This is an interesting math exercise that has been largely replaced (hidden, actually) by web-based spoke length calculators, linked to nice databases for hub dimensions and rim ERD values. I always measure rim ERD for myself and measure at at least 2 different points around the rim. And I always check the supplied hub dimensions for myself. Sometimes a bad value is stored, so checking is worth the extra few minutes it will take. Some hubs and rims are rare enough that they aren’t in the databases at all, so once you’ve determined the measurements, sending them to the various calculator sites is a nice way to pay it forward. Good karma from that can’t hurt. Remember, you are the one that will pay for replacing spokes that are not the correct length, so measure at least twice. Personally, I consider the correct length to be where the top of the spoke is level with the bottom of the slot in the nipple.
A couple of web-based spoke length calculators are:
2/3/4 cross lacing:
The basic formula is:
L = sqrt(R^2 + H^2 + F^2 – 2RHcos(360/h*X)) – shd/2
Where:
L = calculated spoke length
R = rim radius to nipple seat (ERD/2)
H = hub radius to spoke holes (spoke hole circle diameter/2)
F = flange offset from hub centerline
X = cross pattern (2, 3, 4…)
h = number of holes in one side of the hub
shd = diameter of spoke hole in the hub
Check it out, it’s not really that complicated. Most any calculator can handle this fairly easily. When you’re done, try one of the online spoke length calculators to see what lengths they calculate. You can try more than one spoke length calculator and average the results (they will likely have slightly different results, depending on any fudging done during the calculations).
Since spokes are generally available in 2mm increments and some in 1mm increments, you’ll likely need to round the calculated values. For me, I tend to round to the closest available length, but you’ll have to determine that on your own. Experience with calculated lengths on successful wheel builds will tell you which way to go with different length calculators.
Definitely a math geek’s exercise, but sometimes it’s nice to see how the “behind the curtain” work is actually done.
Radial lacing:
For radial lacing it’s much simpler. it’s just a right triangle and you’re solving for the hypotenuse. The formula is:
L = sqrt((R-H)^2 + F^2) – shd/2
Where:
L = calculated spoke length
R = rim radius to nipple seat (ERD/2)
H = hub radius to spoke holes (spoke hole circle diameter/2)
F = flange offset from hub centerline
shd = diameter of spoke hole in the hub
Be aware that there are multiple places in the formula for rounding, and length calculators can and do round differently. This accounts for the differences between methods. All of them should deliver results within a mm +/-.
And you thought you’d never use trigonometry once you finished school.