Replace a broken spoke by matching its length, source and installing it in the same lacing pattern as its neighbours, then bringing it up to their tension and truing the wheel. The length is best copied from an intact spoke of the same side and pattern, or computed from the rim and hub dimensions: for an assumed 600 mm effective rim diameter, 36 holes and 3-cross lacing, it comes to about 288 mm, and a spoke that is too short cannot be used at all.
Part of Bicycle Repair & Fabrication
Spoke replacement is the removal of a broken bicycle spoke and the fitting of a new one of the right length, with the wheel retensioned and trued.
Working Out The Length
The easiest way is to copy an intact spoke from the same side of the wheel: measure it from the underside of its head (where it bends) to the end of its thread. When there is no twin, compute it. For a straight spoke from hub flange to rim hole, the length comes from the law of cosines:
ℓ = √(R² + r² + w² - 2 R r cos θ)
where R is half the effective rim diameter, r the radius of the hub flange hole circle, w the sideways offset of the flange from the wheel’s centre-plane, and θ = 360° × (number of crossings) / (spokes per side). Take assumed figures: an effective rim diameter of 600 mm, so R = 300 mm, r = 30 mm, w = 35 mm, 36 spokes (18 per side).
- R² + r² + w² = 90,000 + 900 + 1,225 = 92,125
- 2 R r = 2 × 300 × 30 = 18,000
| Lacing | θ | 18,000 × cos θ | ℓ = √(92,125 - that) |
|---|---|---|---|
| Radial (0-cross) | 0° | 18,000 | √74,125 = 272 mm |
| 2-cross | 40° | 13,789 | √78,336 = 280 mm |
| 3-cross | 60° | 9,000 | √83,125 = 288 mm |
| 4-cross | 80° | 3,126 | √88,999 = 298 mm |
The rim and hub dimensions are assumed; measure your own, and take off roughly the radius of the hub hole (about 1 mm) from the result. The effect of lacing is clear: going from 2-cross to 3-cross adds 8 mm and from 3-cross to 4-cross adds 10 mm, so a spoke taken from a wheel with a different lacing is the wrong length.
A spoke 1-2 mm too long can be tolerated if its end does not protrude from the nipple into the rim well; a spoke too short does not reach far enough into the nipple to hold, and cannot be used.
What You Need
- A replacement spoke of the same gauge and length: salvaged from a scrapped wheel, if no new one exists
- A nipple to match
- A spoke wrench, and a file
- Wire cutters and eye protection
- A truing reference (Wheel Truing)
Method: Fit And Tension
Step 1: Remove the tire, tube and rim tape to reach the nipple.
Step 2: Remove the broken spoke and nipple, unscrewing the stub from the nipple. On the freewheel side of a rear wheel the hub flange is partly hidden behind the sprockets; removing them may be necessary (recalled practice for rear wheels).
Step 3: Feed the new spoke through the correct hub hole. Spoke heads alternate inside and outside the flange; follow the pattern of the neighbouring spokes.
Step 4: Weave it over and under the crossing spokes in the same way as its neighbours.
Step 5: Thread the nipple onto its end and tighten until the tension matches the neighbouring spokes by feel and by the pitch when plucked.
Step 6: True the wheel, adjusting the new spoke and its neighbours by small turns.
Step 7: File flush any spoke end that protrudes from the nipple, then refit rim tape, tube and tire.
If no spoke exists: a broken spoke can be tied off by wrapping it round a neighbouring crossing spoke to keep it from swinging into the frame, and the wheel ridden gently, with the rim re-trued (recalled field practice; the carrier gives no figure).
Common Mistakes
- A spoke of the wrong lacing length — see the table above.
- Leaving the new spoke slack — it carries nothing and its neighbours carry more.
- Overtightening — the rim cracks near the nipple, or the spoke breaks.
- Leaving the spoke end protruding — it punctures the tube.
Safety
Cut wire and a wheel with a missing spoke
Cutting or filing spoke ends sends fragments flying, and a wheel with one spoke gone is already weakened, so adjacent spokes can break in turn. Wear eye protection, test the repaired wheel at walking pace and then at moderate speed, and re-check the tension after the first ride.
What’s Next
- Learn next: Improvised Parts & Fabrication — when no spoke exists
- Combine with: Wheel Truing — bringing the new spoke into balance