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Wheelbuilding Question

Old 10-20-13, 12:03 PM
  #1  
Meek
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Wheelbuilding Question

Hi All,
I have a Mtn powertap sl+ disc hub on the older ZTR Arch 29er wheel. I want to change rims. Punching in my current set-up 3x rear wheel using http://www.prowheelbuilder.com/spokelengthcalculator/ it spits out a proper spoke length of 291.9 and 291.6. Well I just disassembled the wheel and measured both side spokes and they are 289. How are these not too short? I thought you could only go up or down 1, maybe 2mm? Now that I have the spoke length, am I to trust that site to determine rim compatibility? Any help would be appreciated in determining what 'road/cyclocross' rims may be applicable. FWIW the wheel despite being a 2010 has very, very low miles hence my desire to reuse the spokes. I just need a rim I can pump up properly for road use on my disc cross bike, Stan's said 50psi max and that wont cut it and that why I need a rim swap. Thanks.
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Old 10-20-13, 12:10 PM
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Before disassembling the wheel, did you note how far below the tops of the nipples the spokes ended?

That is key to understanding the issue. Many builders prefer using spokes somewhat shorter than ideal, because it eliminates the risk of the spokes coming up beyond the top of the nipple, or running out of thread. Or it's possible that the ERD value you entered is a bit high, or some other variance in the data input.

In any case, having the old spoke, and knowing where it ended is your best guide to selecting the right spoke length. Either use the same, or go up or down a bit if you want to change the final spoke height in the nipple.

If you didn't note the height, you have to toss a coin and either go with the calculated number or the original length, or you can split the difference, which might be the safest bet.
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Old 10-20-13, 12:17 PM
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They are very deep in the wheel. Does this mean I can use a slightly more aero (deeper) wheel, which would correspond to a rim lesser ERD?
Since I used their input values, can safely look for a rim/hub combo that is between 289 and now 291.6?
Thank you.
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Old 10-20-13, 12:29 PM
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Originally Posted by Meek View Post
They are very deep in the wheel. Does this mean I can use a slightly more aero (deeper) wheel, which would correspond to a rim lesser ERD?
Since I used their input values, can safely look for a rim/hub combo that is between 289 and now 291.6?
Thank you.
Just to be clear, I'm not talking about where the nipples are in the rim, but how close to the top of the nipples the spoke reaches. The ideal height is for the spoke to end between the top of the nipple or up to 1.5mm below that. It's OK to go beyond the top of the nipple by a bit, and I prefer to err high, but to do so you have to make sure there's enough thread for a margin of error. Some spoke and nipple combinations run out of thread and jam once the spoke reaches the top of the nipple, leaving no high side margin for error.
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Old 10-20-13, 03:09 PM
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If you plan to use the same spokes you almost have to use an identical rim. It might be possible to find another rim model that has the same ERD, but rim design accounts for most of the difference in the spoke lengths that are required.
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