A chain that skips only under hard pedalling is almost always a worn drivetrain: a stretched chain whose rollers no longer sit in the worn hollows of the cassette teeth, so load pushes it up and over. The other common causes are a cable or limit adjustment that leaves the derailleur between gears, a stiff link that cannot flex around the small sprockets, and a worn chainring or freehub.
Hard pedalling is the diagnostic clue. Light pressure keeps the chain seated even on badly worn parts, so the skip appears when you stand up on a climb, sprint from a light, or grind a big gear. That means the fault is usually mechanical wear or misadjustment, not something that will settle on its own.
Table of Contents
- Chain stretch and the worn-cassette pairing
- Is it skipping, or is the derailleur poorly indexed?
- Stiff links, dirt, and the single-spot skip
- Cassette, chainring, and freehub wear
- Working through it in order
- Frequently Asked Questions
Chain stretch and the worn-cassette pairing
A chain "stretches" because the pins and rollers wear, which lengthens each link slightly. Over hundreds of miles the chain's pitch grows, so its rollers engage the sprocket teeth slightly off-centre and grind a hollow into each tooth face. The worn chain and worn cassette then match each other, and nothing skips until the pressure gets high. Fit a new chain to that worn cassette and the skipping usually gets dramatically worse, because the fresh short pitch cannot sit in the long, scooped-out tooth gaps.
This is the classic sequence: a bike that ran acceptably starts skipping right after a chain replacement, most noticeably in the gears the rider uses most. Those cogs wear first, which is why one or two sprockets skip while the rest feel fine. Mechanics measure chain wear with a chain checker, a small gauge that drops into the chain and reads elongation as a percentage. The widely used thresholds are around 0.5% for 11- and 12-speed chains and 0.75% for wider 8- to 10-speed chains; past that, the cassette is likely worn too. A ruler works as a rough substitute: twelve complete links should measure 12 inches pin to pin, and noticeably more than 12 1/8 inches means the chain is finished.
Is it skipping, or is the derailleur poorly indexed?
These two faults feel different, and telling them apart saves money. Worn-drivetrain skipping is a sudden forward lurch within one gear — the chain jumps a tooth and the pedals drop — and it repeats in the same cog under the same load. Indexing trouble shows up as the chain sitting half on two sprockets, with continuous rattling, ghost shifting, or a reluctance to change gear in one direction.
Shift cables stretch and their housings collect grit, which pulls the derailleur slightly out of alignment with the sprockets. A barrel adjuster on the shifter or derailleur moves the cage in small increments; a quarter-turn at a time, repeated while pedalling in a stand, usually restores clean engagement. A bent derailleur hanger, the small tab the rear derailleur bolts to, produces the same symptom and needs a shop tool to straighten. Check the obvious before anything else:.
- Worn or seized jockey wheels on the rear derailleur cage
- A bent or damaged tooth from an earlier chain drop
- A loose quick-release or thru-axle letting the wheel shift under load
- Cable housing cracked or kinked near the rear dropout
Stiff links, dirt, and the single-spot skip
If the skip happens once per wheel-and-crank cycle at a regular interval, suspect a single stiff link rather than general wear. A stiff link is a pair of plates pinched too tightly, usually at the joining pin after a repair, or seized by rust and dry grit. It cannot flex sideways through the derailleur cage and pops out under tension. Backpedal slowly while watching the chain run through the lower jockey wheel; a stiff link visibly hops as it passes.
Flexing the link laterally with your thumbs, with a drop of lubricant, often frees it. If the pin is clearly protruding unevenly, the link needs re-seating with a chain tool or replacing. Dirt alone can mimic wear. A chain packed with dried mud and old lube does not seat properly, and degreasing then re-lubricating sometimes cures a mild intermittent skip outright. Clean the drivetrain before measuring anything, because grime hides both worn teeth and a loose pin.
Cassette, chainring, and freehub wear
Worn sprocket teeth lose their square profile and take on a hooked, shark-fin shape, thinner at the tip and leaning in the direction of pedalling. Hold a straight edge or a photo against the teeth of a rarely used cog and compare: the difference is usually obvious. Replace the cassette with the chain when it is worn — fitting one and not the other reintroduces the mismatch. At the front, a worn chainring drops the chain or lets it climb under load, most often on the inner ring or a single-ring setup where every pedal stroke uses the same teeth.
Front wear takes much longer than cassette wear because the rings have more teeth sharing the load. A failing freehub is a different animal and worth ruling out, because no amount of chain work fixes it. Its ratchet mechanism lets the wheel coast; when the pawls or springs wear, the body slips briefly under load and then catches, which feels like a skip but comes with a crunch or a dead spot rather than a clean tooth jump. A useful test: if the "skip" happens with the chain in a gear you can hold under steady pressure and you hear the freehub buzz while still pedalling forward, the hub is the suspect.
Working through it in order
Work from cheapest to most expensive, and stop when the skipping stops. Two warnings. A chain that skips under hard effort can throw you forward onto the bars or off the saddle, so avoid standing sprints and steep climbs until it is fixed.
And a chain worn well past the wear limit will grind the new cassette quickly, so replacing a cheap chain on time is the single best way to avoid buying cassettes. Measuring before buying is the step most riders skip. A chain checker costs a fraction of a cassette and tells you in ten seconds whether you need one part or three.
- Clean and lubricate the chain, then test under load in the problem gear
- Backpedal and look for a stiff or damaged link; free or replace it
- Check wheel axle tightness and derailleur hanger alignment
- Trim the indexing with the barrel adjuster, a quarter-turn at a time
- Measure chain wear with a chain checker or a 12-inch ruler
Frequently Asked Questions
Can I replace just the chain and keep the cassette?
Yes, if the chain is caught early, before roughly 0.5% elongation on 11- and 12-speed drivetrains or 0.75% on 8- to 10-speed. Past that the cassette teeth are usually worn to match and a new chain will skip on them.
Why does it only skip in certain gears?
Those are the gears you ride most, so their teeth are the most worn. A cog that sees little use still has square teeth and holds the chain fine.
Is it safe to keep riding a skipping chain?
For gentle flat riding at low pressure, usually. Under hard effort the sudden pedal drop can unbalance you, so treat standing climbs and sprints as off-limits until it is repaired.
How long should a chain last?
It depends far more on cleaning, lubrication, and conditions than on mileage, which is why measuring beats guessing. Riding in wet grit can wear a chain out in a fraction of the distance a dry-weather rider gets.
