Derailleurs are a fascinating and cherished part of your bike, prone to mood swings and baffling inconsistencies. This piece was first published a few years ago, but with spring around the corner and readers dusting off and tuning up bikes again, we’re putting it back on the homepage. -Ed.
The only things guaranteed in life are death and taxes and lousy shifting. Your bicycle’s rear derailleur can be a fickle beast—there’s little margin for error in its set-up, and time, cable stretch, dirt, and moisture all inevitably transform crisp shifting into just so much skipping, grinding, and stuttering. If you don’t feel the master of your drivetrain, rest assured-you’re not alone on this one. Plenty of riders are flummoxed by this bit of their kit. Fortunately, getting your derailleur back into shape isn’t as hard as it may seem.
THE QUICK-AND-DIRTY TUNE
Let’s say your rear derailleur began its life clicking precisely along but has gradually become fussy when you try to shift the chain onto the bigger cogs in the back. If that’s what happened to you, resolving the problem may just be a matter of slightly increasing cable tension. New cables inevitably stretch over time, which decreases tension in the system and causes the shifting to hesitate and stutter as you attempt to shift to the larger rear cogs (the lower gears).
First, shift down to your smallest rear cog. Now, twist the barrel adjuster (at the shifter) counter-clockwise a quarter of a turn. Go ahead and shift through the gears again. If the chain is still hesitating when you shift onto your bigger rear cogs, give it another quarter turn counter-clockwise and run the derailleur through the gears again. Still hesitating on the upshifts? Give it one more go, then resign yourself to doing the whole enchilada tune-up (see below).
THE WHOLE ENCHILADA TUNE-UP
The first thing you need to do is check to make sure the set screws are properly adjusted. These are the two screws on the rear derailleur (the exact location varies from brand to brand) that dictate how far the rear derailleur can move in either direction.
The screw marked “L” limits how far the derailleur can move inward. When it’s properly adjusted, it keeps your derailleur from shifting into the spokes and devastating your rear wheel and bank account.
The “H” screw limits how far the derailleur can move outward. If it’s not properly adjusted, the chain will dump off the smallest cog and tear up your expensive chainstay.
In short, these two tiny screws are very important, even though they only control how the derailleur behaves in two gears (the largest and smallest rear cogs). The good news is that once you set these screws, they almost never need readjusting (unless someone tries to help you and starts randomly fiddling with them or you crash and bend your derailleur hanger, which is a whole ‘nother world of problems).
Let’s start with the H screw. Put your bike in a work stand-it needs to be elevated so that you can listen to and observe the chain as you turn the pedals. Shift the rear derailleur into the highest gear (the smallest rear cog). Now, make sure that the rear derailleur cable (the actual wire) is just slightly slack. If it’s under tension, turn the adjuster barrel on the rear shifter clockwise until the cable is no longer taut.
Now look at the rear derailleur’s upper pulley: It should line up directly beneath the smallest cog and, when you turn the pedals, you shouldn’t hear any rubbing or grinding. If the derailleur pulley is sitting to the left of the small cog, you’ll probably hear the chain’s inner plates rubbing against the second to smallest cog. Remedy this by turning the H screw counterclockwise a quarter turn at a time while pedaling until the rubbing ceases. Keep eyeing the derailleur pulley-again you want that upper pulley to sit directly beneath the smallest cog and there should be no grinding or skipping from the chain.
Now it’s time to set the L screw. You need to shift the rear derailleur so the chain is riding on the largest cog. Again, you want the derailleur’s upper pulley to sit directly beneath the cog in question. If the pulley sits to the left of the largest cog, even by a few millimeters, the chain can be sent into the spokes and cause a horrible wreck. Consequently, I tend to turn the L screw clockwise by quarter turns (while pedaling the bike) until I hear the sound of the outer chain plates beginning to rub against the second-largest cog. At that point, turn the L screw counterclockwise by 1/8 turns, just until the noise disappears. The pulley should be properly aligned now without any risk of the chain going AWOL into the spokes.
THAT ODD SCREW NO ONE TALKS ABOUT
Next, you want to check the B-screw – that odd, lone screw that juts out near the point where the rear derailleur bolts into the frame. The B-screw dictates how far that upper pulley sits from the cogs. You want about a six-millimeter gap between the upper pulley and the largest rear cog. I often take my chain off to check the gap, but if you don’t want to bother with that step, just make sure that the pulley isn’t rubbing against the cog itself. If it is, turn the B screw clockwise by quarter turns until the noise stops.
SHIFTING FROM COG TO COG
We’re finally ready to perfect how the derailleur shifts between all the cogs. This is probably the step you were really interested in, but if you don’t nail the prior steps, you’re just shooting yourself in the foot.
Perfect shifts are all a matter of achieving just the right amount of cable tension. Start by shifting the front derailleur so the chain is sitting on the outermost (largest) chainring and shifting the rear derailleur so that the chain (at the rear of the bike) is riding the smallest rear cog.
While turning the pedals, push the larger paddle on the rear shifter so that the rear derailleur shifts the chain up to the second smallest cog. If the chain didn’t budge or only haltingly jumped up one cog, keep turning the pedals while increasing the cable tension (by turning the barrel adjuster at the rear shifter counter-clockwise) until it does. Continue to slowly turn that barrel adjuster counterclockwise until the inner plate of the chain just begins to rub against the third cog. At that point turn the barrel adjuster clockwise until the noise stops. Now, go ahead and shift through all the gears. It should be shifting fine now.
Still isn’t perfect? Well, here’s the basic rule of thumb: If the chain is lagging on the upshifts to larger cogs, you need a bit more tension (no more than a quarter turn counter-clockwise at a time). Conversely, if the shifts are slow as you try to shift down onto smaller cogs, you probably have too much cable tension: reduce that tension by turning the barrel adjuster clockwise a quarter turn at a time between testing the gears.
Other things that can stymie shifting performance? Dirt and moisture in the cable housing can make the shifting feel gritty and slow. I replace my cables and housing annually. If you are riding three times a week or more, I recommend you do the same. Remember that new cables inevitably stretch, which means the perfect tension you achieve on installing a new cable will grow sloppy within a few rides. Just do the quick tune outlined at the beginning of this article (counter clockwise turns of the barrel adjuster to add tension).
If your shifting has been flawless for a while and suddenly goes wacky on you, it may be because you bent the derailleur or, more likely, the derailleur hanger during a crash. Don’t remember wrecking? You can also bend the derailleur hanger by simply laying a bike drive-side down on the ground or leaning it against something carelessly, so that the derailleur makes contact with something that isn’t budging. What should you do in that case? That’s the subject for a later piece. Stay tuned.