Bicycle Mechanics - using ERD to determine spoke length
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I have a trashed front wheel which I know the ERD. I Have a new rim that I also know the ERD. Is there a rule or calculation that I can use to choose new spoke length without using a spoke calculator?
It seems like this should be not only possible, but easy, but the head is a bit foggy this morning.
Oh, the same hub will be used to build the replacement wheel.
08-27-07, 10:32 AM
If the wheel's not too old, it may be worth using the original spokes, especially since it's a front wheel. Trashing the rim rarely wrecks the spokes - they're much more likely to go slack, which doesn't hurt the spokes. If you can get a new rim with the same or nearly-the-same ERD as the old one, you can just rebuild using the old spokes. That way you don't even need to re-lace the spokes - just tape the new rim to the old and move the spokes over one-by-one. I've done this a number of times and it's quite easy.
For calculating spoke length you need to know your rim's ERD, and also the flange dimensions of the hub, number of spokes/holes, and the cross pattern. Easiest way is to use Damon Rinard's Spocalc spreadsheet, which you can get here:
08-27-07, 10:43 AM
You might just take the difference in the two rims' erd's and add or subtract that to the length of the original spokes.
On the other hand, since you know the erd of the new rim, it's pretty to plug that number into a spoke calculator. That's what I would do.
it would make sense to simply add or subtract the difference wouldn't it?
that said, the crosses likely complicate matters, so, perhaps there's a ratio or multiplier for that difference in the ERD?
after all that, I'd probably run the numbers through a spocalc just to make sure to appease my OCD :p
just musing a bit, I'd like to hear what others have to say.
08-27-07, 11:58 AM
For radial-laced wheel, you could directly add/subtract - but it would 1/2 the difference in effective rim diameter, as spokes only go one direction (the radius), not the whole diameter across the circle.
Adding/subtracting should work fine on a regular 700c rim with slightly diff ERD's, though, even with 3-cross.
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