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Effective Rim Diameter (ERD) and Spoke Length

When building a bicycle wheel, the Effective Rim Diameter (ERD) of rim is an important measurement used to calculate the correct spoke lengths. It is very important to know that the ERD we list. Below is information on how to properly calculate ERD, and ERD values for our carbon Rims.

What is ERD - Effective Rim Diameter?

At Carbonal 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, we calculate from the top point of the nipple.

The corresponding equation and a representative figure for that definition are as follows:ERD-Effective rim diameter

                               ERD = A + 2B

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

--1. Insert two old spokes into holes exactly opposite each other on the rim. Count holes to be sure.

--2. Screw some nipples onto the spokes.

--3. 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.

--4. 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). 

--5. ERD = A + 2B.

As you can see from the figure, if you use different nipples then we use at Carbonal, 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.  

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