I just purchased this rim, it's a velocity synergy 451. I'm trying to find the ERD for spoke purchase. I can't find the specs anywhere, the velocity site doesn't even list this rim. They only have the A23 in 451, are they the same? They look different when comparing both rim. Any help would be greatly appreciated. Pardon my ignorance but this will be my first attempt at building a wheel.

Bike Friday uses a lot of ISO 451 wheels and they may have the info on this rim. Otherwise measuring the ERD yourself isn't that complicated. A Google search should turn up several methods.

In the 559mm rims, the Synergy has a 1mm greater ERD listed then the A23. I'd expect the same in the smaller size.

Use a steel tape measure to measure overall diameter. Then lay something across the rim and measure down to the top of the nipple stuck in a hole. Add 1mm to the depth (difference between top and slot depth of the nipple) double that (convert radius to diameter) and subtract from the overall diameter, and you have an ERD.

Thanks, I'll try that...

Do the measurements FBinNY suggested at several places around the circumference and average the results. Most rims aren't precisely circular.

i think i would be skeptical even if i found a published ERD, in as much as the rim label looks to be spanning the spoke holes on both ends. not that i think it would cause any functional problem.

Rim labels that are too long aren't that rare. Companies design labels around the popular size, ie 32h 26" or 700c. Then when they decide to produce a smaller, low volume rim with the same extrusion they use the same label, which doesn't fit.

Doesn't reflect on anything except that they're too cheap to buy a short run of labels.

Yes, I should have said that. But IME small rims of toways heavy extrusions tend to be very stiff and round. It's not like 300gr 700c rims that might be oval varying by 10mm depending on where measured.

So play it safe, measure 2 places at right angles, and if they're the same you're good. Otherwise measure one more direction and average the three.

For three of the four sizes Velocity lists an ERD 19mm less than the ISO size, the other one is 18mm difference. In theory the difference between the ERD and ISO size should be consistent for the extrusion regardless of ISO size.

I'd guess 432.

If they calculated it empirically based on the extrusion, they all would have matched. Odds are they had an employe measure, and the real number is very close to a 1/2mm, so a slight error, made him round one down instead of up.

Velocity are good and helpful people. Maybe contact them direct...?

Velocity
2280 29th Street SE
Grand Rapids, MI 49508 US
Phone: 616-243-6564

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I've always found it more accurate to measure the circumference and divide by pi, then subtract twice the depth to the nipple slot. But this requires an accurate flexible tape measure, which are sometimes a little hard to come by...

Sutherand is offering a tape measure depth gauge set exactly as you describe. I believe the tape calibrated in diameters so you don't even have to divide by Pi. (if it isn't he missed a good move) All this for the bargain price of USD $75. I don't own anything dedicated to this job, so use whatever steel tape is handy, and make a depth gauge from a piece of cardboard, which I mark and measure.

Been doing it this way for almost half a century and it still works fine, though when everything is computerized, and pens no longer made, I might have to find a new depth gauge.

Yeah- I use one of those steel wind-up surveyor's tapes, which may not be that accurate after all. And I don't have any desire to buy a $75 tool when the diameter works just fine. But I *do* have a set of dial calipers with a depth tang on the end ($10 at harbor freight) for when they no longer make cardboard and pens

Bingo!! 432 is what I got, doing it FBinNY's way...Really i needed a number for reference (paranoid since this is first build) hence wanted the published data. I've contacted velocity but haven't heard back from them yet. Again, Thanks everyone for their input.