How to Tune a Rear Derailleur
If your chain is skipping over cogs or being unresponsive, chances are your rear derailleur is up to no good. But before you start messing with those tiny screws marked “H” and “L,” try an easier fix—fiddling with the barrel-shaped knob (appropriately called the “barrel adjuster”) located where the cable feeds into the rear derailleur.
First, shift to the smallest cog. Click up one gear while turning the pedals with your other hand. If the derailleur refuses to budge, shift back and turn the barrel adjuster a half-turn counterclockwise. Repeat this process until you get a clean shift, continuing to fine-tune the barrel adjuster as needed. Move up the cogs until your shifting falls into line. Now try again, moving back down the cogset and adjusting less than a half turn. This “indexing adjustment” will be the solution to the majority of your shifting problems.
Slow shifting to a larger cog? Tighten the cable by turning the barrel adjuster counterclockwise.
Slow shifting to a smaller cog? Loosen the cable by turning the barrel adjuster clockwise.
But let’s entertain an alternate scenario—one in which tensioning isn’t your biggest issue. The barrel adjuster will add more tension to your cable, but if the aforementioned H and L limit screws aren’t adjusted right, more tension is never going to guide your chain to the smallest or largest cog. Essentially, the limit screws tell the derailleur where to stop when it’s being moved by the shifter.
To adjust, first shift into the biggest chainring and smallest cog and let the tension out of the derailleur cable by loosening the bolt that attaches the cable to the derailleur. Grab a tiny screwdriver and turn your H screw until the pulleys line up with the smallest cog. Turn the pedals to be sure your chain moves smoothly around the cog. If you get this screw too tight, you won’t be able to get to your little sprocket.
The L limit screw will keep the derailleur from launching your chain into your spokes. To adjust it, pedal forward with one hand and use the other to push your rear derailleur until the chain is on the largest cog. If the chain goes past the largest cog, tighten the L screw until the chain stops jumping off the big sprocket.
Tighten the cable back down again with the cable clamp bolt and start over with the barrel adjuster to dial in all your shifting. That’s really all there is to it.
It’s a simple process—but not one that you want to get wrong because throwing a chain can be pretty hazardous to you AND your bike. If you’re at all nervous about fiddling with your limit screws, here’s more detailed information about how your derailleur works and a step-by-step video guide from Bicycling.
Want more DIY bike fixes? We've got:
Expert Tips from Bike Mechanic Instructor, Dylan Robbins.
How to Fix Your Squeaky Brakes
How to Patch a Tube
How to Measure and Replace Your Chain