The rear mech (or derailleur) is responsible for moving your chain across the cassette (the cluster of gears attached to the rear hub), while a spring-loaded cage containing a pair of jockey wheels compensates for the varying sizes of the sprockets and keeps the chain tight. If you're finding that your gear shifts are less than smooth it may mean that your gears are out of line and your rear derailleur may need checking.
Here are seven steps for smoother shifting.
1. Check your mech
If your shifting is ropey, the first thing to check is that the bolt securing the rear mech to its hanger is tight.
Use a 5mm Allen key (or a T25 Torx for some SRAM mechs) to turn it clockwise until snug. Also check that the mech hanger and mech itself are straight.
2. Check the gear cable
With the bike in a workstand or upside-down, turn the cranks with one hand and shift into the smallest cassette sprocket.
Have a look at the inner gear cable as it enters the mech. If it’s under tension, turn the barrel adjuster on the shifter fully clockwise so it goes slack.
3. Check small sprocket alignment
The mech’s upper jockey wheel should now sit in-line with the smallest sprocket.
If it doesn’t, use a Phillips screwdriver (3mm Allen key for some SRAM mechs) to adjust the screw marked ‘H’.
Turn it clockwise to move the mech towards the wheel or anticlockwise to move it away.
4. Check large sprocket alignment
Turning the cranks, push the mech towards the wheel with your hand until it’ll go no further.
If the upper jockey wheel isn’t in-line with the largest sprocket, turn the screw marked ‘L’ clockwise to move the mech away from the spokes or anticlockwise to move it towards them.
5. Check shifting
Still turning the cranks, use the shifter to shift the chain up and down the cassette.
If it’s reluctant to shift into the bigger sprockets, turn the barrel adjuster on the shifter anticlockwise.
If it’s reluctant to shift into the smaller ones, turn the barrel adjuster clockwise.
6. Adjust shifting
Still not shifting properly? Shift into the smallest sprocket, wind the barrel adjuster fully clockwise and then back it off two full turns.
Loosen the mech’s cable clamp bolt (5mm Allen key, anticlockwise), pull the cable through with pliers until taut, then retighten. Repeat step 5.
7. Replace cables
If you still can’t get your shifting feeling sweet at every part of the cassette and in both directions, it’s likely that sticky cables are to blame.
Worn-out gear cables cause friction that creates havoc with your shifting. In this case, replace the cables, then repeat step 5.