Indexed gearing is great. Each click of the shift lever moves the chain smoothly from one sprocket to the next. When that doesn't happen, you need to adjust the gears. We've described the rear derailleur here, but you can apply the same advice to the front derailleur.
Gear shifting simplified
A gear cable is a thin inner wire that runs from the shifter to the derailleur. For part or all of this distance, it runs inside a hollow outer casing. This means the cable can take a route to the derailleur that involves curves. The gear cable is always under tension. The gear shifter winds the cable in and out, increasing or decreasing that tension. This moves the derailleur sideways. Most shifters are indexed, so that each click winds the cable a set amount... which in turn moves the derailleur a set amount.
The barrel adjuster
The usual problem is that the shifter no longer moves the derailleur the right amount. So the chain rattles on the sprockets and skips in between gears, or requires two clicks to shift. The cause is most likely the cable tension. Find the barrel adjuster: a hollow bolt that the outer casing runs into, located at the shifter, the derailleur, at a metal socket on the frame called a cable stop, or part way along the outer casing.
Unscrewing a barrel adjuster increases the gear cable tension, and vice-versa. Turn the pedals with one hand while changing gear with the other; you'll need an assistant or a workstand to lift the back wheel off the ground. First change gear until the cable is at its slackest; this will be the smallest sprocket unless your bike has a 'low normal' or 'rapid rise' derailleur (in which case it will be the largest). Click the shifter once. If the chain doesn't shift to the next sprocket without hesitation, unscrew the barrel adjuster incrementally until it does. Check that the chain will shift back the other way with one click. This may mean screwing the barrel adjuster back in a little. Shift up and down the cassette to check that all the gears can be engaged with single-click shifts. Further half or quarter turns of the barrel adjuster may be required.
Re-clamping the cable
If the cable was very slack or over tight, the barrel adjuster might not offer enough adjustment. Shift gear until the cable is at its slackest and screw the barrel adjuster(s) all the way in. Using an allen key or small spanner, unscrew the anchor bolt to release the gear cable from the derailleur. Check that the chain is on the smallest sprocket. Hold the end of the gear cable and pull it until it is only just taut, then re-clamp it. Fine-tune the gearshifting as described on the previous page.
Sometimes the derailleur will shift okay one way but is slow to shift the other way; you click and nothing happens and then click again and the chain jumps two sprockets. The problem is friction, probably caused by dirt. You need to lubricate the cable. This is easy on bikes that have the gear cable partly uncovered. Shift gear until the cable is at its slackest. Disengage the outer casings from the gear cable stops on the frame, drawing the exposed gear cable through the slot. The casings can now be slid up or down the inner cable. Lubricate the gear cable and spray lube into the ends of the outer casings, using GT85 or similar. Then slot the outer casings back in the cable stops on the frame. Shifting still sluggish? You may a new gear cable and outer casing. Go to the bike shop.
Sometimes the chain shifts too far and falls off the biggest or smallest sprockets entirely. Or it won't shift far enough, whatever you do to the cable tension. You need to adjust the high (H) and low (L) limit screws on the derailleur. Viewed from behind the bike, the H screw limits how far the derailleur can move to the right, while the L screw limits how far the derailleur can move to the left. Unscrewing lets the derailleur move further in that direction; screwing in restricts movement in that direction. Make small adjustments, half or quarter turns, until the derailleur stops in the right places at either end of the cassette.
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