ted_major

I'm about to upgrade to a generator lighting system which will obviously necessitate wheelbuilding. This isn't my first set of wheels, so that's not my question. Roger Musson's book got me through the last set of wheels with flying colors, and I'm confident it won't let me down this time as well. However, I'd like to go ahead and order spokes ahead of time to minimize the time my bike is out of commission.

I'm planning on using my existing rims (Alex MD-21), and while I'm rebuilding my front wheel, I figure I'll go ahead and re-spoke the rear wheel, both to replace the straight-gauge spokes with butted ones and to match the front. I know from my last pair (that I used Alex Adventurer 2 rims on) that the manufacturer's published ERD was 4mm less than what I measured, so I don't want to rely on the published ERD for these rims. I also don't want to disassemble my wheels to measure spokes and ERD and then wait a week or more for new spokes and nipples to arrive while my bike sits forlorn and unshod. So that leads to 2 questions:

1. Is there a reliable (and not too difficult) method to measure ERD on a built-up wheel?
2. Is there a reliable (and not too difficult) method to measure spoke length on a built-up wheel?

Thanks!

pdlamb

For (2), take a spoke off and measure it. Better yet, on a rear wheel, take one from each side off and measure it. Make sure you replace the first one and re-true before messing things up royally by taking two off at once.

I'd suggest getting new rims, while you're at it. Compared to the cost of a dyno hub, it doesn't add much, but it'll cut your down time way down.

Andrew R Stewart 

With skill and experience a lot of "uncommon" measuring of stuff can be spot on. But lacking that pull a spoke or two and reverse math the ERD. Andy

Russ Roth

Isn't hard if you have one of the sewing tape measures, they come in metric. Take a longish piece of cardboard from one of those amazon boxes you probably have lying about, cut out an arch from it so it can sit over the hub and axle, mark the inside edges of the rim on the cardboard (this is easiest if the tire is off and the cardboard edge can sit right on the rim) from two spots directly opposite each other. Measure the distance between the marks, and add 3-4mm to the length to cover the thickness of the rim and get the threads into the spoke nipples.

ted_major

I've considered that option, but rim availability seems pretty spotty these days. Any recommendations for an available, reasonably-priced 32-hole 650b disc rim for 42-47mm tires?

Using new rims would also tempt me to further expand the scope of the project: once I'm buying new rims, I might as well replace the rear hub, too...

Dan Burkhart 

It depends what you consider reasonably priced, but the Velocity Cliffhanger is available (just brought in 2 of them in that size) and you can just go with a non machined brake track and call it a disc rim.https://www.velocityusa.com/product/...liffhanger-584

Moe Zhoost

Personally, I would skip rebuilding the back wheel. The marginal gains are just not worth the effort. OTOH, if you just want the practice or if you have some damaged spokes, go for it.

What hub are you planning to use? I've used Shimano, Sturmey, and Shutter Precision (which I like the best). SON was a bit too expensive for my budget.

cyccommute 

I’ve built a lot of wheels and seldom do I measure the ERD myself. The published ones are good enough or won’t make enough of a difference to matter. Additionally, if I were measuring the ERD, I would suspect my own measurement over that of the manufacturer. They measure ERD all the time. I would trust that they know what they are doing because they do it far more often than I do.

The other thing to consider is how much of a difference will it really make? Using the ERD for a 622mm Alex MD-21 of 608mm and an ERD that is 4mm less (604mm), Prowheel Builder’s spoke calculator returns lengths of 298mm and 296mm, respectively, for the two ERDs. Often I’ve had to compromise on spoke length of about the difference due to availability. I’d probably err on the long side rather than the short one, however.

If you really have to let your OCD kick in, you could measure from the j-bent to the outside of the rim, then measure from the spoke end to the outside of the rim and subtract the difference. Do it more than once (or even twice or even 3 times) and average the results. But that’s a lot of work and is likely to have a whole lot of variance. Probably more than just going with the manufacturer’s ERD

ted_major

I get that a probably won’t be able to tell any difference on the rear wheel, but I have a feeling I’m not going to like the look of shiny new butted spokes on the front and black straight gauge on the back. I enjoyed building my first set of wheels, so I’ll probably do it as much for skill development as anything else.

As far as hubs, I’ve got a Kasai FS on the way. I’m a little leery of components that need to be shipped overseas for service, even if that service isn’t likely to happen soon. The serviceability aspect was enough for me to finally give it a shot. (I can’t quite bring myself to shell out for a SON either.)

ted_major

You make a good point, and I have wondered how much difference 4mm makes in the end. As a baby wheel builder, I’m still just following directions.

Musson said not to trust the mfr and to take my own measurements, so that’s what I did and it worked. I figured I’d do the same thing this go ‘round, but using the mfr specs would be easier, and you’re absolutely right that trying to measure a built rim by adding and subtracting several measurements would probably be less reliable than the published numbers. Tolerance stacking in action!

mpetry912 

Tolerance stacking or uncertainty stacking, exactly. Going to be really hard to come up with ERD or spoke length measurements by measuring a built wheel.

I always measure ERD myself. Errors of 2-3 mm are not uncommon. On the DTs in particular they may be using the proprietary nips (Squorx) as the basis of measurement.

you can make up your own ERD measuring tools as shown. Thread nips onto 2 spokes and cut them at exactly 250 mm. Drop them into 2 rim holes 180 deg. apart. Measure the gap as shown and add that to 500.

/markp

ted_major

In the end, I've decided that pdlamb has some of the best advice in the thread. As my wife pointed out, "Go ahead and try that if you want, but in the end it probably won't work out right and you'll just end up ordering new rims anyway, so you might as well just get them now and save yourself time and trouble." So now I've got a pair of WTB KOM i23 rims and a Bitex MTR 12 rear hub on the way. Once they're here, I'll confirm measurements and order spokes and nipples (14/15 ga butted).

As far as my original question goes, I found that Musson describes how to measure the ERD of a built rim in his book. He says to measure the OD of the rim by rolling 1 revolution on a flat surface, measuring that distance, and dividing by pi. Next, measure the distance from the rim to the top of a nipple. Subtract twice that distance and then subtract another 2mm to get to the bottom of the slot in the nipple.

I came out with an answer that was off from the published ERD by 2.8mm. Maybe close enough, but I think I'll just make this easier on myself.

Didn't take too much fooling around with measuring tools to destroy what little confidence I had in my own measuring ability. I've got a pair of spokes I prepared in similar fashion from the last wheels I built that I'll use when my rims come in.