I'm about to build my 7th wheel and still learning. I bought a nice shiny Pacenti rim and the sticker reads "ERD 611." Because I'm still trying new things, I measured it, several times, in several locations, and I get an ERD of 615.
I cut 2 spokes to 200 mm and screwed a nipple on each one until the spoke was flush to the top of the nipple. I put 'em in opposite holes on the rim and carefully measured the space between them. When I total the numbers, my measurements are always higher than the published ERD.
If I calculate spoke length with the published numbers the spokes would probably reach to the bottom of the screw slot.... Why would I waste the last remaining bit of threads for such a thing?
Am I missing something simple?

I've always hated published ERD measurements, and would greatly prefer rims to publish the actual measurement of the diameter at the spoke nipple seat.

I believe ERD fails as a standard because it includes an assumption about the desired height of the spoke in the nipple. You measured to the top of the nipple, and called that the ERD. The rim maker probably assumed the builder would want the spoke ending short of the top, possibly at the height of the screw slot and called that the ERD. The difference of 1mm on each end means a difference of 2mm in ERD. A different assumption about the desired spoke height would likewise mean different ERD.

This is the core of my objection to published ERD. It's like playing the Newlywed game, where the wheel builder has to guess what the rim maker thinks about his desired spoke height.

ERD is specific to the way the wheel will be built. If you want your spoke ends at the top of the nipple (or maybe a little above), then that is how ERD should be measured. Apparently it is becoming common to publish ERDs based on spokes ending somewhere in the driver slot. This is partly because there is no tension on the spokes when you are measuring. Thin gauge spokes can stretch as much as 2 mm under full tension. Hence starting a little lower. If you don't mind the spoke sticking over the nipple top a little, no problem, but you risk running out of threads and being unable to fully tension the wheel. About halfway into the driver slot is a good compromise. But it is strictly user specific. Make your own decision and measure that way.

Also every rim is different, but not every rim is actually measured. So you have to expect some discrepancies from the published value. That is why the experts say measure every rim according to how you want your wheel to come out. Then there are no oops-es.

The two last brands of rims I built on used the surface where the nipple contacts the rim as the published ERD.

Eons ago, the nipple seat diameter was the ERD, but that concept is pretty much gone these days. Most spoke calculators will give you a spoke length value that will have the spoke ending at the same diameter as the ERD value entered.

So it's important that one knows what to expect, and if using a calculator that asks for ERD, either add the allowance for the nipple headto the actual diameter (doubled) to get the ERD, or use the actual diameter, and add the nipple head allowance to the result, which is what I do.

It doesn't matter what you call things, or how you prefer to measure, as long as the builder, rim maker, and spoke calculator are all using the same assumptions, or compensate accordingly.

The problem is that these days, people are doing things differently, but calling it the same, so spoke calculating has become a GIGO process. This is why I and many experienced builders, measure everything ourselves rather than using an unreliable spec. and use the same calculator every time.

https://www.sutherlandsbicycle.com/rimdiametersystem/

There's no need to spend any money because it isn't a problem of how to measure, but what to measure. The OP's two spokes are perfectly fine as a measuring tool.

In this thread I explain what ERD really is and remind folks that it is NOT a static reference, white paper spec, etc.:

https://www.bikeforums.net/showthread...?highlight=erd

Post #56



In this video I explain how certain variables influence aiming points when measuring your own ERD:

https://www.youtube.com/watch?v=gCXFKN9JvCQ


Most important thing of all is the reminder repeated often here on BikeForums:

"It's always best to measure your own ERDs."

=8-)

Cute system. But three flaws: first unnecessary, second way overpriced, and third still just one guy's definition of ERD. It you want the spoke tip to end up in a different place, you have to measure to a different place. And you have to account for stretch.s

It does have an upside I like:

It will measure accurately a rim that is warped - because it's measuring circumference to arrive at an ERD estimate.

That's similar to the benefit of a dishing tool - with a dishing tool you don't have to worry so much about axle locknut imperfections or warped axles and the effect they have on flip flop dishing.

=8-)

Didn't we talk about inventing another word - like Nipple Seat/Support Diameter a while back?
Give us the true geometrical reference and adapting for preferences becomes a lot easier.

"Because I'm still trying new things, I measured it, several times, in several locations, and I get an ERD of 615."
Hats off to you for respecting the art enough to take responsibility for taking your own measurements.
I use your exact same method for measuring ERD and have always gotten accurate spoke measurements using Roger Munson's calculator, https://www.wheelpro.co.uk/spokecalc/.

Yep, you're absolutely right about that. The device has a built in averaging feature so only one measurement need be taken to get the best average diameter. That is neat. But on the other hand doing it that way you don't get an appreciation for how good or bad the rim is. You don't see how round or not the rim is based on multiple diameters taken at different points around the rim. Flatness of course is easy. Just lay the rim on a flat surface to see any rises and dips.

My main complaint is how expensive it is compared to other wheel building tools that provide more necessary functionality like a spoke tensiometer. For about $20 less than this device you can get a Park Tensiometer that is essentially indispensable for good wheel building, at least for most mortal humans who can't feel or hear spoke tension accurately. For the higher price of this device you get something you don't need in the first place. Your traditional method of measuring rim diameter works just fine even if it just a wee bit less convenient.

I don't know about a "we", but I've been calling it the actual rim diameter for years. I also proposed the term nipple seat diameter here on BF and elsewhere. It isn't about names. It's about clarity. For example if a rim maker saif ERD = 605mm including allowance of 3mm for nipples, I'd be very happy.

The problem is that some Europeans publish the actual diameter, an most others publish some sort of ERD but nobody says how they're measuring.

It won't measure rims like Mavic's UST tubeless and Fulcrums 2-way fit.

nothing is perfect...

I must be missing something. Why not?

tubeless, nipple is internal.

only hole is for the valve in the tire bed.

I've always used the method explained by FBinNY, and I always measure every rim before buying spokes. That combined with the simple Edd spoke calculator hasn't failed me yet.

What happens when you have to replace a nipple in one of these rims?

More to the point, how do the nipples get into the rim in the first place?

Screw-on eyelets most likely?...similar to the 819 Disc?

=8-)

yes.

Fulcrums you have to use a magnet to drag the nipple into place.
Pretty easy, I had no problems.

https://www.youtube.com/watch?v=nC8fV5nxJww