Classic & Vintage - Wheel questions -- from a book
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08-17-12, 10:56 AM
A while back, I attempted to build a front wheel using online spoke calculators. I think I tried 3 of them, and two gave me a 284 length, and the other 283.6, so I bought 284mm spokes. They were too long. Per my usual method, I got mad at myself, and shelved the whole thing.
Recently -- a year or so later -- I'm trying to look at the situation again, and try to figure out what went wrong. In the process, I've acquired a copy of Jobst Brandt's "The Bicycle Wheel," and am finding it really interesting, if a bit over my head. In the spoke length calculation section, however, he has an example, and while the numbers make sense, and plugging in my numbers makes some sense, there is one value I don't understand.
If anyone has the book, the section I'm referring to is pages 133 and 134. He uses the term, "w" to refer to the distance from the center of a flange to the center of the hub. Actually, on page 133, he uses a capital,"W," and I'm assuming they are the same thing. Anyway, in his example, the other values make sense, but he has a value of 71mm for this center of flange to center of hub measurement. This book was written in 1981, I think, so even rear hubs were only maybe 126mm? I measured a 126 hub I have, and the flanges are 58mm apart, and the non-drive side flange is 38mm from center and drive side 20mm. What kind of hub had a flange that was 71mm from center? I'm not trying to question this well respected book, it's just that my not understanding this value has me questioning my own measurements.
Oh, for what its worth, the front wheel in question is a fairly odd, and cheap, attempt at a 590 rim, 90mm OLD wheel. It uses an old Union hub from some old cruiser, and one of the steel 590 rims sold under the name Wheel Master -- actually has no markings on it. Cheap.
Also, in Mr. Brandt's book he refers to the elongation of the spoke under tension, and that it can be around 1mm. Is it a good idea to subtract 1mm from calculated length?
I'm pretty sure the biggest part of my problem is incorrect measuring of the hub and maybe the erd of the rim. I'm just going to start all over with the measuring, but I would like to know what you knowledgeable folks have to say about the 71mm value.
08-17-12, 01:14 PM
There are a variety of 'online' databases and reference material with accurate hub and rim dimensions. I uses Damon Rinards 'spocalc' but the others are just as good. i haven't read the book but 71 seems like a typo, heck, a tandem hub doesnt have a flange to center distance that big!!
Keep in mind that due to angles and triangulation measurements can be off my a mm or so with no ill effects to the overall spoke length. The best advice I can give you is to NOT over annalyze wheel building and spoke length.
I also use spocalc and cross check with the DT Swiss or other sites (I thought wheelsmith had one, can't find it).
I'd also recommend The Art of Wheelbuilding: A Bench Reference for Neophytes, Pros & Wheelaholics by Gerd Schraner
I find it a much easier read than Brandt's book.
08-17-12, 02:05 PM
I'm surprised 3 spoke calculators all steered you wrong to the same number (unless they all used the same faulty database/algorithm). Did you put in them to do a 3 cross and then lace it as a 2 cross or something?
And as far as books go, I read Jobst Brandt's book, but then just followed Sheldon's instructions from his website.
Try and find a copy of Sutherland's to download online, they have an understandable proceedure for sizing spokes.
I read Brandt's book years ago. There's lots of information in it, but not a lot useful for building a wheel.
It's not hard. Assemble slowly and methodically and remember it only goes together right one way. I showed my son once how to lace a wheel, and he laced two for me that afternoon.
Mostly I've built the same wheels for 40 years, small or large flange, so I know what length spokes I need for which hubs and cross pattern.
08-17-12, 07:40 PM
I too thought the 71mm figure a bit ridiculous, but chickened out on replying because I don't have the book.
When you run the spoke calculator make sure you set it for the correct number of spokes. The more spokes in the wheel the shorter they must be. If you try to string a 32-spoke wheel with spokes computed for 36, they will be too short.
08-17-12, 07:47 PM
Thanks for the responses. I don't think the calculator sites led me wrong, I think my measurements were a little inaccurate. I believe a couple of the ones I used were the one on the UBI site and the one called "EDD." I got the Brandt book because I remember somebody referencing it on the forum, and thought I might get a better explanation from it. I've also found a version of the formula on wikipedia under "spoke," and even asked a former math teacher to help me with the law of cosines, since I didn't remember that from my own school years. I also thought that if the math was just solving for the side of a triangle, wouldn't the tension have to shorten it a little? I've already remeasured what I could get to on the wheel and put the numbers into Mr. Brandt's calculation, and came up with 281.6. I will have to dismantle the wheel anyway, so I can hopefully be more accurate with measuring. I've just been puzzled by the 71mm figure. And I also saw that the spoke is going to elongate, so I kind of wondered if that might be where the tension element comes into calculating. It's just not stuck in the formula.
I agree that the text is a bit confusing - bad job of editing perhaps. In any case "w" is the distance between the centers of the flanges. If you look at page 134 there is a variable "C" that is the hub half breadth and is calculated by dividing W by 2. In this case the half breadth would be 35.5. You can't easily measure the distance between the flange and center of the hub and so halving the distance between flanges makes sense.
How much too long were the spokes. Most of the calculators I have seen do a great job as long as you input the right measurements. If you think you put in accurate numbers, they I would suspect that you tried to lace the wheel with a different pattern than you input. If you planned initially for a 4x pattern then tried to lace 3x, the spokes would be too long.
08-18-12, 12:05 PM
The spokes are just long enough to not tension. A few are sticking through the nipples, and I've probably tightened them enough to mess up some nipple threads. I checked again, and the wheel is laced 3X -- haven't messed up that badly. I think my measurements are just off that much. When I re-measure, I may check to see if they are actually long enough for 4X, but they're probably somewhere in between.
08-18-12, 12:31 PM
Like a lot of things in life, the most important value is the one that's hardest to measure accurately: the "effective rim diamter" -- that is, the diameter described by the heads of the spoke nipples when the wheel is assembled and fully tensioned. This is a few mm more than the actual diameter of the rim bed itself because you want the spoke threads to come up into the screwdriver slot of each nipple.
When building up a wheel from scratch I have always relied on the published ERDs of my planned rims. It is difficult to estimate the ERD of a loose unassembled rim to the required accuracy.
08-18-12, 12:54 PM
A few are sticking through the nipples, and I've probably tightened them enough to mess up some nipple threads. I checked again, and the wheel is laced 3X -- haven't messed up that badly. I think my measurements are just off that much. When I re-measure, I may check to see if they are actually long enough for 4X, but they're probably somewhere in between.
It's not likely you messed up the nipple threads unless you tightened them down past where the spokes were threaded. As for 3x vs. 4x, you can go back to the spoke calculator and see the difference. Most likely the difference is a few mm, though I may have misremembered it from my last such effort. (I've gotten spokes where the bike shop made a similar mistake, or somehow came up with spokes too short.) In any case, it's no problem if they stick up past the nipple heads a little as long as you have enough thread on the spoke to tighten them enough and the spoke doesn't poke through the rim tape.
Like a lot of things in life, the most important value is the one that's hardest to measure accurately: the "effective rim diamter"
I recall a thread on this very subject from a year or so ago. If I had to do one because the rim wasn't listed in a database I'd wrap a non-stretching thread about the rim and mark it with a Sharpie where it overlaps itself, unwrap it and measure the distance between marks, then divide by PI. From that value you can compensate for the depth of the spoke well, if any, and the nipple head thickness both easily measured.
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