If you're looking to find the spoke length for a wheel build, then you've come to the right place. When it comes to calculating the correct spoke length, getting it right the first time is such a time saver. Who wants to re-lace a wheel two or three times because the spokes were the incorrect size?

It's now possible to save the measurements of your hub and rim to the database for everyone to use.

If you have an account, you'll be able to save your wheel build measurements for future use.

How to measure rim diameter

Measuring the rim's diameter, isn't a case of measuring from one spoke hole to it's opposite. In fact we need to measure a internal position within the rim, where the end of the spoke will be once the wheel has been built. This measurement is known as the Effective Rim Diameter, or ERD for short. There are plenty of ways to measure the ERD and some may be more easy than others and this is the way I do it. One a side note, some time int the future, I'll add the other ways too.

All good spoke length calculators require the Effective Rim Diameter. This is where the spoke will finish, which is usually flush with the top of the nipple.

What we really need is the Effective Rim Diameter, ERD. This is where the
          spokes will be once the wheel has been built.

Note:
A is usually the length of the spoke nipple. However, it's also perfectly ok to measure the nipple height to the base of the slot. Either measurement will result with enough thread contact between the spoke and nipple.

For this example I'm measuring a Mavic XM119 29" rim. Measuring the gap between the spoke nipples comes in at 575mm. The spoke nipples are 12mm long. However, you can use any length nipple as the end result will be the same. 12 * 2 + 575 = 599mm.

  • Lay a tape measure on the workbench, or even the floor, making sure its longer than the rims outer diameter.
  • Get two spoke nipples and securely place them in apposing holes in the rim.
  • Place the end of one spoke nipple at the start of the tape measure and then take a measurement at the end of the apposing spoke nipple. This will give you measurement B.
  • ERD = A * 2 + B.

Finally, as some rims are not truly round it's best practice to repeat the measurement at different points around the rim two or three times, and using the average of the measurements.

What is Offset Spoke Bed?

The idea behind the OSB is to even the spoke tension between the two
          sides. This can lead to a bicycle wheel being more laterally stronger.

The OSB is the distance from the center of the spoke hole to the center of the rim. When you look at the spoke holes, you'll notice they are staggered either side of the rims center line. This staggering with a symmetrical rim cancels it's self out, so for these rims we consider the OSB to be 0mm.

With an asymmetric rim, the spoke holes are moved to one side of the rim's center line. Traditionally, they were used for rear wheel were the rim is off center from the hub and helped to even the spoke tension between the two sides. Today with fat bikes, wider axles and disc brakes, asymmetrical rims are becoming more popular.

This is the formula to measure the spoke hole offset

  1. Measure the rims outer width and divide by 2. This is W.
  2. Measure the distance from the closest sidewall to the center of the spoke hole. This is L.
  3. OSB = (W/2) - L.
  • For a rear wheel enter the the measurement into the Right Spoke Offset box as 2 and -2 into the Left Spoke Offset box
  • For a front disc wheel enter the measurement into the Left Spoke Offset box as 2 and -2 into the Right Offset box

Basically, we're adjusting the flange distance to the hubs center line, which the rim should be aligned too, With this example we are reducing the left flange distance and extending the right flange distance.

For our example, the Mavic XM119, the OSB is 0mm.

How To Measure a hub

The main rule here is to always measure to the center of the spoke holes for DL and DR, and to measure to the middle of the flanges for LFO and RFO. For our example we'll be using Hope's Pro 4 front hub.

There are a total of five measurements for front and read hubs. The most
          notable differences will be the left and right flange offsets.
  • OLD Is the measurement from lock nut to lock nut
  • LFO Is the distance from the lock nut to the center of the left flange.
  • RFO Is the distance from the lock nut to the center of the right flange.
  • DL Is the diameter of all the spoke holes in the left flange. Measured center to center.
  • DR Is the diameter of all the spoke holes in the right flange. Measured center to center.
  • Spoke Hole Diameter The diameter of the spoke hole in the flange. Normally in that range of 2mm to 2.5mm.

For our Hope Pro 4 front hub, the measurements are OLD: 100mm, LFO: 57mm, RFO: 57mm, DL: 30mm, DR: 16.99mm and a Spoke Hole Diameter of 2.6mm

*Some sites have measurements from the flange center to the hubs middle, known as Wl and Wr, which can lead to your spoke calculation being 0.5mm to 1mm out! You can easily convert these to flange offset. flange offset = (OLD / 2) - Wl

Spoke length Formula

The default calculation is from Sheldon Browns website, which hosts Damon Rinard's Spocalc.xlm excel file. The formula is below.

pi2 = 6.28318530717959

a = (DL / 2 * sin(pi2 * Cross Pattern / (Spoke Count / 2))) ^ 2
b = (ERD / 2 - ((DL / 2) * cos(pi2 * Cross Pattern / (Spoke Count / 2)))) ^ 2
c = ((OLD / 2) - LFO + OSB) ^ 2

Spoke Length = sqrt(a + b + c) - Spoke Hole Diameter / 2

Using our measurements for Mavic's XM119 rim and Hope's Pro 4 front hub, with 32 spokes, the measurements are 289.18mm for the left and 290.37mm for the right spoke lengths. Now, most local bike shops keep spokes of even numbered lengths, for example 288mm, 290mm, 292mm. So we need to do some rounding, and a good rule of thumb is to round to the nearest even numbers. For us, this results to 290mm for both sides.

Final Notes

You can leave comments over on the message board, with comments about spoke length calculations and any recommendations you may have