The gears on your bike help you cycle through various conditions without losing efficiency. Whether climbing a steep hill or getting the most out of a sprint, being able to shift to the proper gear is important. A couple of signs that you need to adjust your gears include sluggish shifting (when you shift there seems to be a lag time before anything happens) or the inability to get into a gear when you shift.
Mount your bike on the bike stand to allow easy access to the gears. Slowly turn the crankarm and use the shifters, noting where your gears are having trouble. Examples could be an inability to get into top gear, the chain falling off going to a low gear or not being able to shift quickly between gears.
Locate the limit screws on your rear derailleur. There should be an H for high and an L for low. On Shimano derailleurs the top screw will be the high limit screw and the bottom one will be the low limit screw.
Shift onto the large chainring (chainrings are the front "gears"), using your left gear shifters. Turn the crankarm and use the right gear shifter to get into the smallest cog on your cassette (the back gears). Turn the high limit screw clockwise if the chain does not move easily (or at all) onto the smallest cog of the cassette. Continue making small turns until it does. If the chain falls off the smallest cog when you shift onto it, turn the high limit screw a quarter turn counterclockwise. Make small adjustments until the chain seats properly onto the cog.
Shift until you are on the smallest chainring and the largest cog on the cassette. If the chain is pushed beyond the largest cog or touches the spokes of your wheel, turn the low limit screw a quarter turn clockwise. If the chain isn't able to shift onto the largest cog, turn the low limit screw counterclockwise. Make small adjustments until the chain seats properly onto the cog.
Run through your gears once more. If the shifting still feels sluggish adjust the cable tension at the cable barrel. Most Shimano shifter have this where the frame has a cable stop (a notch to guide the cable). Tighten or loosen in small increments until the shifting issmooth and feels well-adjusted for your needs.
Locate the high limit and low limit adjustment screws on the front derailleur. They should be marked with an H for high and L for low. If there are no marks, a Shimano front derailleur will have the screw closest to the frame as the low limit screw and the screw furthest from the frame as the high limit screw.
Shift between your chainrings using the left-hand shifter. If the chain falls off the smallest chainring, turn the low limit screw a quarter turn clockwise. If the chain won't move over to the smallest chainring, turn the low limit screw a quarter turn counterclockwise. Make small adjustments until the chain seats properly onto the smallest chainring.
Shift between the chainrings again. If the chain is unable to climb onto the biggest chainring, or there is a delay between shifting and the chain moving over, turn the high limit screw a quarter turn counterclockwise. If the chain is jumping over the big chainring, turn the high limit screw clockwise a quarter turn. Make small adjustments as needed until the chain seats properly onto the biggest chain ring.
Run through your gears once more. If there is still a sluggish feel to the shifting adjust the cable tension at the cable barrel, located where the frame has a cable stop (a notch to guide the cable) for most Shimano shifters. Tighten or loosen in small increments until the shifting is smooth and feels well-adjusted.
You will need to change gears on the shifters and turn the pedals to do this project. Using a bike stand is the most convenient way to work on your gears. These instructions are for a road bike. Mountain bikes and hybrids will be similar except the cable barrel adjustment may be at the handlebars. This process takes patience and fine tuning, be sure to only make small adjustments at any one time.
Watch your fingers as you turn the crankarm, it is too easy to get a finger caught in the spokes of the spinning back wheel.