Indexing your rear derailleur

One of the most common issues you’ll experience whilst out and about on the bike is that your gears start to misbehave, jump about or not shift up or down when you ask them to.

Unless you are blessed with an electronic group set like Shimano’s Di2 system, your gears are going to be controlled by cables. Unfortunately, over time these cables can stretch, fray and drag muck and grit into the plastic cable outer, meaning that they no longer work as efficiently as they should.

If you’ve just bought a new bike, or recently had your gear cables serviced, you’re probably going to be asked to go back to the shop after about 6 weeks to have your gears indexed again. Fortunately, you should be able to have a good go at doing this yourself, even while you are out on a ride. It may only need a half-turn of the barrel adjuster…

  1. Turn the bike over, or if you have a stand handy, mount your bike onto that.
  2. Select the smallest cogs at the front and at the back (Don’t ride like this though, ever! This is known as cross-chaining and it’s a pretty bad idea…)
  3. Turn the pedals forwards and ensure that on the lowest cog the chain is sitting quietly & not jumping about. If it is trying to shift upwards, slacken the cable using the barrel adjuster near the rear derailleur. If the cable is slack but the chain still isn’t sitting on the cog, you will need to adjust the “L” limit screw until it does.
  4. Shift up one and keep turning the pedals. If the chain doesn’t jump immediately into the 2nd cog, you’ll need to dial in some extra tension using the barrel adjuster. Keep turning the pedals whilst turning the barrel adjuster gradually until the chain is sat quietly on the 2nd cog.
  5. Shift down again whilst still turning the pedals, ensuring that it shifts down quickly, before shifting back up again. If it is still a little reluctant to shift, dial in a little more tension on the barrel adjuster.
  6. Repeat this process until shifting up & down happens quickly & smoothly, before shifting up to the top of the cassette and back down again one click at a time to make sure it’s right.
  7. Shift into the top ring on the front (if you have one) and repeat the process.

A couple of things to note. If you are riding in the smallest cogs at the back, you should be in the largest chainring at the front. The same rule applies the other way around –large cogs at the back, small on the front. You want to avoid cross chaining, which causes undue stress on your components by forcing your chain into a sort-of “S” shape when it should be a straight line. Chances are, your front derailleur will rub against the chain whilst in this position anyway.

If you find your chain falling off a lot, chances are your limit screws need adjusting. The screw marked “H” controls movement over the largest cog, “L” the smallest.

You should also keep an eye on the shape of the teeth on your rear cassette. Your chain should be replaced every 6-12 months depending on mileage to ensure the lifespan of your rear cassette. It stretches over time, meaning that your chain no longer sits snugly over the teeth of your chainrings & cassette. If the teeth start to resemble shark fins or the chain starts to slip whilst pedalling, you’ll be needing a new chain and cassette pretty soon.

Finally, if nothing you do will get the derailleur to behave, take it to your local bike shop. It’ll likely cost no more than about £15 to have your gears serviced and should tease out other issues like spring tension, wear and tear and cables that are past their best.

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