Bicycle Mechanics - Spoke Length

I'm building a wheel at the moment (my third ever) and while i have the spokes I need as supplied by my LBS I am interested in how the spoke length is calculated. I'm not quite interested enough to work it out from first principles but I know there is a formula for it. Does anyone have it?

ooops, I just found a load of info in the barnets manual thanks to Mr Gardner's post on the Barnett's manuals thread and it looks like I need to read that before bothering you all again

Cheers :)

you can download a spoke calculator.. I think its in sheldon brown's webpage..

I have used that.. And its good.

You can also go here and use a spoke calculator

DT Swiss (http://www.dtswiss.com/index.asp?fuseaction=category.bikes)

:beer:

Here's a website that has formulas for the DIY'er mathematicians.
http://www.stormpages.com/spokeanwheel/

what wheel are you building up?

Thanks for the advise - some great info there. To answer your question Danka24, I have a front and back to build up. I have xtr disc hubs and mavic 317 rims. I'll have to check the spokes 'cos i didn't get the make - think they are DT black stainless? I confess i just asked the shop for what he recommended - I trust him. (I suppose they must be annodised black?)

Also I have Stan's no tube kits for them on the grounds it would be cheaper and lighter than the mavic x3.1 rims. Got shafted by the uk inland revenue and messed about by the post office but that's another story.

I'm building a wheel at the moment (my third ever) and while i have the spokes I need as supplied by my LBS I am interested in how the spoke length is calculated. I'm not quite interested enough to work it out from first principles but I know there is a formula for it. Does anyone have it?

The other day while taking a coffee brake from working on bikes, the combination of bike-brain and caffeine buzz inspired me to sit and work out the spoke-length calculation formula from first principles. I've been out of practice with math, but I knew I could handle the light trig and geometry it'd take.

Anyway, I sat down, drew some diagrams, and came up with the formula, given all the appropriate measurements. I had my PDA handy, so I had a regular ol' calculator and a spoke-calc program. So I came up with some example values, plugged 'em into my formula, and went to compare the answer w/the spoke calc, and what's this? the spoke calc program has a field for spoke hole diameter, which I hadn't figured in at all.

Well, it turns out my formula was right for an idealized one-dimensional spoke. That is, I got the same answer as the spoke calc with spoke hole dia. set to zero. And for my examples, when I set the hole dia. to 2 mm, the end result was around 1 mm off, so my formula should have an acceptable margin of error.

But now I can't figure out how I would change the derivation of my formula to account for the spoke hole diameter. I checked out the site Bigwheel mentioned (http://www.stormpages.com/spokeanwheel/), and its author came up with the same calculations I did, leaving out the spoke hole diameter..

Does anyone here have any idea how it changes the calculation? Feel free to get technical; if I don't get what you mean, I can probably find a book to look it up in..

Thanks in advance.
Y.

Maybe spoke length is measured from the inside of thespoke under the head, rather than from the centre of the spoke under the head. This would be easier to measure as you can push a ruler against that corner.

Maybe spoke length is measured from the inside of thespoke under the head, rather than from the centre of the spoke under the head. This would be easier to measure as you can push a ruler against that corner.

Hmm.. In which case it would be a simple matter of subtracting half the spoke hole diameter from the result of the prior calculation to get the measured spoke length, wouldn't it? I'll have to try a couple test runs to see if that's the only difference..

aybe spoke length is measured from the inside of thespoke under the head, rather than from the centre of the spoke under the head. This would be easier to measure as you can push a ruler against that corner.
Hmm.. In which case it would be a simple matter of subtracting half the spoke hole diameter from the result of the prior calculation to get the measured spoke length, wouldn't it? I'll have to try a couple test runs to see if that's the only difference..

Yup, that was it. Duh.

Thanks, Andrew.

I'm building a wheel at the moment (my third ever) and while i have the spokes I need as supplied by my LBS I am interested in how the spoke length is calculated. I'm not quite interested enough to work it out from first principles but I know there is a formula for it. Does anyone have it?

I have it around here somewhere, but I haven't used it for years. I actually used it semi-successfully once to lace up a rim on an Atom hub brake. The spokes came out to be a few millimeters too short so I compensated by using longer nipples. The formula is a slight derrivation of the long form of the Pythagorean theorem (Law of Cosine). The hardest part of using it is making sufficiently accurate measurements of all of the component parts.

Much easier are the variety of computer spoke calculators that now abound. The easiest ones to use have data bases of hubs and rims so all you have to do is to enter the number of spokes and number of crosses. They're usually more accurate too.