Originally Posted by mrrabbit

Not surprised at the Shimano weirdness....

They measure not to the center-of-flange for offsets - but somewhere past - just like Campagnolo. Campagnolo is pretty easy to adjust - just subtract 1.6mm. Shimano seems inconsistent where on the outside they are measuring so you can't just subtract a regular number...

For Shimano, I've have to include their spec in my spreadsheet - then update as I encounter them...covered about 2/3's, still have a way to go...

http://www.mrrabbit.net/wheelsbyflemingapplications.php

Also, myself and a few others here have caught a few flange diameter mistakes on their PDF - one affecting a disc one and another affecting some Dura Ace hubs.

Question for you:

Are you sure that was FH-7900 rear? (Not FH-7850?)

=8-)

Originally Posted by FBinNY

The center to flange measurement is the least critical of the measurements.

ERD measurements affect spoke length on a 2:1 (diameter to radius) ratio, ie. +2mm ERD means +1mm spoke length

Flange diameter affect length -2:1 for a radial pattern, (-2mm dia. means +1mm spoke length), and not at all with a true tangent, ie. 4x 36h, or 3x 28h, and to varying degrees for other patterns.

Center to flange affects length on a 10:1 ratio. a 1mm difference changes the spoke length by 0.1mm

The exact ratios vary slightly within the general bands I described, but the concept holds. I offer this to spare folks the effort of agonizing over details that don't mean anything in the real world. For example a 2mm difference in CTF changes the spoke length by 0.2mm which doesn't factor when you're going to round to 1mm increments anyway.