Calculating Spoke Lengths

The formula for calculating spoke lengths is complicated and I’ve made no effort to try to understand it. Fortunately there are some excellent web-based resources that mean we don’t have to worry about it at all! The best known of these are listed in the “RESOURCES” post, and Damon Rinard’s site also includes the proof of the spoke calculation formula for the insomniacs out there.

Rinard’s free “Spocalc.xls” (full strength version) is an indispensable point of reference which you should definitely download. In addition to a multi-function spoke length calculator it includes fairly comprehensive hub and rim databases covering loads of different wheel types and sizes and it’s spreadsheet format means you can save your workings without having to re-key everything. It’ll be bad luck if your components aren’t listed, and from past experience I know that the stored dimensions and resulting calculations are sufficiently accurate that it can be trusted. It would however be good practice to check Spocalc’s default measurements against the actual components with which you will be building your wheels (my natural indolence means I haven’t bothered doing this!).

Another well-known resource is the DT Swiss spoke calculator, which we can access for free from the DT Swiss homepage. This also contains in-built hub and rim databases but is not as comprehensive as Spocalc. It’s easy to use but you can’t save your work, although I should think you can print it out. Interestingly, it generates different spoke lengths than Spocalc for the same components, but only a millimetre or so either way and this is not enough to have any significant effect on its own.

The other commonly used spoke calculator is on the Wheelpro website, but as this doesn’t include component databases I didn’t use it.

Primarily I use Spocalc, but I run my numbers through the DT Swiss tool by way of a check. On dished wheels (most rear wheels and any front wheel with a disc hub), the spokes required on the left and right of the wheel will be slightly different lengths. This is usually only by a couple of millimetres, but it’s enough to cause problems if you don’t get it right. Spokes that are too long will protrude from the nipple and may stick into the tube, rendering the wheel useless. Too short, and they’ll not be long enough to screw entirely into the nipples which might leave the spoke threads showing (unsightly) and could over stress the nipples causing them to fail (they’re only soft brass, after all).

An added problem is that you’ll often find that shops only sell spokes in even-numbered lengths, and even those that do all lengths may not have your exact requirements in stock. In such cases do you go long or short? I have no idea, and guess what? Yep, one of the spoke lengths I need is out of stock! The shorter in-stock length is only 1mm shorter than my calculated length where as the longer alternative is 2mm longer. So, worried about spoke ends protruding from my nipples (I mean, who wouldn’t be?!?!) I’m going to take a gamble here and round my spoke lengths down to the nearest length available from www.rose.de, and instead of buying 12mm nipples for these I’m going to get some 14mm versions. I’ve never seen nor read of this option before (Rose also sell 16mm nipples, which are intended for use in deep-section rims) and I hope it works! It’s nice to have got in a nipple gag too ;-)