The humble rear derailleur (or ‘mech’ to its friends) is a remarkable beast. When you think about what it’s trying to do it’s a wonder it works at all, let alone as well as it does. They don’t like going without periodical TLC, though, so if your shifting’s gone a bit awry here’s how to sort it.
1. It’s a nice straightforward job tools-wise, this one. You’ll need a 5mm Allen key, a cross-head screwdriver and a pair of pliers (unless you have fingers of steel).
2. First things first – get your bike in some sort of stand that lets you spin the pedals and back wheel. It makes things a lot easier. If you don’t have a workstand you can try hanging the bike off a loop of rope, drafting in a willing volunteer or quickly growing a third arm.
3. The first thing to get right is the upper and lower limits. Left to its own devices the rear mech’ll happily shift the chain beyond the top and bottom sprockets on the cassette. This is clearly a bad thing. Fortunately it comes equipped with two screws to limit the throw of the mech. The high limit is the easiest to set. Shift into the smallest sprocket (also known as ‘top’ or ‘high’) and twiddle the high limit screw (the one marked ‘H’ until the top jockey wheel is directly below the sprocket. Cable tension may prevent it moving out far enough, in which case you can undo the clamp bolt – we’ll be setting that bit up in a minute anyway.
4. In theory you should do the low limit screw next, but we always find it easier to do that after the cable tension’s sorted out. So that’s the next job. Make sure the cable’s clean and running smoothly. If it’s sticky or gritty you’ll never get it to work. Once you’ve got free-running cables, make sure the barrel adjusters on the mech and shifter are wound nearly all the way in (one turn out gives a bit of useful scope for adjustment).
5. Then thread the cable through the barrel adjuster and cable clamp, pull it taut with pliers and do up the clamp bolt with a 5mm Allen key (or an 8mm spanner on some mechs). Don’t pull the cable too tight, just take the slack out of it. And make sure that all the cable housing is properly seated in the cable stops.
6. Now it’s time to see how close you are. Spin the cranks and try shifting from the smallest sprocket to the next smallest. If it goes straight in, shift back, wind the barrel adjuster in (clockwise) a quarter turn (the adjuster’s usually indented so you can easily count turns) and try again. At some point it’ll refuse to shift, so turn it out (anticlockwise) again. Similarly, if it won’t shift on to the next sprocket straight away, turn the adjuster out until it does. If you have to turn it miles, wind it all the way back in and retension the cable at the clamp bolt.
7. After a bit of twiddling you should have a mech that happily shunts the chain up and down the cassette without hesitation either way. Now it’s time to sort that lower limit screw out. Carefully shift into the biggest sprocket. Go one click at a time, if the limit screw’s way out you could easily shift over the end into the wheel. Once the chain’s on the big sprocket, turn the lower limit screw clockwise until the mech just starts to move outwards, then turn it back a whisker.
8. All we’re left with now is the mysterious B-tension screw. This controls the angle of the mech body itself and thus the vertical gap between the top jockey wheel and the sprockets. You should get the best shifting with the smallest gap you can manage without the jockey wheel actually touching the sprockets. Turning the B-tension screw anticlockwise allows the mech to swing further forwards, closing the gap. If the jockey wheel gets too close, turn it clockwise until it clears. And that should be it…
Usually that’s all there is to it. Sometimes, though, it just won’t work. Here’s the common problems and some possible answers:
|Mech won’t shift into smaller sprockets||Turn barrel adjuster clockwise|
|Mech still won’t shift into smaller sprockets||Clean and lube cables|
|Mech won’t shift into larger sprockets||Turn barrel adjuster anticlockwise|
|Mech won’t shift into smallest sprocket||Check high limit screw|
|Mech won’t shift into biggest sprocket||Check low limit screw|
|Constant clicking in low gears||Jockey wheel hitting cassette – adjust B-tension screw|
|Mech won’t work no matter what I do||Jockey wheels possibly worn. Replace.
Mech itself possibly worn – can you wobble the cage from side to side lots?