One of the most annoying things that can plague a bike ride is shifting that isn’t working properly. When a derailleur has been adjusted correctly, a bike’s shifting will work flawlessly providing quick and accurate shifts whenever you need them. But when your derailleur’s screws are loosened by trail vibrations or your gear cable is at the wrong tension, a good ride can turn mediocre very quickly.
While there are a number of different derailleur manufacturers, they all use the same basic principles for adjustments and adjusting your bike’s shifting is a relatively simple process - but it can be tricky. Even for experienced mechanics, fine-tuning smooth shifting performance can be a trial-and-error process to diagnose what’s causing the shifting problem.
Luckily, you don’t need any complicated tools. You’ll really only need either an Allen key or Phillips head screwdriver that fits your derailleur’s bolt heads.
1. Rule out other problems
Before adjusting your shifting, check to see if the problem you're experiencing isn’t being caused by something else. One possibility is a bent derailleur or derailleur hanger. The hanger is bolted onto a bike’s rear triangle and is what the derailleur mounts to. It can be bent in crashes or impacts from trail debris. If the derailleur or its hanger is bent, they can sometimes be straightened. In the worst cases, the hanger or derailleur will have to be replaced, with the derailleur being the more expensive of the two.
It’s also worth checking if the bolt connecting the derailleur to the hanger and the hanger to the frame are tight. In rare cases, they can loosen by themselves, which will cause shifting problems. This can also damage the derailleur and hanger.
Another thing to check is the gear cable. If the cable is frayed at the end, it’s time to replace it. Plus, if the cable and housing have been ridden for a while and are getting old, replacing them will also improve shifting performance, and is relatively cheap to do.
Finally, shifting will be impacted by worn-out drivetrain components, like the cassette, chain, and chainring. A chain wear tool can be used to check this. If the drivetrain is worn down enough, some problems like skipping gears will never be solved without replacing old components.
2. Limit screws
There are two limit screws on a derailleur that sit right next to each other. The screws set how far the derailleur will travel up or down the cassette. They stop the chain from being thrown into the wheel’s spokes or jammed into the chainstay. They will often be labeled H and L, but if they aren’t, you can twist them and see which way the derailleur travels.
The L or Low-gear screw stops the chain from shifting into your spokes. This unpleasant occurrence will happen if the bolt is too loose. If your chain is being thrown into the spokes, tighten the screw until your chain can shift properly onto the largest sprocket, but no further.
The H or High-gear screw controls how far down the cassette the derailleur can shift. On modern derailleurs, looseness is not much of a problem, but if this screw is too tight, you won’t be able to shift into your smaller sprockets. Loosen it until you can.
3. Cable adjustment
If you can shift into both your highest and lowest sprockets correctly, the shifting problem will be one of two things. More than likely, it will be an issue with the cable tension, or indexing. All modern bikes have what’s known as index shifting, as opposed to the friction shifting of yesteryear.
Index shifting means that the shift lever will have a detent, or small groove, which tells the lever to stop. This way, each click of the lever corresponds with a sprocket on the derailleur.
Shimano was the first brand to bring index shifting to the mass market in the ‘80s, so your bike will have this type of shifting unless you’re working on a very vintage bike.
The indexing is adjusted by how long the gear cable is, a.k.a the cable’s tension. If the cable is too loose or tight the shifting will be off. Tension can be fine-tuned by barrel adjusters. Modern mountain bikes won’t have a barrel adjuster on the derailleur, but they will have one on the shifter.
Turning the barrel adjuster counterclockwise will tighten the cable and help the derailleur travel up the cassette. Turning the barrel clockwise will loosen the cable and help the derailleur travel down the cassette. This adjustment can be used when you feel the derailleur hesitating before it pushes the chain up or down a gear when you shift.
4. B screw
The other screw on a derailleur is the B screw, which controls the distance between the derailleur and the cassette cogs. If it’s too loose, the upper pulley wheel and chain will rub against the cassette. If this happens, tighten the screw, which will alleviate the problem and allow you to shift into your bigger sprockets. Too tight and the derailleur won't shift as smoothly as it could. There is no exact science to the correct position of the B screw but you should aim for a gap of around 5mm between the jockey wheel and largest cassette cog.
Modern mountain bikes rarely have front derailleurs, but a similar process is used to adjust them. Front derailleurs also have L and H limit screws, which work just as described above. The positioning of the derailleur is controlled with a cable. In even rarer cases, a bike with a 3x drivetrain system will have an indexing adjustment.
If you follow these steps, your shifting should be running smoothly. Even if it is a bit daunting, remember that practice makes perfect, and you can learn a lot by getting your hands greasy by experimentation.