Bicycle Mechanics - Spoke length for Ultegra 6600 hubs to Mavic Open Pro rims

Dear wheelbuilding pros,

I'm ready to order spokes. I'm 3x lacing 36H Shimano Ultegra 6600 hubs to Mavic Open Pro rims using DT Competition spokes (2.0/1.8/2.0) and DT 12mm brass nipples. There doesn't seem to be exact agreement about the ERD of these Mavic rims. What spoke lengths should I order? I don't mind ordering three lengths of spokes if that's best.

DT Swiss calculator (uses 604mm ERD):

Front left+right: 293.8mm
Rear left: 292.7mm
Rear right: 290.9mm

Spocalc spreadsheet (uses 605mm ERD):

Front left+right: 294.3mm
Rear left: 293.2mm
Rear right: 291.4mm

Mavic dealer calculator (does not disclose ERD but I infer a value of ~604.5):

Front left+right: 294.2mm
Rear left: 292.9mm
Rear right: 291.1mm

So I'm tentatively thinking:

Front left+right: 293mm?
Rear left: 292mm?
Rear right: 290mm?

(Inputs from Shimano's site: front hub spoke hole circle = 38mm; front hub centre to flanges = 35.8mm; rear hub spoke hole circle = 45mm; rear hub centre to left flange = 38.4mm; rear hub centre to right flange = 20.8mm. Inputs from http://lenni.info/edd: front hub spoke hole diameter = 2.6mm; rear hub spoke hole diameter = 2.5mm. I don't trust the exactness of these spoke hole diameter measurements but they impact spoke length very little. The Mavic calculator does not factor in spoke hole diameter.)

Thanks.

I would go with the final set of values you have there, 293mm, 292mm and 290mm.

Looks good. Go for it. You could build with 15-16 spokes and not lose strength.

Thanks folks. I got to build wheels under the eyes of a professional at UBI but they reserve spoke length calculation for their advanced wheel class. It doesn't seem like rocket science but I just wanted to double check before I open my wallet (or, worse, invest my time and effort).

If you already have the rims with you, measure them to be sure. Otherwise, I agree with using the final values. Being off by 1mm is not a bid deal anyway. I started ordering my spokes through wheelbuilder.com and they only offer even sizes in stock, with a 20 cent charge for cutting odd sizes, so I just round up or down and stick with even. Hasn't been a problem yet.

I think , if I recall correctly, I went with 293/291 on a rear ultegra/open pro wheel. But 292/290 would be fine.

Edited to add: As has been noted before on BF, you can go to the Mavic site for info on their stuff. You need to login to http://www.tech-mavic.com (http://www.tech-mavic.com/) (Login: mavic-com; password: dealer). Mavic, for reasons I do not undrstand, lists spoke support diameter rather than ERD for open pros. This number for open pros is 602mm so you have to add in two or 3 mms to account for the nipple head. Some sites add 2mms while others add 3mms, which is why you find differences. I measured myself and came in at 605 for ERD.

As has been noted before on BF, you can go to the Mavic site for info on their stuff. You need to login to http://www.tech-mavic.com (http://www.tech-mavic.com/) (Login: mavic-com; password: dealer). Mavic, for reasons I do not undrstand, lists spoke support diameter rather than ERD for open pros. This number for open pros is 602mm so you have to add in two or 3 mms to account for the nipple head. Some sites add 2mms while others add 3mms, whic is why you find differences. I measured myself and came in at 605 for ERD.

It seems kind of consumer-unfriendly of Mavic to specify dimensions this way. They must have an "official" value for the ERD because it's one of the inputs necessary to calculate spoke length. The outputs from their calculator (available on the very site you suggest) lead me to believe they use ~604.5 for the ERD although this number is fuzzy because they may use 604 or 605 depending on what value they assign to spoke hole diameter. I guess it's splitting hairs at this point.

Looks good. Go for it. You could build with 15-16 spokes and not lose strength.

I wish I could find a U.S. source for 15-16ga. butted spokes anymore. Got any leads?

I wish I could find a U.S. source for 15-16ga. butted spokes anymore. Got any leads?
http://www.cambriabike.com/shopdisplayproducts.asp?id=29&cat=Spokes+and+Nipples
Not always in stock, but they can get them. I would personally rather use Revolutions than 15/16, but ymmv.

Open Pro ERD = 602 min /603 max (Aiming for nipple flat - Do measure though - cause once in a rare while you'll get 601mm.)

Open 4 CD ERD = 604 min /605 max


If you use 15g head in a standard 14g drilled hub - subtract another .5mm from the final spoke length.

=8-)

http://www.cambriabike.com/shopdisplayproducts.asp?id=29&cat=Spokes+and+Nipples
Not always in stock, but they can get them. I would personally rather use Revolutions than 15/16, but ymmv.

I did order 15-16ga butted from CBO once, but they managed to mess up the order :(

So I'm tentatively thinking:

Front left+right: 293mm?
Rear left: 292mm?
Rear right: 290mm?

You're thinking correctly. I trust the DT Swiss calculator and always round down.
You could save a little weight at a good location with aluminum alloy nipples.

Using Shimano's PDF for hub specs and 603 ERD:

Front = 293.65 @ 293.50

Rear Opp = 292.43 @ 292.50

Rear Drive = 290.65 @ 290.50

That'll go to a thread above the nipple flat...rounding down another .5mm shouldn't kill you. Be aware though that in the last year I've run into new boxes of DT spokes whereupon the spokes inside are .5mm shorter than what is listed on the label. Hopefully it's just a fluke, not an old habit returning.

=8-)