# Spoke Length for Internal Nipple Rim



## SBH1973 (Nov 21, 2002)

I'm trying to figure out the ERD for a 38mm carbon clincher rim that uses internal nipples. The stated ERD (not accounting for internal nipples) is 564.9mm. According to the manufacturer, "It is not include the internal nipple. Owing to there are too many nipples. We can not know the length of each nipples." Okay, fair enough.

So, I used the very helpful instructions at:

How to measure SED

I measured the rim's outer diameter at 24 15/16", or 633.2mm. The distance from the rim edge to the top of the internal nipple was 22.05mm. So 633.2mm - 2x22.05(44.1) = 589.1 ERD.

So, when I calculate spoke lengths for a radial lacing of a front hub with a PCD of 31mm and flange-to-center of 39.25mm, I'm coming up with 280.3mm for the internal nipple (12mm Sapim inverted alloy, 589.1 ERD) and 267.9mm if it were an external nipple (using ERD of 564.9mm). In other words, the internal nipple requires a c.12mm longer spoke than the external. 

Does that seem right? Seems a like a lot. I hung a spoke through a Kinlin, one on an internal and one on an external, and the difference was about 8mm, even though the measurement wasn't very precise. But that seemed more believable than a 12mm difference.

Then again, the rim is 37.6mm deep and about 3.4mm thick at the edge. So, I suppose if you consider those 3.4mm and the 8 or so mm that the internal nipple takes vs. the external nipple, well, there's your roughly 12mm difference, right? Maybe?

Thanks!


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## valleycyclist (Nov 1, 2009)

If you have the rim, why can't you measure the ERD using two spokes threaded onto internal nipples?


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## SBH1973 (Nov 21, 2002)

valleycyclist said:


> If you have the rim, why can't you measure the ERD using two spokes threaded onto internal nipples?


Oh, yeah - that would make a lot of sense, wouldn't it? So, hang a couple of spokes from opposite ends and then measure the distance remaining between them and voila, ERD. Is that right?


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## Lectron (May 29, 2005)

Ask the guys at bikehubstore...They should know :smilewinkgrin:


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## valleycyclist (Nov 1, 2009)

SBH1973 said:


> Oh, yeah - that would make a lot of sense, wouldn't it? So, hang a couple of spokes from opposite ends and then measure the distance remaining between them and voila, ERD. Is that right?


Thread each of the spokes most of the way through internal nipples, and measure the distance between them. It is useful to mark a line before the spoke head to make the measurement easier (I mark it at 200mm from the end of the spoke threads). It is better than guessing.


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## SBH1973 (Nov 21, 2002)

Thanks for your help, Valley. I'm coming up now with 592-593 ERD using two 295mm spokes. There's about 3mm between the center of the J-bends on each spoke (see picture), so 590+3. 

Per Mike T's instructions, I also cut the j-bends off two 279mm spokes, measured them at 255mm and 256.5mm, threaded nipples on each, and found an average of 82mm between them at to opposite sides of the rim. Result: ERD at 593. 

So, that's what I'll go with. 

If this is right, the internal nipple rims require spokes almost 13mm longer than the same rim with external nipples. Wow - that's more than I expected.


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