Bit of a niche one, this, but if you find yourself needing to measure rim ERD this is one way to do it accurately.
You’ll be needing either:
- Two long straight spokes with nipples
- Some universal glue
- Two long conventional spokes with nipples (I’m using a pair of 314mm DT Champions from SJS Cycles)
- Some universal glue
- A tape measure
- A boltcutter or similar
- A file
Step 1: Think of a number
The first task is to think of a length, namely the total length that your two spokes will end up. Longer is better.
If you have straight spokes, it’s easy: just add the two spoke lengths together.
If you have conventional spokes, it’ll need to be at most 4mm less than the total length of the two spokes you start with. This is because you’ll be cutting off the heads and elbows, which means losing at least 2mm off each spoke.
I’ve picked 622mm, because it’s the ETRTO size of a 700c rim, which makes it easy to remember.
If you’re using straight spokes, skip to Step 4.
Step 2: Off with their heads
Now divide your number by two and cut your first spoke to that length.
Measure it after you cut it, because you probably didn’t get it spot on. But it doesn’t matter: it’s the total length of the two spokes that counts, they don’t have to be equal.
Now account for any error in your second spoke length, add 0.5mm for luck, and cut that.
Step 3: File down
Next step is to file the ends. Firstly to fine-tune the lengths so they add up to your desired number; secondly to remove any sharp bits.
If you end up missing your desired number for the total length of the pair, it’s no big deal. Any number will do, you just need to be able to remember it.
Be sure to clean the bits off the kitchen table before you get found out. (If you do get found out, claim that your daughter was playing with glitter again.)
Step 4: Sticky nipples
Next, put some glue on the thread of the first spoke.
Feed it into the nipple until the end of the spoke lines up with the bottom edge of the well in the head.
Now do the same with the other spoke.
Step 5: Wait
Now let the glue set overnight.
Step 6: Measure
Measuring ERD is now pretty straightforward. Feed your sticks into the rim through an opposing pair of spoke holes and bring them together. Measure the overlap and subtract it from the total length of your spokes.
So, let’s say I measure an overlap of 76mm which I subtract from my total stick length of 622mm, leaving me with an ERD of 546mm.
It’s worth repeating, using a second pair of spoke holes at 90 degrees from the first, just in case your rim is slightly oval. Take the average as the ERD.
And then off to your favourite spoke calculator you go with a skip and a hop.