Indexing gears at home

Almost everyone who brings a bike in for skipping gears has already tried turning the little barrel adjuster, found it did not help, and given up. That is usually because the barrel adjuster is the last step, not the first one, and turning it while something else is wrong just moves the problem to a different sprocket.

Here is the order that actually works.

First, check it is not something else

Indexing cannot fix a bent hanger, a worn chain or a frayed cable, and trying will waste your afternoon.

Look at the rear derailleur from directly behind the bike. The two jockey wheels and the sprockets should all sit in one flat plane. If the derailleur is leaning inwards, the hanger is bent, and no adjustment will hold. That is a workshop job because straightening it needs a gauge and doing it by eye usually snaps it.

Then check the chain. If it is worn past 0.75 percent it will skip under load no matter how well indexed it is, and a new chain on a worn cassette will skip too. Checking chain wear covers how to measure it in about a minute.

Finally, shift into the smallest sprocket and squeeze a loop of the cable between the derailleur and the frame. It should feel smooth. If it feels gritty, the cable or outer is dying and needs replacing before anything else is worth doing.

Set the limit screws before you touch the cable

The two small screws on the back of the derailleur, usually marked H and L, decide how far it can travel. They have nothing to do with indexing and everything to do with whether your chain ends up in the spokes.

Shift to the smallest sprocket. Turn the H screw until the upper jockey wheel sits directly under that sprocket, no further out.

Now, and this is the safe way to do it, undo the cable pinch bolt so the derailleur springs to the small sprocket on its own. Push the derailleur by hand towards the wheel as far as it will go and set the L screw so it stops directly under the largest sprocket. If it can go past that, it can go into the spokes, and that ends a ride and often a wheel.

Now the cable

With the shifter in the position for the smallest sprocket, pull the cable through the pinch bolt with your fingers until it is just taut, not tight, and do the bolt up. Finger tight on the cable is genuinely correct here. If you pull it with pliers you will over tension it and every following step will fight you.

Then the barrel adjuster, one click at a time

Turn the pedals and shift up one sprocket. If the chain does not move, or moves reluctantly, turn the barrel adjuster anticlockwise by a quarter turn and try again. If it jumps two sprockets, turn it clockwise.

The rule worth memorising: anticlockwise adds tension and moves the chain towards the wheel.

Work up the block a sprocket at a time, then back down. Adjust in quarter turns and stop as soon as it shifts cleanly in both directions. It is normally three or four quarter turns in total.

Check it under load

A drivetrain that shifts perfectly in a stand sometimes skips on a hill, because pedalling hard flexes the frame and the chain. Ride it, get out of the saddle in a middle gear, and push. If it skips there and nowhere else, the chain is worn.

If it still will not hold

Then it is one of the three things at the top, and almost always the hanger. That is worth a workshop visit rather than more adjusting, and it is what our brakes and gears service exists for.

There is no shame in that. We check the hanger first on every single bike that comes in for gear trouble, because it is the thing you cannot see and cannot fix by feel.