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Is there anything I should know when calculating the ERD or spoke lengths when building with an offset rim? I ask as I'm trying to build the following rear wheel but the spoke lengths are definitely too short (I cant even finish lacing!):
RIM
Ritchey Girder OCR
ERD: 537 (my measurements, using the Musson method of 2x200mm spokes to the nipple slot head + difference)
HUB
Shimano M475 (measurements taken from Shimano Tech Doc)
Flange PCD 61mm (both sides)
L flange to centre 35.35
R flange to centre 21.75
36 hole, 3 cross.
I make that 256(L) and 254(R), to which (having read on http://lenni.info/edd/howtomeasure) I have added 1mm to the drive and take away 1mm for the non-drive, so 255mm on both sides. But this is definitely too short - I'm only on the 3rd group of 8 spokes when lacing and am already finding them too short.
Most likely problem is with the ERD, but every other rim I measure and have built with is fine, ie matches 'known'/manufacterers printed ERDs. I havent measured the hub entirely accurately myself, as its hard to, but it is definitely is 61mm, +/- 1mm at most, similarly the centre to flange measurements look accurate.
I tried varying the rim offset from 0-4mm using the option on http://lenni.info/edd/ but I still only get a variation of +/- .5mm spoke length. The spokes look to me to be at least 2-3mm too short.
I'm stumpted. Any ideas? I am right in thinking that the rim should be positioned so that the side wall that is closer to the spoke holes sits on the non-drive side with an offset rim, yes?
Thanks!
I think you may be missing the obvious, you're telling us that you're on your 3rd set of 8 spokes when you find they're too short by about 2-3mm, but you've based your calculations on a 36 hole set-up which requires sets of 9 spokes(usually 2-3mm shorter than a 32 hole set-up).
Sorry, meant to correct the '8' to '9' after I first posted, then forgot. I was using the correct number of spokes in my calculations. Thanks for suggesting that none the less.
It transpired the nipples simply weren't seated properly in the rim holes. So I'm a bit of an idiot.