Quick diagnosis
“Slipping” usually means one of two things, and they have different causes:
- The chain jumps or skips under load, often when you push hard or climb. This is commonly a sign of worn parts.
- The chain will not stay in, or moves to the wrong gear, regardless of effort. This is commonly a shifting adjustment problem.
Start by noting when it happens: in one particular gear, in all gears, only under load, or only after a shift.
Slipping under load: worn parts
If the chain jumps when you press hard on the pedals, check the drivetrain wear first:
- Worn chain. A chain stretches slightly with use, so it no longer fits the teeth snugly. A chain-wear gauge is an inexpensive way to check it, and our guide to the signs of a worn bike chain lists what else to look for.
- Worn cassette or chainrings. The teeth wear into a shape that lets the chain ride up. This is more likely if the chain has been worn for a long time. Often a new chain on a worn cassette will still skip, so cassette and chain may need replacing together.
- Stiff link. A single link that does not flex can cause a regular skip once per chain revolution.
Dirty or dry chains can also contribute. Cleaning and lubricating the chain is cheap to try; see how often to lube a bike chain for sensible intervals. Be aware that a worn drivetrain will not be cured by lube alone.
Shifting problems: cable tension and indexing
If the chain hesitates, shifts late, or drops back, the usual culprit is cable tension. Cables stretch and housing settles, especially on a new bike. Other causes are a frayed or sticky cable, and housing that is dirty or kinked.
A basic rear-derailleur adjustment
This is the general idea. Models differ, so check your manufacturer’s instructions (for example, for Shimano).
- Shift the chain onto the smallest rear cog.
- Check that the limit screws stop the derailleur from moving past the smallest and largest cogs.
- If the chain is slow to climb to a larger cog, turn the barrel adjuster at the derailleur or shifter a small amount (about a quarter turn) counterclockwise to add tension.
- If it climbs readily but is slow to drop to a smaller cog, turn the barrel clockwise slightly.
- Test while pedaling, in a repair stand or by lifting the rear wheel, and repeat with small changes.
Bent hanger, dirt and other causes
- Bent derailleur hanger. After a fall or knock, the hanger that holds the derailleur can bend and make indexing impossible to get right. Shops have a tool to check and straighten it.
- Wrong parts. A chain, cassette and shifters need to be compatible in number of speeds.
- Chain too short or long, or a worn jockey wheel, can also affect shifting.
If you are not sure what each component is called, see bike gear names and the bike gears diagram.
Can you spray WD-40 on bike gears?
WD-40 is a water-displacing spray and light solvent rather than a long-lasting chain lubricant. It can help loosen grime (more in using WD-40 on a bike chain), but it should not replace a bike-specific chain lube. If you use it, clean it off and apply proper lube, and keep it away from brake surfaces, since contamination can reduce braking.
When to see a shop
Take the bike in if adjusting the cable does not help, the hanger looks bent, you suspect worn parts but are unsure, or you do not have the tools. Replacing a worn chain early (see how many miles a bike chain lasts) generally costs less than also replacing a cassette and chainrings later. If your gears are slipping on a bike you recently converted, see adding gears to a single speed bike for setup points to double-check.