Spoke size

Effective Rim Diameter
I created a measuring device for Effective Rim Diameter (ERD) following Roger Musson's book. The only thing I didn't do was glue/crimp one of the nipples onto one of the spokes.  If you glue/crimp then your elastic bands must be able to fit through the holes in the rim.  The bungee cord that I used was a bit thick, so I had to screw both nipples on/off for each measurement, which doubles the uncertainty associated with adjusting the nipple on only one end. I took 5 measurements on each rim.

Measurements for the first rim: 148 , 148, 149, 149, 149 mm; average = 148.6 mm

For the second rim: 148, 149, 149, 149, 148 mm; average = 148.6 mm.

Adding the 400 mm for the two spokes in the measuring device and we get an ERD of 548.6 mm, which is within my measurement precision of the official ERD of 549 mm (see http://sun-ringle.com/wp-content/uploads/2012/10/SUN_website_rim_chart_2012.pdf )

Front hub dimensions
The web site for Shimano has the following data on the HB-M8000 front hub (http://bike.shimano.com/content/sac-bike/en/home/components11/mountain/deore-xt-m8000/hb-m8000.html)

• Axle length 100 mm
• Axle diameter 14 mm
• Center to Flange Right 41 mm
• Center to Flange Left  44 mm

I checked the measurement between opposite holes on the flange. The distance between opposite spoke holes on the brake side of the hub is 44 mm, whereas the similar measurement on the non-break side is 41 mm, so this appears to be what they are calling the "Center to Flange" distances (which seems odd to me).  Measuring the hub using the nomenclature on Musson, pg 56 - with the brake side being A and the gear side B.

• A = 44 mm (left brake side)
• B = 41 mm
• F = 53.6 mm
• Z + Z = 99.6  so  Z = 49.8
• X = 29 mm
• Y = 16.5 mm
• C = Z - X = 20.8 mm
• D = Z - Y = 33.3 mm

However, with the above C+D = 54.1, which is 0.5 mm longer than F (the flange to flange measurement). Given my set of calipers, the measured F is likely correct, but the X and Y are likely too short.  If we distribute the error with 0.3 mm on X and 0.2 mm on Y, we have the modified set

• A = 44 mm (left brake side)
• B = 41 mm
• X = 29.3 mm
• Y = 16.7 mm
• C = Z - X = 20.5 mm
• D = Z - Y = 33.1 mm

And C + D = 53.6 mm = F, which is what we wanted.

Going to the spoke calculator at http://www.wheelpro.co.uk/spokecalc/ , and here's the result

So for a 3-cross (which is my plan), I need 16 spokes that are 266 mm and 16 spokes that are 268 mm (or 267 mm) for the front wheel.

Now for the rear wheel.

Rear hub dimensions
The web site for Shimano has the following data on the FH-M8000 rear hub
http://bike.shimano.com/content/sac-bike/en/home/mtb1/wheels---hubs/rear-hubs/fh-m8000.html

• Axle length 146 mm
• Axle diameter 15 mm
• Center to Flange Right 45 mm
• Center to Flange Left  44 mm

Taking measurements - here my measuring gage couldn't reach then entire Z+Z length, so I've used a ruler for this measurement

• A = 44 mm (left brake side)
• B = 45 mm (right gear side)
• F = 50.0 mm
• Z + Z = 135 mm  so  Z = 67.5 mm
• X = 35.5 mm
• Y = 48.5 mm
• C = Z - X = 32 mm
• D = Z - Y = 19 mm

So we have C+D = 51, which is 1 mm to large from the F measurement. Splitting the difference on both X and Y provides

• A = 44 mm (left brake side)
• B = 45 mm (right gear side)
• F = 50.0 mm
• Z + Z = 135 mm  so  Z = 67.5 mm
• X = 36 mm
• Y = 49 mm
• C = Z - X = 31.5 mm
• D = Z - Y = 18.5 mm

which gives us C+D = 50 mm = F.

So back to the spoke computer:

Again doing a 3 cross, I'll need 16 spokes of 267 mm (or 268 mm) and 16 spokes of 266 mm for the rear.

Buying plan:

• 32 x 266 mm spokes
• 32 x 268 mm (or 267 mm) spokes

Let's make it an even 40 of each so that I have spares.

UPDATE (April 13, 2016):
I bought Sun RhynoLite rims, which are a little bit wider as I decided I wanted 1.75" tires on my touring bike, and will use the 1.5" tires on my spouse's.  The online ERD for the RhynoLite is 549 mm, which checks against my measurements.  I'm going with the same hubs, so everything is pretty much the same as above and I can use the same spokes as on the CR18 rims.