Effective Rim Diameter and Spoke Length

When building a cycling wheel, the Effective Rim Diameter (ERD) is an important measurement used to calculate the correct spoke lengths.  A lot of rim manufacturers will list the ERD of their rims (we do too), but it is very important to know that the ERD we list, can be different from the ERD you will calculate for your specific wheel build.  The short answer is because you may be using different nipples and components for your build then we use with our wheel builds.  Below is information on how to properly calculate ERD, and ERD values for our FLO Rims.

What is Effective Rim Diameter?
At FLO we use the technique defined on Sheldon Brown’s website to measure ERD.  That technique is defined as follows:

Effective Rim Diameter (ERD) is the diameter on which you want the ends of the spokes to lie. Most people prefer it near the end of the spoke nipple.

The corresponding equation and a representative figure for that definition are as follows:

ERD = A + 2B 

If you want to measure your own rim (recommended, just to be sure), then follow these instructions:

  • Insert two old spokes into holes exactly opposite each other on the rim. Count holes to be sure.
  • Screw some nipples onto the spokes.
  • Pull them tight and measure dimension A in the figure (the diameter to the edge of the nipples, where the spokes disappear into them). Do this at several spots around the rim and average the measurements.
  • Measure the length of a nipple (dimension B in the figure) and add it twice (once for each nipple). The result is Effective Rim Diameter (ERD). Thus,
  • ERD = A + 2B.

As you can see from the figure, if you use different nipples then we use at FLO, your ERD value could be different.  As a result, we recommend that you always measure your own ERD value before building your wheels.  For more specific information on how to properly complete the measurements above, we suggest you thoroughly read through Sheldon Brown’s website, or take your wheels to an experienced wheel builder.  

FLO ERD Values
Assuming you’ve read the information above, it is safe to say that you already know the values I am listing below are subject to change for your specific build.  That said here are the ERD values for FLO rims, and the components we use to calculate these values.  

FLO 49 AS: 541.0mm
FLO 64 AS: 510.2mm
FLO 77 AS: 485.8mm
FLO G700: 529.2mm
FLO G650: 483.4mm
FLO 45 Carbon Clincher: 548.4mm
FLO 60 Carbon Clincher: 513.9mm
FLO 90 Carbon Clincher: 452.3mm
FLO 30 Aluminum Clincher: 577.5mm

Spokes: Sapim 12mm Hexagonal Brass

Note: We build all FLO wheels with nipple washers and recommend you do the same.  We use Sapim HM Washers.

I hope this article has been helpful.  If you have any questions or comments.  Please feel free to leave them below.