The Short Answer
If your bike chain skips, jumps, or slips off a gear the moment you stand up and mash the pedals, the cause is almost always one of five things: a stretched-out chain, loose or maladjusted derailleur cable tension, a chain that's simply too loose, worn cassette or chainring teeth, or a bent derailleur hanger. The good news is that this is one of the most diagnosable problems in bike maintenance — each cause has its own tell, and you can usually narrow it down in five minutes without any tools beyond your hands and eyes.
First, Figure Out Which Kind of Skip You Have
Not all "chain skipping" looks the same, and the pattern tells you a lot before you touch a single bolt:
- Skips only in one or two specific gears: almost always a worn or bent tooth on that one cog or chainring, or cable tension that's slightly off for that part of the range.
- Skips across most or all gears, especially under hard pedaling: usually a stretched chain that no longer matches the tooth profile of your cassette, or a chain that's too loose overall.
- The chain jumps sideways off the big or small cog entirely: this points to derailleur limit screws or a bent derailleur hanger, not the chain itself.
- Skipping got worse right after you (or a shop) installed a new chain: the new chain is very likely paired with an old, worn cassette — a classic and completely avoidable mismatch.
Check for a Stretched Chain First
Chains don't stretch in the sense of the metal itself getting longer — the pins and rollers wear down, which effectively lengthens the distance between links. A worn chain no longer sits properly in the valleys between cassette teeth, so under load it can hop or skip forward. This is the single most common cause of skipping in bikes with more than a season of regular riding.
You can check this yourself with a $10-15 chain wear indicator tool, or do a rough check by pulling the chain away from the front chainring at the 3 o'clock position — if you can pull it far enough to see daylight behind more than half a tooth, it's worn. Most manufacturers consider a chain worn out at around 0.5% to 0.75% elongation, well before it looks visibly damaged.
Here's the part that surprises a lot of people: if you replace a badly worn chain but leave an equally worn cassette in place, the brand-new chain will often skip worse than the old one did, because the new chain no longer matches the wear pattern grooved into the old cassette's teeth. If your cassette has more than a few thousand miles on it and you're installing a new chain, plan on checking (and likely replacing) the cassette at the same time.
Rule Out Cable Tension Next
Derailleur cables stretch slightly over the first few rides after installation, and gradually after that. When tension drops even a little, the derailleur can't hold the chain precisely over each cog, so it hovers between gears and skips or makes noise under load — especially in the middle of the cassette.
Most bikes have a barrel adjuster where the cable enters the rear derailleur. With the bike in a stand (or up on its back wheel) and pedaling by hand while shifting through the gears, turn the barrel adjuster a quarter-turn counterclockwise at a time if shifts feel sluggish or the chain hesitates before settling on a gear, then test again. This single five-minute adjustment fixes a surprising share of "my chain keeps skipping" complaints, and it's the first thing worth trying because it costs nothing and doesn't risk damaging anything.
Check Chain Tension and Line
On a derailleur bike, chain tension is mostly handled automatically by the rear derailleur's spring, so true looseness is less common than on single-speed or hub-gear bikes — but it does happen if the derailleur spring is weak, damaged, or gunked up with old grease and dirt. A chain that's too loose will slap against the chainstay and can skip over the teeth when you hit a bump or stand and pedal hard. Cleaning and lightly lubricating the derailleur pivot points, and confirming the spring snaps the derailleur cage back briskly when you pull it and let go, rules this out.
Inspect the Teeth Themselves
Cassette cogs and chainrings are designed to wear alongside a chain over thousands of miles, and eventually the teeth develop a shark-fin or hooked shape instead of a straight, even profile. A hooked tooth can't hold the chain link securely under load, so the chain rides up and skips off, particularly in your most-used gears (often the middle of the cassette and the big chainring, since that's where most people spend the most pedaling time). If skipping is isolated to one or two gears and a fresh chain doesn't fix it, worn teeth on that specific cog are the likely culprit.
Check for a Bent Derailleur Hanger
The derailleur hanger is a small, intentionally sacrificial piece of metal that attaches the rear derailleur to the frame — it's designed to bend before your frame or derailleur does in a fall or a bike knocked over in a garage or car. A bent hanger throws off the derailleur's alignment with the cassette, which can cause skipping, slow or sluggish shifts, or the chain jumping between gears on its own. If your bike was recently dropped, transported, or leaned against something that knocked the derailleur, this is worth checking before you assume the problem is the drivetrain itself. A hanger alignment gauge is the accurate way to check it, though a visual sight-line down the cassette from behind the bike can reveal an obviously bent one.
When to Just Learn to Do This Yourself
A shop tune-up runs $40-100 and fixes the symptom for a while, but skipping chains, cable stretch, and worn drivetrain parts are recurring maintenance, not one-time repairs — they'll come back every few thousand miles for as long as you own the bike. Understanding how to check chain wear, adjust cable tension, clean a drivetrain, and true up a derailleur is one of the highest-value skills a regular rider can pick up, since it turns a $60 shop visit into a 15-minute driveway job you can do whenever something starts feeling off.
If you'd rather work from clear video instruction than trial and error, the DIY Bike Repair Course walks through exactly this kind of diagnosis — chain wear, derailleur adjustment, gear tuning, and the other maintenance basics — so you can troubleshoot a skipping chain (and most other common bike issues) at home with confidence instead of guessing.