Bike gears skipping or not changing: causes and fixes

Short answer: start with cable tension, because it is the quickest check and fully reversible. Find the barrel adjuster where the gear cable leaves the shifter (or enters the rear derailleur), turn it anticlockwise half a turn, and test through the gears. Repeat until each click gives exactly one clean shift. If the chain slips under hard pedalling while staying in the same gear, that is wear rather than tension. Read on before you touch anything, because more tension will make it worse.

What is actually happening: skipping, ghost shifting or not shifting

These get lumped together, but they are three different faults, and naming yours correctly is half the diagnosis. Skipping is the chain jumping to a neighbouring sprocket, or slipping forward under load. Ghost shifting is the bike changing gear when you did not touch the shifter, because the derailleur is sitting between two indexed positions and hops over bumps. Not shifting is clicks at the lever with nothing, or a long delay, at the wheel.

As a rough map, skipping under load is usually wear, ghost shifting is usually indexing, and not shifting is usually the cable itself. Diagnose before you adjust, because the fix for one fault can make another worse. Adding cable tension to cure a worn-chain slip, for example, just moves the derailleur off line and adds ghost shifting on top.

The five usual causes, and how to check each one

1. Cable stretch

Gear cables settle into their outer casing and stretch slightly with use, most noticeably in a new bike's first few weeks. That is why brand-new bikes come back shifting badly for the free first check that Halfords, Evans and many independent shops offer a few weeks after purchase. The symptom is indexing that has drifted everywhere at once: hesitant climbs to easier gears and rattly running in the middle of the cassette. Check: if half a turn on the barrel adjuster makes everything crisper, this was the cause. The barrel adjuster exists precisely for this drift.

2. Indexing out of adjustment

Indexing is the alignment between the shifter's click positions and the actual sprockets. When it is out, the derailleur stops between gears: shifts hesitate in one direction, ghost-shift in the other, and the drivetrain ticks in gears that used to run silently. Check: with the bike in a middle gear, look at the derailleur's upper jockey wheel from behind. It should sit exactly under the sprocket the chain is on. If it sits slightly to one side, the indexing is out, and the barrel-adjuster steps below are the fix.

3. Bent derailleur hanger

The hanger is a soft aluminium tab between the frame and the derailleur. It is soft on purpose, so that it bends instead of your frame. It bends in falls, in train luggage racks, and simply from leaning the bike drive-side against a wall. Check: sight from directly behind the bike. The two jockey wheels and the sprockets should line up in one vertical plane. A visible tilt, or shifting that is fine in the small sprockets and chaotic in the big ones, points at the hanger. A badly bent hanger can put the derailleur into the spokes on a downshift, which turns a cheap part into a destroyed wheel. If you suspect it, stop adjusting and get it aligned.

4. Worn chain and cassette

Chains lengthen as their pins and rollers wear (this is what "chain stretch" means), and a lengthened chain no longer meshes with the sprocket teeth. It rides up and slips forward under load, classically in the one or two gears you use most. Check: a chain-checker tool gives a yes or no in seconds. The ruler method works too: on a new chain, twelve full links measure exactly twelve inches pin to pin, and noticeably more than that means the chain is done. Two things to know. A new chain on a worn cassette often still slips, because they wear as a pair, and no amount of adjustment fixes wear. This is a parts job rather than a tuning job.

5. Friction in the cable run

Dirt, water and corrosion inside the outer casing make the inner cable drag. The spring can no longer pull the derailleur back smoothly, so shifts lag in one direction and mysteriously improve when the bike is clean and dry. London winters, with their salt, grit and rain, are exactly what causes this, and internally routed cables hide it well. Check: click the shifter while watching the derailleur. A crisp click followed by lazy movement means friction. The real fix is a new inner and outer cable. Lubricating an old corroded run buys weeks rather than months.

Which one do you have?

The barrel-adjuster fix, step by step

1. Find the adjuster

A knurled barrel where the cable leaves the shifter, or where it enters the rear derailleur. Some bikes have one in-line on the cable run instead. No tools are needed, because it turns by hand.

2. Get the wheel turning

You need to pedal while shifting, so use a workstand, ask a friend to lift the saddle, or do short test rides up a quiet street between adjustments. Do not try to judge shifts while wheeling the bike by hand, because the chain needs to be moving under a little load.

3. Work in half turns

If the chain is slow to climb to bigger sprockets (easier gears), turn the adjuster anticlockwise half a turn. If it is slow to drop to smaller sprockets, turn it clockwise half a turn. Test the full range after every half turn. Stick to half turns, because it is easy to sail straight past the sweet spot and conclude that the adjuster does nothing. Half turns suit the 7 and 8-speed Shimano Tourney and Altus gears on many Carrera, Apollo and Decathlon commuters. On 11 and 12-speed road and gravel groups (Shimano 105 or GRX, SRAM Apex or Rival) the sprockets sit closer together, so work in quarter turns.

4. Stop at one click, one clean shift

Every click should move the chain exactly one sprocket, promptly, in both directions, and the drivetrain should run silently in each gear. Do a final check in the gears you actually commute in, under real pedalling load on the street.

5. If it never settles, the fault is elsewhere

If you keep winding without ever reaching "crisp", the problem is not tension. Go back to the hanger, chain-wear and cable-friction checks above, because more tension cannot fix any of those three.

Limit screws: what they do and when to touch them

The two small screws on the derailleur body, usually marked H and L, set the physical end stops of its travel. They stop the chain going past the smallest sprocket into the frame, or past the biggest into the spokes. Most guides skip one important point: limit screws do not drift. If your shifting got worse over weeks, the limits are not the cause, and turning them will add a new problem. Touch them only if the chain actually falls off the ends of the cassette or chainrings, and then in quarter turns, remembering which way you went. If the chain has been going into the spokes, check the hanger first, because a bent hanger changes where the limits point.