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Discussion Starter · #1 ·
Got a pair or carbon rims in to build for my wife and I (50mm and 38mm).

ERD listed for the 50mm is 540. I measured to double check before ordering spokes and came up with 547mm.

My method was stringing 2 spokes opposite each other (in 2 locations). Measured 519mm between spokes. Added the 2x14mm for the spokes. Which gave me (519+28)=547

Did the same technique for the 38mm rim, and came up with 572. Which seems about right, a difference of 25mm, which would be within 1mm of the 2x12mm difference in depth.


On the rim alone, measuring across is 533mm and according to this picture, the nipple bed is 3.35mm deep which would come out to (533+(2x3.35))=539.7. I am guessing this is how they calculated the ERD.

View attachment 277022

Now, since I was using a 14mm DT swiss nipple, the thread doesn't go all the way to the end, but it should not have made a difference on my measurement, only matters if I run out of room in the nipple... always wondered why 12mm can go all the way in, but the 14 and 16mm have a little lip on the flat-head end... curious, but not the point


Which number would you go with?
 

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I can't say which numbers are right for you. I tend to do my measurements outside in:
1) measure outer diameter of the rim.
2) screw a nipple onto a spoke backwards.
3) seat the nipple, and measure the depth from the outer edge to the screwdriver flat.
4) subtract depthx2 from OD.

At least you'll have three answers, and so be able to triangulate the answer. I would tend to trust a measurement over a spec, provided I was confident with the measuring method. Another choice might be to use you method on a known measurement rim and see if it works out.
 

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Discussion Starter · #3 ·
Thanks danl

I did your method, and came up with 547.5. From what I can figure is that the spec'd number is to the nipple bed, and my measurements are accounting for the 'head' of the nipple, all the way to the top, which according to my calipers adds about 3mm per side (head and tapered section of nipple).

DT swiss spokes will bottom out on my 14mm nipples before the spoke sticks out the back end of the thread, so I think I will use a bit smaller number than my 547, because I fear most bottoming out before tension is reached (using dt comp on the DS, so shouldn't be too much problem/stretch at 110kgf). I do want the most thread engagement that I can, but obviously bottoming out is a problem. I'll run the numbers with 540 and 546 or so and see what difference comes out in the end
 
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