Short answer: gears skipping when pedaling hard almost always comes from one of four things: a worn chain and gears, a shift cable that has stretched or is out of adjustment, a bent derailleur hanger (the small bracket holding the rear derailleur), or a slipping rear hub. You can usually ride home seated and gently. What you should not do is keep standing on the pedals, because a skip under load can drop your foot off the pedal. If the skipping started after the bike fell over or took a knock, avoid the largest rear gear and get it checked soon.
It only happens going uphill for a reason, and that reason is the key to working out which one you have.
Why It Only Happens Uphill
When you pedal gently, the chain sits on the teeth of the gears with very little force trying to pull it off. Worn teeth, a slightly stretched cable or a slightly bent hanger can all get away with it.
When you push hard, climbing or starting from a stop, the force through the chain rises sharply. Now the same small problem is enough to let the chain ride up and over a tooth, or jump to the next gear. That is the skip: a sudden jolt, often with a clunk, and a moment where the pedals give way under your foot.
So "it only does it uphill" does not mean the problem is small. It means the problem is hiding the rest of the time.
Which of the Four Causes Is Yours?
| What you notice | Most likely cause | How urgent |
|---|---|---|
| Skips in the gears you use most, bike has lots of miles, chain looks dark or dry | Worn chain, possibly worn gears too | Soon. Wear gets worse and spreads to other parts |
| Skips in one or two gears, or the bike seems to shift by itself; bike or cables are fairly new | Shift cable stretched or out of adjustment | Low. Often a small adjustment |
| Started suddenly after the bike fell, was transported or was knocked; shifting is worst at the biggest or smallest rear gears | Bent derailleur hanger | High. Avoid the biggest rear gear until checked |
| Pedals briefly spin forward with no drive, sometimes with a clunk, in any gear | Rear hub's freehub or its internal parts slipping | High if it keeps happening |
| Skipping started right after a new chain was fitted | Old gears worn to match the old chain | Soon. The new chain and worn gears will not mesh |
| A jump at a regular interval, whatever the gear | A stiff chain link | Low to medium. Often fixable |
Worn chain and gears
This is the most common cause on a bike with real miles on it. As a chain wears, it effectively gets longer and stops sitting snugly in the teeth. The gears wear to match it. Under light load it still works; under hard load it skips. The League of American Bicyclists' Basic Bike Check (read September 25, 2026) asks riders to "look to see that the chain is not rusted and it is free of gunk," which is a good first look, but wear itself needs measuring. A shop can check it with a chain wear tool without taking anything apart. We are not quoting a wear limit here, because the right figure depends on your chain and gear system, and the tool maker and the chain maker publish their own.
Our guide to what a worn drivetrain costs explains how a worn chain takes the gears with it, and why catching it early is cheaper.
Stretched or misadjusted shift cable
New shift cables settle and stretch over the first rides, which is why a new bike's shifting often needs a small adjustment after its first few weeks. Older cables can also get gritty or corroded inside their housing, so the derailleur does not move to where the shifter tells it. The result is a chain that sits between two gears and skips across under load, or seems to shift by itself.
This one is often fixed with a small turn of the barrel adjuster, where the cable enters the rear derailleur or shifter.
Bent derailleur hanger
The hanger is a small metal bracket that holds the rear derailleur to the frame. On most modern bikes it is designed to bend in a knock, so the derailleur and frame are spared. A bike that tips over on its right side, gets bumped in a rack or goes through a crash can bend it without anyone noticing.
A bent hanger throws off the shifting, and cable adjustment cannot correct it. It is also the one cause here with a real safety angle: a badly bent hanger can let the derailleur move into the spokes when you shift to the largest rear gear. That can damage the derailleur, the wheel and the frame, and can stop the rear wheel suddenly.
Stop condition: if you hear a tick-tick-tick from the rear wheel, or can see the derailleur sitting close to the spokes, do not shift into the largest rear gear. Ride home in a middle gear and get the hanger checked.
A slipping rear hub
The rear hub has a ratchet inside (on most bikes, the freehub) that drives the wheel forward when you pedal and lets it freewheel when you coast. If it wears or gets dirty, it can slip under load. It feels different from chain skip: the pedals spin forward briefly with no drive at all, whatever gear you are in.
This is a shop job. The fix depends on the hub, and a slipping hub is not something to keep testing on climbs.
Is It Safe to Keep Riding?
The answer depends less on the cause and more on how you ride until it is fixed.
Why a skip can hurt you. When the chain skips, the resistance under your foot vanishes in an instant. If you are seated and spinning gently, you barely notice. If you are standing and pushing hard, your weight drops with the pedal. Your foot can come off, your shin or knee can hit the bike, or you can be thrown forward toward the handlebars. Starting from a stop in traffic, the same moment costs you momentum right where you least want to lose it.
Until it is fixed:
- Stay seated on climbs and pedal in a lower gear than usual.
- Avoid hard starts from a stop; shift to a low gear before you stop.
- Ease off the pedals for a moment each time you shift.
- Shift one gear at a time, not several at once.
- If a bent hanger is possible, keep out of the largest rear gear.
That makes most causes safe enough to ride home and to a shop. It does not make them safe to ignore for weeks, because wear costs more the longer it runs.
What You Can Check at Home
These are checks where a mistake shows itself straight away rather than on the road.
- Look at the hanger from behind. Crouch behind the bike and look at the rear derailleur and gears. The two small pulley wheels on the derailleur should sit in a straight vertical line under the gear the chain is on. If the derailleur looks tilted inward toward the wheel, suspect the hanger.
- Clean and oil the chain. A filthy, dry chain is rarely the whole cause of skipping, but it hides wear and makes everything else worse.
- Look for a stiff link. Turn the cranks backward slowly and watch the chain pass through the derailleur's pulleys. A link that does not straighten out as it passes will show as a small kink.
- Try a small cable adjustment. A small turn of the barrel adjuster can fix a gear that hesitates or skips. Turn in small steps, test after each, and if it gets worse, turn it back and stop.
Straightening a hanger, replacing a chain or cassette, and anything inside the rear hub are better left to a shop unless someone has shown you how. The U.S. Consumer Product Safety Commission's 1997 bicycle safety tips (read September 25, 2026) still put the basic question well: "Is your bicycle properly adjusted?"
What to Tell the Shop
Describing the skip well helps the mechanic find it faster. Say:
- Which gears it happens in, all or just some.
- Whether it happens only under hard pedaling or also on the flat.
- Whether the bike fell over, was transported or crashed before it started.
- Whether the pedals slip forward with no drive at all, or the chain seems to jump.
- When the chain was last replaced, if you know.
If this is your first visit, our guide to what to ask at a bike shop has more.
If Your Rides Are Mostly Flat
Riders on flat routes sometimes decide that gears are more upkeep than they need. A single-speed bike has no derailleur, no shift cable and no hanger to bend, though its chain still wears. Our comparison of single speed vs geared bikes for commuting explains when that trade makes sense and when hills make it a poor one.
FAQ
Why do my gears slip when I pedal hard?
Hard pedaling puts much more force through the chain. A worn chain and gears, a stretched or misadjusted shift cable, a bent derailleur hanger, or a slipping rear hub can all hold under light load and let go under heavy load.
Is it dangerous to ride with a skipping chain?
It can be, mainly if you are standing and pushing hard. The pedal can give way suddenly and your foot can slip. Ride seated in lower gears until it is fixed, and avoid the largest rear gear if the hanger might be bent.
Will a new chain fix skipping gears?
Sometimes. If the chain is worn but the gears are not, yes. If the gears are worn too, a new chain can skip even worse on them, and the gears need replacing as well. A shop can tell you which by checking the chain and looking at the teeth.
Why does my chain skip only on hills?
Because that is when you push hardest. Small amounts of wear or misadjustment hide under light load and show up under the extra force of climbing.
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