bikerbobbbb

Say it's a brand new set up -- rim, hub... but then you need to know the length of spokes. And you need to know spoke length pretty much down to the millimeter from what I see. I've got a double walled rim on one wheel, so that would allow for some cheating, ie the spoke could stick out a bit but it's inside the rim so it's not going to poke the rim tape or tube.

It must just be measuring, right? You know the hub. You know the rim. And then you should end up with shorter drive side spokes and longer non-drive side spokes. How do you get that spoke length though, esp. if the rim is free and the hub is free?

I imagine there's a correct way. I'm picturing doing that correct measurement but then only ordering the "first round" of spokes for each side (every fourth one or something?). Order fewer of what I think is the correct length to make sure it actually is the correct length. Then once the first round of spokes are on, see that it is the correct length, and then order the rest. But that's just me.

I imagine there's a way to know that, "Yes, this is the exact, correct length for sure."

prathmann

There are lots of spoke length calculator sites online if you don't want to go through the geometric calculations from scratch. Here's one example:
Spoke Calculator for bicycle wheels | Prowheelbuilder.com

bikerbobbbb

Very cool. I took a guess on remembering exactly what rim and hub I've got and I got the same spoke dimensions that I have now. That's a good sign.

What do you do if the parts aren't listed on a site though? Are there calculators for that? Just put in hub measurements, rim measurements... Same idea, the site does the rest?

prathmann

Sure, here's one site:
Spoke calculator | Sapim

jyl

Run measurements through a couple different calculators, just to be careful. Precut spokes usually come in 2 mm lengths, like there might be 290 mm and 292 mm but not 291 mm. If in doubt, get the shorter length.

JohnDThompson 

It takes a little trigonometry:


Source: Sutherland's 4th Edition

BITD, most mechanics looked up spoke lengths in published tables of hub and rim models. If you couldn't find it in the tables, you'd break out the calipers and slide rule. These days the formula can be embedded in a spreadsheet or javascript function.

Dan Burkhart 

I still do my own measurements and long form calculations on the formula published in Barnett's manual which is a variation of that formula. No reason to change when the success rate is pretty much 100%.

Bill Kapaun

I eally don't like to generalize too much on hubs, but if for example, both hubs are rear Shimano, the slight variations in CTF distances don't change spoke length to a noticeable degree. Maybe .1 to .2mm?
As long as the spoke hole "circle" is the same diameter. (PCD)

Some of us have pairs of "test spokes" of a known length. (nipple "glued on" so it doesn't move.
We put them in opposite holes and measure the distance between the elbows with a caliper where they "would" enter the hub.
Check in multiple places around the rim and average the length. (in case the rim is a bit oval)
We put t

Drew Eckhardt 

You measure the ERD of your rims using your nipples - insert spokes in opposite holes, thread nipples to your target depth (usually the slot bottom), measure across the elbows with calipers, and add twice the spoke length. Repeat 90 degrees away. If that differs do the remaining 45 degree increments. The average is your ERD.

Then you drop that into a calculator - I like spocalc.xls.

Adjust for stretch. Very roughly in the front and rear drive side 2.0/1.5mm spokes stretch about 1mm at full tension and 2.0/1.8mm 0.5mm. NDS spokes stretch half that far.

DT spokes have 9.5mm of thread and 12mm nipples 8mm to the top, so you bottom 1.5mm past the top. Alloy nipples will break if you don't make it through the rim so you have about 1mm below the nipple slot and it's still not a good idea to rely on brass nipples to do the spokes' job. Aiming for the bottom that's -1mm/+2.5mm of tolerance.

Then you use logic to accommodate available spokes and rim diameter variation. Spokes come in 2mm increments; so in some cases your choices are either the nipple top or 1mm below the slot and prefer the top. If your rim is bigger than average you might get a spoke cut 1mm shorter instead so that when it bends/wears out you won't need new spokes (or to drill your nipples) if the replacement is smaller.

Drew Eckhardt 

It depends on where you're aiming. Use a 290mm spoke not a 291mm ending at the slot bottom risks broken alloy nipples especially if you neglected to measure and have a rim + nipple combination with larger than advertised ERD. 292 is better in that case assuming you correctly accounted for stretch because it'll end at the nipple top where you have -2.0/+1.5mm of tolerance not -0/+2.5mm with the 290 which will end 1mm below the slot if everything works as planned.

davidad

Damon Rinard's free spoke length calculator

Don't overthink this. It ain't rocket science. I use this spoke calculator and have had good luck.