CYCLE CALCULATORS

CalculatorsSpoke Length Calculator

Spoke Length Calculator

Enter the rim ERD and one hub flange. Left and right flanges often differ on a rear hub or a disc hub, so calculate each side.

Inputs

Units
Distance from the hub centreline to the flange centre.

Results

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Please tick the box above to display the Result.

How the number is made

Length = √(R² + F² + O² − 2·R·F·cos θ) − 1.3 mm, where R is ERD/2, F is flange diameter/2, O is the flange offset from the hub centre, θ is 720° × crosses ÷ spoke count, and 1.3 mm is half a standard 2.6 mm flange hole. ERD is the diameter where the spoke ends should sit, so nothing is subtracted at the rim. Example: ERD 600, 58 mm flange, 18 mm offset, 32 spokes 3-cross → 289.4 mm.

One flange per calculation. A rear or disc hub needs a second run with the other flange diameter and offset. The 1.3 mm comes off because the spoke elbow bears on the edge of the flange hole, not its centre. Spoke stretch and rim compression under tension are not included; together they are worth about 1 mm, so round down when you fall between sizes. A 1 mm miss is usually tolerable; 2 mm short leaves too little thread in the nipple and 2 mm long can poke through the nipple head.

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Questions

What is ERD?

Effective rim diameter: the diameter of the circle where the spoke ends should sit, normally flush with the nipple heads inside the rim. Use the rim maker’s figure or measure it with two spokes and nipples. It is larger than the nipple-seat diameter, so do not measure the seats alone.

Do I run the same length both sides?

Only if both flanges match. Disc and cassette bodies usually offset one flange.

Why does the result subtract 1.3 mm?

The spoke bends through the flange hole and its elbow bears on the hole’s edge, so its working length starts about half a hole diameter closer to the rim than the hole centre the geometry uses. Most hubs drill 2.6 mm holes for 2.0 mm spokes. Nothing comes off at the rim, because ERD already marks where the spoke should end.

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