It is amazing how deep you sometimes have to go to get really basic information about relatively common bicycle parts, especially wheel components. When you are going to assemble a wheel yourself, you need to know a bunch of dimensions in order to determine what length of spokes you will be using. How far are the holes in the rim from its center? How high are the flanges on the hub, and how far are they from the center of the hub? How many times will the spokes be crossing other spokes from the same side of the hub? OK, so that last one is up to the wheelbuilder and easily answered.
Backing up for a moment. I had a clipped in fall to the right side on my Scattante a couple of mornings ago. It was very similar to the fall I experienced earlier this year, where my foot rubbed on my front fender, stalling the front wheel and sending me forward (and in this case on to my back in some sort of bizarre roll) and off the right side of the bike. I hopped back on, and other than a bruised ego and a smashed apple in my bag, all seemed to be well. Before I left work for the day, though, I noticed a significant wobble in my rear wheel and by the time I got home six miles later, I was riding cautiously and at about half speed as the wobble worsened.
I pulled the wheel off the bike and cleaned it, expecting to spend some after-dinner time at the truing stand. Once I got it off the bike though, I discovered that the damage was more extensive. I had snapped two spokes, one from each edge of the rim (and from opposing sides of the hub). The spokes broke right where the spoke head meets the hub. The rim had been torqued pretty badly in my fall, but appears to be something I can true. I thought for a while about simply replacing the two broken spokes, but I really don't trust this wheel anymore in its current configuration. I built it up in a three-leading, three-trailing pattern and have since read in a couple of places that while it might work (and isn't considered "plum stupid," like a radial-spoked rear wheel), it is generally not advised due to the stresses that a driven wheel experiences. What if some of the adjacent spokes are also ready to go? Better to strip the whole thing down and rebuild it in a plain-vanilla three-cross pattern and reap the benefits of added wheel strength. With the the cassette on, you could never really see the nice pattern of the woven spokes anyway.
Which brings me to ordering spokes and finding dimensions. I must have had the dimensions for the hub and rim at some point, but they are lost to me now. A short Internet search ought to bring me the right information, though. That was true for the Velocity Fusion rim, who unlike Weinmann, have spread their rim ERD information far and wide. 591 mm ERD in this case, and away we go to find the Sora hub dimensions.....
It took some digging. None of the catalog pages I found for the hubs gave the numbers you need to build a wheel on them. Shimano's own web site provides a lovely exploded diagram of the hub with exactly zero dimensions identified on it. Why do this? I'm not sure, but it certainly isn't done to encourage home wheelbuilders to use Shimano's products. Why not just look up what length spokes I ordered last time? Because Ebay doesn't see fit to make more than 60 days worth of purchase history available. What about measuring what I have? Well, I don't have the wheel apart yet and would just as soon wait until I get new spokes to take it apart. I don't have a good set of metric calipers to get on the hub, and I don't trust myself measuring the spokes, bent as they are through the now-failed pattern.
A Google search specifically on Sora (The componet group the hub is from) will not get you there, but after about an hour I finally found what I needed on Velospec. Without further ado, I'll get that information out in a way that the next person who Googles finds it more easily:
Shimano Sora 8/9 speed rear Hub Dimensions:
Spoke Hole Diameter: 45mm
Center to Right Flange: 21.3mm
Center to Left Flange: 38.7mm
That's it. That's all there is to it. Plug those numbers, along with the ERD of the rim (in my case, 591mm for a Velocity Fusion) in to any of a number of online spoke length calculators, and there you go. For a three-cross wheel I need 284.14mm spokes on the drive side and 285.95mm spokes on the left side. Done.
..And now I have it all archived in this blog post, in case I ever need the information again!