Get your gears shifting sweetly: How to tune a rear derailleur

Making sure your rear gear indexing is properly adjusted means that next time you reach for the shifter to make a gear change you'll be guaranteed one click, one clean, crisp shift. It's a relatively simple job. Here's how to get it right.w

Our guide below shows you what we think is the best way to adjust and tune your rear indexed gears. We've included a list of the tools and materials that you will need to complete the job and in some cases where you can buy them. If there are others that you prefer then feel free to let everybody know in the comments below.

Indexed gears require accurate cable tension to deliver crisp, clean gear shifting. An under- or over-tensioned gear cable will throw the shifting out. The difference between perfect shifting and skipping can be as little as a single millimetre of cable so make small adjustments of a quarter turn to screws and barrel adjusters and test the effectiveness of the change before making further changes.

Tools 

• Allen keys (some derailleurs use Allen bolts instead of screws)
• Small crosshead screwdriver
• Cable puller
 

Checking for cable slack
If your bike is relatively new, or even if only the gear cable is, there’s a good chance that the metal inner cable has developed some slack (often referred to as 'stretch' although technically it's bedding in, as the cable ferrules, housings and stops all snug up to their minimum length). This is often enough to throw gear shifting out. If the gear cable has developed slack, you’ll need to remove it by pulling the inner cable through at the derailleur clamp bolt (see below).

An easy way to check is to put the deraileur in the smallest sprocket and use a steel ruler as a datum as shown. If the cable's not prallel to the ruler, then it probably needs tightening.
 

Barrel adjuster tension
You might also find that over time, cable tension adjustments have used up the range of adjustment provided by the barrel adjusters (at the derailleur and/or at the shifter depending on how your bike is equipped). As you can see above, this rear derailleur barrel adjuster is almost all the way out. This needs resetting at the cable clamp bolt (see above).

With the cable tension removed at the cable clamp bolt, re-set the barrel adjusters. Screw them all the way home (clockwise) into the derailleur or shifter body. Then screw them out (anticlockwise) one full turn. This will give you leeway to make fine adjustments in either direction as required.
 

High gear adjustment
With the tension removed from the cable, turn the pedals. The rear derailleur should move the chain to run on the smallest rear sprocket, and the upper jockey wheel of the rear derailleur should be directly underneath the smallest sprocket.

If it's not, make any adjustments necessary to sort out this alignment with the set screw marked 'H' on the derailleur body. Turning the screw limits the extent to which the rear derailleur cage, and therefore the chain, can move outwards. Turn the screw anti-clockwise to let the derailleur move further out. 
 

Low gear adjustment
Setting the low gear limit so you can shift cleanly onto the largest rear sprocket is important because failure to do so can allow the chain to over shift off the top of the cassette and into the spokes causing (occasionally terminal) damage to the rear wheel. The furthest inboard the rear derailleur should be able to move is the point where the upper jockey wheel is sitting directly underneath the centre line of the largest sprocket when viewed from the rear. Make any adjustments to achieve this position with the set screw on the rear derailleur body marked ‘L’. 
 

Setting cable tension
Use a cable puller (sometimes called a 'third hand tool') to pull the inner gear cable gently through the housing, making sure all sections of cable outer are located in their stops and that any cable guide grooves on the derailleur body are being correctly followed.

It’s the smallest things that can foul up indexing so take a second to be sure. You’re looking for just enough tension to take the slack out. As you make the adjustment, watch to see that the upper jockey wheel isn’t being pulled inwards from under the smallest sprocket. When the cable is free of slack and the derailleur and sprocket are still lined up, tighten the cable clamp bolt to hold the cable in place. Don’t over tighten the bolt – it's easy to strip threads and tear heads.
 

Check it
Rotate the cranks. The chain should roll cleanly on the smallest sprocket and through the rear derailleur cage without catching or skipping. Now try the full range of gears one at a time. You can occasionally get the cable tension spot-on first time, but you'll usually have to make some fine tuning adjustments to the cable tension with the barrel adjuster. You're looking for one single click of the shifter to deliver one clean gear change. A single index click, which delivers a gear change that is slightly over or under shifting means the cable tension needs fine tuning at the barrel adjuster.
 

Fine tuning with the barrel adjuster
Shift one gear from the smallest sprocket. The chain should move smoothly onto the adjacent sprocket. If it doesn't reach the adjacent sprocket, add a quarter turn of tension at the barrel adjuster (turn it anticlockwise). Similarly, if it shifts to the adjacent sprocket, but is making a noise as it tries to reach the third sprocket, you should release a quarter turn of tension at the barrel adjuster to remove the over shift (turn it clockwise). Whichever way you need to make adjustments, make them small; quarter turn adjustments are plenty. Continue until one click at the shifter delivers one clean (quiet, no skipping) shift to the adjacent sprocket.
 

Test ride your indexing
Ride the bike close to home and test the gears. Occasionally, freshly tuned gears can work faultlessly on the workstand, only to exhibit tuning small tuning niggles when on the road. Bikes with lots of sections of cable outer (including over or under length runs of cable), internal cable routes with funky stops, rear suspension or convoluted cable routing can all suffer in this regard. Make whatever remedial adjustments necessary.