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# Could someone check my math please? (wheelbuilding)

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# Could someone check my math please? (wheelbuilding)

02-21-08, 09:51 PM
#1
Kinetikx
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Could someone check my math please? (wheelbuilding)

I'm finally going to try my hand at wheelbuilding. Since this is my first build I'm not entirely confident in the calculations I used to get spoke length. I'm getting some Mavic Open Pro rims and using a set of 36 hole Shimano 600 hubs that were made in 1985. The wheels will be 3 cross using DT Swiss DB Competition spokes.

The E.R.D. I found for the Open Pros is 602

Rear Hub
Here are all the measurements I came up with for the rear hub:
Spoke hole size= 2.5mm
Hub Flange Diameter= 45mm
O.L.D.= 126mm
Left locknut to center of left flange= 25mm
right locknut to center of right flange= 44mm
Left hub center to flange center= 38mm
Right hub center to flange center= 19mm

Using this calculator http://www.bikeschool.com/spokes/ I came up with these lengths:

Spoke length left side 291.5 (292)
Spoke length right side 289.8 (290)

So I'm assuming that a length of 291mm would work for both sides of the wheel, correct?

Front Hub
The front hub was much easier but I'll include the figures anyway just in case:
Spoke hole size= 2.5
Hub flange diameter= 38
O.L.D.= 100
Left locknut to center of left flange= 14
Right locknut to center of right flange= 14
Left hub center to flange center= 36
Right hub center to flange center= 36

Using the calculator I came up with a spoke length 292.9 (293)

Does all that look about right?

My next question- Could I get away with using a length somewhere in between that would work for both wheels or should I get different lengths for each wheel?

Last edited by Kinetikx; 02-21-08 at 10:06 PM.

02-21-08, 10:14 PM
#2
waterrockets
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I'd round down rather than up on the rear spokes, but I also got 291/290mm using DT Swiss' spoke length calc.

On the front though, I'm getting 281mm, which is way off from what you got. You might double-check with a different calculator.

02-22-08, 12:04 AM
#3
Kinetikx
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Originally Posted by waterrockets
On the front though, I'm getting 281mm, which is way off from what you got. You might double-check with a different calculator.
Weird. I just did it with the the DT Swiss calculator and I got 294mm (this was using the generic "Shimano front 100mm" which has really close specs to mine). Any idea?

02-22-08, 05:57 AM
#4
Retro Grouch
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I just ran your numbers through the QBP spoke calculator.

I got 289.53/291.38 for the rear and 292.68 for the front. That's pretty close to your numbers.

FWIW I've ordered spokes from wheelbuilder.com for a few builds. He'll sell you the exact number of spokes that you want. Since this is your first try at wheelbuilding, I'd bite the bullet and get rear spokes that are 2mm different lengths.

02-22-08, 06:47 AM
#5
Ex Pres
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Originally Posted by Retro Grouch
I just ran your numbers through the QBP spoke calculator.

I got 289.53/291.38 for the rear and 292.68 for the front. That's pretty close to your numbers.

FWIW I've ordered spokes from wheelbuilder.com for a few builds. He'll sell you the exact number of spokes that you want. Since this is your first try at wheelbuilding, I'd bite the bullet and get rear spokes that are 2mm different lengths.

+1
My first rear build I tried the using the same spoke length (6/7s 80's DA hub) for both sides and I never got the dish exactly right. The wheel still worked just fine, but I had every drive side spoke out of threads and when the NDS was tensioned the dish was just off (probably a mm+/-)
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02-22-08, 07:33 AM
#6
waterrockets
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Sounds like I got something wrong on the front then.

02-22-08, 05:01 PM
#7
Kinetikx
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Thanks for everyones help on this, I appreciate the advice.
So just to be sure I'm thinking of going with:
289/291 rear
293 front

Luckily it looks like Cambriabike.com has all those sizes. I'll shop around a few more places and hopefully before I decide to pull the trigger on these spokes someone will come here to stop me before I get the wrong size

02-23-08, 10:17 AM
#8
Al1943
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Always round down spoke length to avoid running out of threads and to allow for stretch.

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