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.
When a spoke length calculator is used - it is averaging the center-to-flange distance for purposes of spoke length calculation. The reality is that you are really calculating 2 center-to-flange distance for EACH side of the hub.
For example - if the center-to-center_of_flange distance is 34.00mm for a modern day hub - what you really have is roughly 36.6mm for outside spokes and 31.4mm for inside spokes - or something like that.
Futhermore - that slowly starts to change the further you move the flange out assuming the flange diameter stays the same. The 10:1 ratio you introduced actually drops to 9:1...slowly but gradually...when working with CTFC's of 38mm and 40mm like with the old Maillard's...
What happens when you are that far out? You can get as much as a 1.00mm difference in spoke length between the outside and inside spokes for the same flange. It can cause a situation where half of the spokes can run short or run out of threads for a given nipple.
Keyword is can...pretty rare though...we're talking extremes here...
My problem with Shimano is they are measuring to the outside - making the insides spokes longer than necessary - and making the outside spoke even more longer than necessary. I want the average - which is what I get when the measurement is to the center of the flange.
Overall it's really not a big deal...unless you go all elbows in for radial...or when you have a flange that's way way out there....45.00mm is the worst I've seen - electric motor hub I think...
American Classic and Bullseye have had CTFC's of 40.00mm to 42.00mm in the past...
Basically it'd be nice if Campagnolo and Shimano would do the same as most of us when we are measuring hubs for spoke length calculation purpose - reference the center of the flange.
I can dream can't I?
Now back to ERD...
=8-P