Spoke length calculator

Ever found yourself with a rim and a hub, but no idea what length spokes you will use to join them together? Each time I dismantled a veteran wheel, I would dutifully record all the appropriate measurements, so that after a few years I had gathered a good database of wheel info and spoke lengths. When I reached for my sheet a couple of weeks ago, I found that something (a snail? a mouse?) had eaten half of it! In despair, I have had to retreat to calculating spoke lengths using a formula from Jobst Brandt's book The Bicycle Wheel. Before you use the calculator, make sure you read the info below, particularly the hints, the disclaimer and my wife's disclaimer.

Effective rim diameter: This is the most important measurement! It is the diameter of the circle made by the ends of the spokes when the wheel is built. Note that a drop-centred rim, a dimpled rim, and an undimpled rim may have the same outside diameters, but that their effective diameters may vary. Try to estimate this measurement to an accuracy of 1 mm or so.

Hub diameter: This is the diameter of the circle on which the spoke holes are drilled: i.e. the centre-to-centre distance between two opposite spoke holes.

Flange spacing: Measure the distance from the centre of one spoke flange to the centre of the other.

Diameter of a spoke hole: Usually, this is just a little larger than the diameter of a spoke.

Cross Pattern: This is just the number of other spokes that a spoke crosses on its journey from the hub to the rim.Other than veteran Rudges ( 0 cross in a Multi back wheel, 2 cross in a veteran front wheel ), most wheels use 3 or 4 cross.

Number of spokes: Sure to be an even number! On a "typical" veteran motorcycle, you might find 36 at the front and 40 at the rear, but BSA owners will delight in 32 at the front, and NSU owners will be busy with 48 in the rear. The time Rudge Multi owners save by spoking 0 cross will be lost as they cut and thread the 80 spokes for the back wheel!

Spoke length: Is measured from the inside of the spoke elbow to the end of the spoke.

Hints for advanced uses: For asymmetric spoke flanges, you can use the calculator to do one half of the wheel at a time. Start with one flange, but instead of using the real flange spacing, use twice the distance from that flange to the centreline of the rim. The calculator will give the spoke length for that side. Repeat for the other. Remember that some asymmetric wheels have different cross patterns on each side. For any wheel where the rim is drilled far from the centreline, you will get a better estimate of the spoke length if you decrease the flange spacing by an amount equal to the spacing between the left and right spoke holes in the rim.

Disclaimer: I'm not a real expert at either wheel-building or programming in JavaScript, so use the calculator at your own peril. I usually carefully work out the correct length then cut the spokes a millimetre or two longer longer to be safe, build the wheel, then end up grinding the extra length off the spoke ends! Seriously, the formula works well for bicycle wheels. It seems pretty good for veteran wheels with light spokes ( 12 or 10 gauge ) and skinny hub flanges. For later wheels, you will have to use your judgement to make allowance for fat spokes, thick flanges, funny rim drillings  etc.

My wife's disclaimer: My wife has read this page over my shoulder, and would like everyone to know that my data sheet was eaten by vermin in the shed. Jazzy and Flippy (the house cats) would like everyone to know that they would gladly eat vermin daring to enter the house.

Copyright Leon Mitchell 2000

Home | Leon's Motorcycles | Australian motorcycles | Miscellany | Buy swap and sell | Links