Here's a handy little tool for anybody trying to find the right length spokes for a BMX wheel. Ok, it doesn't have every combination of hub and rim but at least you'll find the most popular ones listed. Hopefully you'll find it useful (if you're building a wheel) and more accurate than some of the other information out there.

Why write our own spoke calculator?

It's so frustrating to see manufacturers/shops/distributors/websites giving out spoke lengths which seem to be plucked out of thin air. For example, one famous bike company says on the packaging of their rims "most front hubs: 190mm spokes, most cassette hubs : 188mm". How can anybody say that when it also depends on the dimensions of the hub you're running, not just the rim? And another famous brand suggest you use the same length spokes on both sides of a wheel when the hub in question has a much bigger flange on the drive side than the non-drive side?

We've been using this calculator in Pijin for the last 2 years without any problems. Might as well make it available to everybody who has internet access. Should save us a few phone/email enquiries about spoke lengths!

Spoke Length Calculation

Ok, here's the formula we use to calculate the spoke length. I guess that trigonometry I learnt at school wasn't so useless afterall! I'm not going to go through the proof of this formula... I'm sure you can search for it on Google if you're really mathimatically inclined.

l is the spoke length
r_r is the radius of the rim
r_f is the radius of the hub flange (measured to the centre of the spoke hole)
f is the offset of the flange from the centre of the wheel
x is the number of times a spoke crosses over another spoke
h is the total number of spokes in the wheel
d_h is the diameter of the spoke hole in the hub

I guess there is a slight inaccuracy in the formula in that it assumes the actual diameter of the spokes to be infinitely thin. This will give spoke lengths which are a fraction of a millimetre too short. However, this is balanced out because in the real world, the holes in the hub stretch when the spokes bed in, so I guess these 2 factors pretty much cancel each other out...

- John