How exactly to measure ERD (Effective Rim Diameter) for wheel building

[BikeGremlin]

I got a question via email, and I hope the answer will help anyone with a similar dilemma:
(the original email is redacted for privacy)

Hi Relja,
Thank you for posting your web page on wheel building, it's very informative..
Unfortunately, in spite of your detailed explanations I'm still confused as to where you are taking the measurements.for ERD

Is the ERD taken from:
the seat of the nipple (where the nipple contacts the rim) as in Picture 1?
the top of the nipple as in Picture 2?
the end of the spoke as in Picture 5?

Thank you for your help,
Regards,
redacted

 

[BikeGremlin]

Hi,

Section 3 of the ERD article explains how I do it in detail (at the end of the section I've added a video demonstration). Briefly put:

- I set the "tool" as shown in pictures 4, 5, and 6 of the article.
- Then I measure the distance and add the "tool's" length to the result
(I add it twice, as I've made two of the exactly same length and use them on opposite ends of the rim - with a double check measurements in two places, 90 degrees apart).

This basically measures what's shown in picture 5 and works with my calculations (I use nipples that I will be using for wheel building, so it minimizes the error margin when "exotic" rims and nipples are used).

I hope this makes it a bit clearer. Let me know if you have any other questions.

Relja