
Lubricate cable housings, typically black tubes, before installing new shifter and cable.
Reattach new shifter and cable exactly like the old shifter and cable.
Ensure cable tension by pulling cable taut before tightening, turning the hex head bolt clockwise (this is the cable attachment bolt at the derailleur).
Set the derailleur high-limit screw. Turn the derailleur screw marked "H" clockwise until the chain remains on the highest gear without jumping off when the shifter is set to high. If the chain attempts to move to second highest gear or rubs against second highest gear, turn the screw counterclockwise at quarter-turn intervals until the chain remains on the highest gear.
Set the derailleur low-limit screw. Turn the derailleur screw marked "L" clockwise until the chain remains on the lowest gear with shifter in low setting. If the chain attempts to move to the second-lowest gear or rubs against the second-lowest gear, turn the screw counterclockwise at quarter-turn intervals until the chain remains on the lowest gear.
Shift smoothly through the gears one click at a time. Adjust cable tension using barrel adjusters, threaded cable stops allowing for minute cable adjustments, attached to the cable housing at various points of the bicycle. These points may differ from machine to machine. If the chain attempts to move two gears with a single click, loosen the barrel adjuster to add slack to the shift cable. If chain does not move one gear with each click, tighten the barrel adjuster to add slack to the cable.
Lubricate the derailleurs, chains and cables to ensure smooth shifting. Allow penetration for several minutes and wipe excess lubricant from parts. Repeat lubrication often. Many common squeaks arise from poor lubrication, according to bicycle mechanic and former editor of Bicycling Magazine Jim Langley.
Shimano typically warranties only parts worked upon by professional bicycle mechanics in authorised bicycle shops. Check warranty before beginning any Shimano shifter work.