How it works:
The rear derailleur is there to move your chain onto different gear sprockets. A cable runs from your gear selectors (shifters) to the derailleur (mech). Each ‘click’ on your shifters should pull (or release) the cable the exact amount required to move the mech to the next gear. The amount of movement is fixed, according to your shifters. 8-speed shifters will move the cable the right amount for 8 geared bikes. Likewise for 5,6,7,9,10 and 11 speeds.
The gears on your back wheel come as a set of sprockets (cogs), called a cassette. If the cogs are worn out they can be replaced with special tools. Sometimes it’s possible to replace individual cogs, but most people will replace the whole cassette. You need to fit a new cassette the same number of gears as the old one. It may be possible to fit a cassette with more (or less) gears, but you will also need to buy a matching shifter.
The cables are normally made up of two parts: the inner cable, a shiny wire cable made of twisted steel strands; and an outer cable made of straight steel strands coated in tough plastic. The inner cable slides in and out of the outer cable, and it’s this movement that transmits the gear selection from the shifters to the mech.
The mech itself is made of many parts, but these can be simplified into the body, jockey wheels (the small pulley wheels that guide the chain) and adjustment screws.
Possible adjustments you can make:
There are quite a few adjustments you can make so, before we get down to how to set up your gears, here’s a quick run-down of what each screw does.
B-Screw: this adjusts how close your mech get to the gears on your cassette. You should tighten/loosen this screw so that the Jockey Wheels are clear of the Cassette, normally you aim for a gap between 1mm and 6mm, depending on manufacturer.
H-Screw: this is a ‘limit’ screw. It stops the mech travelling too far off the end. Without this it would be possible for the chain to be shifted off the cassette to get stuck in the frame.
L-Screw: another limit screw, but for the other side of the cassette. It stops the chain being shifted into the spokes.
The H and L screws will often be placed next to each other, with the B screw out on it’s own. Actual position will vary between each mech.
This lets you fine-tune your gears using just your fingers and, once the limit screws are properly set, is probably one you will fiddle with most. Sometimes you get a barrel adjuster on either the shifter, the mech, inline with the cable, or a combination. Winding the barrel adjuster IN will move your mech & jockey wheels towards the smallest cogs. Wind it OUT to move towards the bigger cogs (opposite way round for the few people who use LowNormal or RapidRise). The aim is to use the barrel adjusters to get the jockey wheels lined up with one of the cassette sprockets for perfect shifting. Once it’s lined up with one sprocket the shifter will ensure the others line up.
Before you start:
Make sure that your cables are free of rust, gunk, kinks and frayed ends. Anything that impedes the smooth running of your cables will cause your gears to misbehave.
Also make sure that the jockey wheels are perfectly straight. With the bike vertical the jockey wheels should be perfectly vertical too, see pic ->
Cables on poor condition can be cleaned or replaced, and bent mechs can be straightened. If in doubt contact your LBS (Local Bike Shop), they’re there to help.
When changing gear (while riding or while setting up) you must always keep the pedals spinning slowly! Use the brakes to slow/stop the wheel if it’s spinning too fast.
How to adjust the indexing of your gears:
Pop your bike in a stand so you can spin the pedals while stationary. If (like most people) you don’t have a stand then turn your bike over and rest it on it’s bars & saddle, making sure you first remove any lights/bell/gps etc. that could get damaged if resting on the ground taking the weight of your bike.
While slowly turning the pedals click your rear gear shifters to release the tension in the cables. On most bikes this will change gear on to the smallest cog (highest gear). If you have three gears on the front then select the middle one using the same process.
Screw the barrell adjuster(s) all the way in (clockwise) and turn the pedals again a few times.
If the chain is not on the smallest cog then you need to unscrew the H-Screw until it is. The chain needs to be able to drop onto the small cog but not be able to overshoot so it drops in betwen the small cog and the frame.
If you still can’t get the chain on the smallest cog then you will have to undo the cable from the mech to release any tension and re-tighten.
Once you have the smallest cog selected it’s time to adjust the indexing. These next steps can be carried out when you are out on the road/trail if you need to fine tune further.
Now click the shifter once to select the next gear. If the next gear is not selected then un-twist the barrel adjuster until the gear changes (remember to keep slowly turning the pedals, and watch your fingers).
The aim is to use the barrel adjuster to get the jockey wheels in perfect alignment with one of the middle gears, as in the picture. Try again for a few more gears, going up and down the gears until you get it right. It’s often best to listen for perfect shifting, which will be crisp and precise. Badly aligned gears will make a clattering noise.
Now we can check the L-Screw – Carefully shift onto gear number 1 (the easist/largest rear gear).
Make sure that the chain can’t over-shift into the spokes. If it can, then tighten the L-Screw until movement is limited.
If you can’t get into the biggest cog then you may need to loosen the L-Screw, but first run through all the previous steps again to make sure that your highest/smallest gear is selectable and that all the others line up.
There are a number of free videos on the internet. As a starter, check out this short and easy to follow video: http://www.youtube.com/watch?v=SjJfKO_tAo0