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Chain Repair

5 min read

Parent
🔧
Common Repairs
Fixing the most frequent breakdowns
Current
⛓️
Chain Repair
Removing broken links, reattaching, tension

To repair a broken chain, find the damaged link, push its pin partly out with a chain tool (or a nail and hammer), remove the damaged link or links in an even number so the chain can rejoin, and push the pin back until it is flush. Each pair of links removed shortens the chain by 25.4 mm and so needs the wheel moved forward 12.7 mm or a gear combination that tolerates it, and because a hard pedal stroke loads the chain with about 1.5 kN, a re-pinned joint should be treated as the weak point until replaced.

Part of Bicycle Repair & Fabrication

Chain repair is the removal of a damaged link from a bicycle chain and the reconnection of the ends with its own pin.

Some Arithmetic Behind A Short Chain

The chain’s pitch is 12.7 mm (1/2 inch), so a pair of links (one inner and one outer, which is why the count removed must be even) is 25.4 mm. The chain length in links that fits two sprockets of N1 and N2 teeth, a centre distance C apart, is

L = 2C / p + (N1 + N2) / 2 + ((N1 - N2) / 2π)² × p / C

For an assumed single-speed bicycle with a 44-tooth ring, a 16-tooth sprocket and a 420 mm chainstay:

  • 2C / p = 840 / 12.7 = 66.1
  • (N1 + N2) / 2 = 30
  • ((44 - 16) / 6.283)² × 12.7 / 420 = 19.9 × 0.0302 = 0.6
  • L = 96.7 links, so 96 links

Taking out 2 links changes L by 2, which changes the centre distance by p × 2 / 2 = 12.7 mm. On a bicycle whose wheel can slide in its dropouts, take the 12.7 mm up by moving the wheel forward. On a geared bicycle the derailleur absorbs only a limited amount. The parent article’s limit is that a chain shortened by more than 4-6 links probably needs a replacement section spliced in; 4 to 6 links is 51-76 mm of chain, and a chainstay change of 25-38 mm.

How hard does the chain pull?

Take an 80 kg rider standing on one pedal, force 784 N, on a 170 mm crank:

  • Torque = 784 N × 0.170 m = 133 N·m
  • Pitch radius of the 44-tooth ring = 44 × 12.7 / (2π) = 88.9 mm
  • Chain tension = 133 N·m / 0.0889 m = 1,500 N

That is about 1.5 kN on the tight side of the chain, and harder pedalling or a sprint is more. A joint closed with the original pin or a nail is weaker than a new link, so after an improvised repair ride gently and replace the joint with a proper link as soon as you can (recalled practice; the carrier does not state a strength).

For gear arithmetic, the distance covered per crank turn is the ratio times the wheel circumference: 44 / 16 = 2.75, times an assumed 2.1 m circumference, is 5.8 m. At 60 pedal turns a minute that is about 347 m a minute, or 21 km/h.

What You Need

  • A chain tool, or a nail and a hammer on a hard surface
  • Spare links or a spare chain section
  • Light oil, as listed in step 7
  • Pliers and a rag

Step 1: Find the damaged link, usually a bent or cracked outer plate.

Step 2: Push the pin partly out with the chain tool, or the nail and hammer, until the link comes apart.

Step 3: Remove the damaged link or links, an even number of them, so the chain can rejoin.

Step 4: Rejoin. Push the pin back through to reconnect. Do not push it through the far plate; leave it flush.

Step 5: Free the stiff joint. Flex it by hand until it moves freely.

Step 6: Check the length and tension. A single-speed chain should have a little slack; move the wheel back until the chain is just snug.

Step 7: Lubricate. Any light oil works: motor oil, vegetable oil (temporary, as it gums up), 3-in-1 oil, or rendered animal fat in an emergency. Apply to each link and wipe off the excess.

Common Mistakes

  • Removing an odd number of links — the inner and outer links no longer alternate, so the ends cannot meet.
  • A stiff joint left stiff — it skips under load.
  • Too short a chain on a geared bicycle — it pulls on the derailleur and can break it.
  • Hard pedalling after a nail-pin repair — the 1.5 kN stroke finds the weak joint.

Safety

Hands, spinning chain and broken links

A chain tool can slip and drive a pin into the hand, and a chain breaking under load throws the rider forward onto the top tube. Keep fingers out of the chain and sprockets while the wheel can turn, hold the chain on the tool, and do not stand on the pedals or sprint on a repaired joint.

What’s Next