Why does my bike chain skip or slip when I pedal hard?

Skipping has two quite different causes that need entirely different responses. A five-minute check tells you which one you have, and getting it the wrong way round wastes either time or money.

Safety

What you'll need

• A chain wear checker
• An allen key set
• A chain tool and a new chain (optional)

Step by step

1. Notice when it skips.Under hard pressure in most gears points at wear. Only in certain gears, or a hesitation when changing, points at indexing. That distinction determines everything that follows.
2. Check the chain for wear.A chain checker tool costs a few pounds. Chains stretch as the pins and rollers wear, and past about 0.75% the chain starts wearing the cassette teeth to match itself.
3. Replace a worn chain promptly.A chain replaced in time costs a fraction of a chain plus cassette plus chainrings. Leaving it is what turns a cheap job into an expensive one.
4. Check the cassette teeth.Worn sprockets look hooked or shark-finned rather than symmetrical. If a new chain skips on an old cassette, the cassette is worn to the shape of the old chain and both must be replaced.
5. Adjust the indexing if it is gear-specific.Turn the rear barrel adjuster a quarter turn at a time with the bike in a stand or the rear wheel lifted, and shift up and down until the chain sits quietly in each gear.
6. Know which way to turn the adjuster.If the chain is slow to move to a larger sprocket, turn the barrel adjuster anticlockwise to add cable tension. If it is slow to come down to a smaller one, turn it clockwise.
7. Check the cable and outer.A frayed inner cable or a compressed, dirty outer causes vague, sticky shifting that no amount of adjustment fixes. New cables and outers transform an older bike.
8. Check the hanger alignment.The derailleur hanger bends easily if the bike falls over. A bent hanger causes skipping and poor shifting across several gears, and it needs a specific tool or a shop to straighten.
9. Check the derailleur limit screws.If the chain overshoots into the spokes or off the smallest sprocket, the high and low limit screws are set wrong. These set the ends of the range, not the indexing between.
10. Look at the chainring teeth too.Worn or bent chainring teeth cause skipping at the front. A single bent tooth after an impact is a common and easily missed cause.
11. Replace as a set when worn.Chain, cassette and often chainrings wear together. Replacing only one on a badly worn drivetrain usually means it skips immediately on the old parts.

Tips

• Skips in every gear under load: wear. Skips in specific gears: indexing.
• A chain checker is the cheapest tool in cycling and saves the most money.
• A new chain on a worn cassette will skip. They wear as a pair.

Common mistakes

• Running a chain until it visibly sags — A worn chain reshapes the cassette and chainring teeth to match it. What would have been a cheap chain replacement becomes a full drivetrain, which can cost several times as much.
• Adjusting the limit screws to fix indexing — The limit screws only set how far the derailleur can travel at each end. Using them to try to fix gears in the middle of the range makes the shifting worse and risks the chain going into the spokes.

If it doesn't work

Questions people ask