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How To Measure Rim and Hub To Calculate Spoke Length

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 length spoke, getting it right the first time is such a time saver. Who wants to re-lace a wheel twice because the spoke length was incorrect the first time. The database of hubs and rims should make the process a lot quicker too.

If you cannot find your desired hub or rim model within the database, it'll be possible in the future, to add them for everyone to use. However, in the mean time using the process below, you'll be able to add the measurements manually.

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

How To Measure Effective Rim Diameter for Wheel Building

Measuring the rim is one of the most crucial parts. for every incorrect millimeter, the spoke length will be in the region of 0.5mm out.

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.

Effective rim diameter is the measurement where the spoke will finish when the wheel is build. Do
        not mix this up with the rims internal diameter.

ERD = A * 2 + B.

Note:
A is usually the length of the spoke nipple. I say usually because we want the spoke to be flush at the top of the nipple when the wheel has been built and tensioned. However some wheel builders prefer the spokes to finish at the base of the slot in the nipple. If you prefer the latter, then measure the spoke length to that point.

There are different ways to measure the ERD. For myself, I've found this method the most convenient.

  • 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.

How to calculate Offset Spoke Bed (OSB) for Asymmetric rims?

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.

Before disc brakes became a thing for bikes, asymmetric rims were mostly used on rear wheels. As we know, spoke tension is higher on the right of the wheel compared to the left, and this can lead to a weakness with the wheels lateral strength. To overcome this the spoke tension needs to be more even between the two sides, and to do that, the wheel requires spoke lengths which are very close to each other.

With asymmetric rims for the rear hub, the spoke hole is offset to the left of the center of the rim. This intern reduces the distance the spoke needs to travel and shortens it length and almost evens the spoke tension between the two sides.

Since front disc hubs have a greater left flange offset compared to the right, we can use asymmetric rims to achieve better spoke tension too. However, the spoke offset is set to the right side this time around.

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 our example, the OSB is 2mm.

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

Finally this is where the real magic comes in. Very wide rims can come with an OSB very close to the rims edge on both sides, a good example are these rims from Sarma. Also some electric bike wheels are starting to cross the spokes to the furthest OSB to laterally strengthen the wheel. These will either require positive or negative measurements for both offsets. And guess what? this site can handle those 🔥

Which hub measurements do you need?

The main rule here is to always measure to the center of the spoke holes for DL and DR. These should be measured as accurately as possible. When measuring the flanges LFO and RFO the accuracy isn't as important. But whats the point in that!

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.

*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) - width

The default calculation is from Sheldon Browns website, which hosts Damon Rinard's Spocalc.xlm excel file. The formula is below. Also the website database holds all of the rim and hub measurements from the file. Looking at the file data attributes, this was last updated back in 2005.

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

Final Notes

If you have any suggestions towards spoke length calculations, you're more than welcome to add a message on the message board. Also if you have other spoke length formula's or cross patterns you would like to see added, please leave a message.



This page was updated on 29th July 2020

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