ERD Measurement Using the Elastic Band

written on Saturday, February 1, 2014

Being an amature wheelbuilder, you must really carefully measure the rim's ERD (effective rim diameter), which is the fundament of the success. For practical purposes, ERD could be defined as the distance between opposite ends of the spokes with nipples screwed on them until the nipple's thread is fully covered and not more. In other words, you should:

  • take 2 spokes
  • protrude them through the opposite holes of the rim
  • screw on them the nipples to achieve the following state (a bit more then on the photo, to be honest), in principle - to achieve the desired state: nipple
  • measure the distance between spoke ends

So, you've put the spokes and nipples into the rim, but how to measure the distance? Roger Musson in his great book "The Professional Guide to Wheel Building" - which I would like to recommend to any beginner as the really, really well-written practical manual, - proposes to use two cutted up to 20cm spokes, measure the difference between them and then calculate the ERD using the formula ERD = 2 x 20 + measured distance.

Nevertheless, practical application of this trick in home conditions was a bit difficult for me - you should keep the rim stable, hold the spokes as straight as possible, hold the ruler and them make the correct measurement - all in the same time. So, I was literally forced to engage my brain and - bingo! - I've realized, that to free hands and not hold the spokes all the time I can just use the simple small elastic band on uncutted spokes for holding them together!

el_band

So, now the ERD measurement algorithm for me looks like this:

  • take 2 spokes
  • mark 20 cm from the threaded end
  • screw on them the nipples until the full thread engagement
  • stretch an elastic band between the bent ends of the spokes
  • measure the distance between marks and then calculate the ERD using the aforementioned formula.

The overall picture looks like this:

rim_measurement

As the result, you can make the measurements with ruler confidently and with great precision :)))

I hope you've liked the trick!

Categories: wheels

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