Derailleurs are the transmission of your bike. When you shift, they guide the chain to the correct gears. Whether you’re fixing a particular problem with your chain or simply giving the system a tune-up now that spring is here, here’s how to do it. (Our expert: Savannah Quandt, bike mechanic at Scheels in Johnstown, Colorado.)
1. Start by putting your bike on a bike stand to get the rear wheel off the ground. Turn the pedals by hand and shift through your gears. If there’s a lag between when you shift and when the chain moves, the tension on the cables going from your shifters to the derailleurs is off, and you need to unscrew the barrel adjuster (the black cylinder on the derailleur cable) to increase the tension. If the chain jumps between gears on its own, tighten the barrel adjuster to add tension.
2. Shift to high gear. The chain will be on the smallest ring in the rear and the largest ring in the front. On the rear derailleur, find high and low limit screws (labeled H and L). They control how far the derailleur can move. Tighten the H screw on the rear derailleur until the chain does not click as it moves through, but leave it loose enough that the chain can still shift from the second smallest gear to the smallest.
3. Finally, shift to low gear. The chain will be on the largest ring in the rear and the smallest ring in the front. Tighten the L screw until the derailleur can move only as far as the largest gear and not past it. But keep it loose enough that the chain still moves from the second largest ring to the largest ring with ease when shifting. It might take a few tries.
4. On the front derailleur, adjust the L screw until the inner side of the cage that encircles the chain is as close as possible to the chain without scraping against it. Shift both derailleurs into high gear. Adjust the H screw until the outer edge of the cage is as close to the chain as possible without scraping.
Want more Popular Mechanics? Get Instant Access!
James Lynch is Popular Mechanic’s Field Editor looking for stories across the U.S. about those building a better America. He’s passionate about the Erie Canal and the Apollo Space Program.
