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When to replace a bicycle chain, and how to fix poor shifting

A worn chain left too long ruins the sprockets and chainrings, and a cheap chain becomes an expensive repair. Measure the chain every month and replace it in time.

Why chains wear

As a chain is used, its pins and rollers wear and the chain gets longer. A worn chain:

• skips or jumps under load, especially on the smaller sprockets;
• shifts poorly;
• wears the sprockets into a hooked shape, so that even a new chain skips on them.

Replace the chain early and the sprockets will last through two or three chains.

How to measure chain wear

• With a chain checker: the tool drops fully into the chain when the chain has reached the limit marked on it, often 0.5 or 0.75%. Narrow 11- and 12-speed chains are usually replaced at 0.5%. A checker costs less than one new sprocket.
• With a ruler: on a new chain, 12 full links measure 304.8 mm (12 inches) from pin to pin. Replace the chain when they measure about 1.5 mm more.

Replacing a chain

1. Buy the right chain for your number of gears.
2. Shift to the smallest sprocket and the smallest chainring.
3. Remove the old chain by opening its quick link, or by pushing a pin out with a chain tool.
4. Set the length by counting the old chain's links, if the old chain was the right length.
5. Thread the new chain through the derailleur and join it with the quick link or joining pin supplied. Never reuse an old pin.
6. Check that the join moves freely, shift through all the gears, and lubricate.

Fixing poor shifting

Most shifting problems are cable tension.

1. With the back wheel off the ground, pedal and shift to the smallest sprocket.
2. Shift one click.
3. If the chain hesitates or will not climb, turn the barrel adjuster anticlockwise a quarter turn. If it overshoots or is noisy, turn it clockwise a quarter turn.
4. Work through all the gears until each click moves the chain exactly one sprocket, quietly.

Leave the limit screws (marked H and L) alone unless the chain goes past the end sprockets or cannot reach them.

If the bicycle has fallen on its right side and nothing you adjust helps, the hanger that holds the derailleur may be bent. Take it to a workshop, because straightening it needs a special tool.

Habits that make a chain last

• Shift while pedalling lightly, not while standing hard on the pedals.
• Shift before a hill or a stop, not halfway up.
• Avoid using the biggest chainring with the biggest sprocket, or the smallest with the smallest. The chain then runs at a sharp angle and wears faster.
• Shift one or two gears at a time.
• Keep the chain clean and lubricated.

Single-speed bicycles

A single-speed chain needs the right tension: about 1 cm of up-and-down movement midway. Set it by moving the rear wheel back in its slots, keeping the wheel straight.

This guide is based on Chapter 6 of Bicycle Maintenance and Repair, which also covers front derailleurs and hub gears.