Jobst Brandt's book The Bicycle Wheel is a classic. Despite being fairly old (first published in 1981) the information remains up to date, and it applies just as well to mountain bike wheels as to road wheels. The writing is terse and dry, but the information is valuable enough to make it worth a read for beginner and experienced wheelbuilders alike.
The book has three parts. Part I discusses the various aspects of wheelbuilding. These include choice of materials as well as different numbers of spokes and lacing patterns. For each aspect, Brandt discusses a number of alternatives, then states his preferred method and gives reasons for his preference. However, he gives you the information you need to allow you to construct wheels with any method and materials you choose.
Part II is a step by step guide to building wheels. Here he sometimes restates the conclusions he came to in the first section and shows how the steps give the correct result. Part III is the most technical one. He shows his test apparatus and describes the experiments and theoretical analysis he did to derive the results presented earlier.
All through the writing is precise and direct. No attempt has been made to try to make the book fun to read. This is a valid approach to the subject, but in my experience the addition of the occasional story and joke can help make technical material more digestable. It's still perfectly readable, but it's not as enjoyable as it might have been had a few lighthearted bits been included.
One of the best features of this book is the multitude of drawings. They help greatly in explaining the concepts. For example, Figure 45 below shows a partially laced wheel with the third set of spokes inserted but not yet laced. It clearly shows (in the book, although not necessarily in the reproduction shown here) which side of the flanges the spoke heads are on, which side of the hub is the drive side (the side with the threads), which spokes cross which other ones, and the offset of the spoke holes in the rim.
These line drawings are far clearer than a photograph could be. Consider a photograph of a wheel in the same state as the drawing above. The photo would show a silver rim, silver spokes, silver nipples, and a silver hub, and you wouldn't be able to distinguish anything. Furthermore, you wouldn't be able to see the offset of the spoke holes as well in the photo, since they have been exaggerated in the drawing.
Another positive feature of the book is the great wealth of technical information. Most of the descriptions given, although terse, explain clearly what's going on. This is true even when the concepts are non-intuitive, such as "a wheel supports its load on the bottom few spokes". This may seem strange, since spokes don't have any compressive strength: if you push on the ends they just bend. But this concept is explained with the help of lots of diagrams and analogies, and you gradually begin to understand what he's saying. By the time you get to the end of the section you should be convinced.
However, in a few places the book could use a bit more explanation. Usually you can figure out what is meant if you stare at it and mull over it long enough, but a few extra words here and there wouldn't go amiss. For an example where I think some extra explanation could be useful, look at the graph below.
This graph is an example of the topnotch information you'll find in the book. Once you figure out what it says, it is an elegant and compact way of displaying a great deal of data. It is the sort of diagram that Edward Tufte, author of The Visual Display of Quantitative Information, would be very pleased with: it is a simple, uncluttered, easy to read (once you understand it) presentation of relevant facts. However, like many of the better graphs in Tufte's book, it needs some explanation, and I felt that the explanation given in the book (below) isn't enough.
A typical six-speed rear wheel is shown in cross section. The horizontal scale in this graph (mm) represents the actual hub dimensions. The curves show the change in spoke tension and the force required to displace the rim and cause these tension changes. The left spokes become slack for even small rim deflections to the left while the right spokes remain tight for the range of displacements shown. This wheel is twice as stiff for deflections to the right as to the left. The graph can be verified by observing that, when unlacing a rear wheel, the right spokes do not become slack if the left side spokes are removed first.
It took me a fair bit of time to see how the graph works, but now that I understand it I quite like it. Maybe I was just being thick when I came across it: I'm not sure now why it took me so long initially to figure it out. At any rate, an example of reading the graph would have helped me to get it much more quickly.
A quick look through the book will show you that all the drawings show road wheels (with screw-on freewheels at that). However, almost all the principles described work just as well for mountain bike wheels. The only one that doesn't (the rim overloading method described in Finding the Right Tension in Part II) is explicitly stated as not being applicable to mountain bike wheels, and alternative methods are given.
Many of the topics covered in the book are surprisingly relevant to current trends in spoked mountain bike wheels. For example, number and thickness of spokes, cross sections of rims, anodized and ceramic coated rims, alumimun vs. nickle-plated brass nipples, and radial spoking are all discussed. There are only a few topics that are not discussed. One is asymmetric rims, in which the spoke holes in rear rims are offset from the center line of the rim, as in Bontrager ASYM or Ritchey OCR rims. Another is the process of machining sidewalls of rims, such as on many Mavic rims. I would like to know what Brandt thinks of these.
[After writing this review I wrote to Brandt to ask him the questions above. He doesn't like machined sidewalls at all. He writes "I don't know what advantage is gained by machining rims other than expense, making the side walls variably thin and leaving little wear margin. I have used more rims than I'd like to think of and have never had any problem with the smoothness or continuity of the side walls." He also doesn't think that asymmetric rims are such a great idea. His main complaint is that since the spoke holes are nearer to the edge of the rim, double eyelets can't be used. However, since MTB rims rarely have double eyelets in the first place, this doesn't seem to me to be a good reason to avoid them.]
There are several ways to approach this book, depending on the level you're at.
If you know nothing about building wheels, you'll find this an immensely useful book. I wish I had read it before building my first wheel. I learned how to build wheels by looking at a couple of how-to guides: Sheldon Brown's instructions and the notes from Edinburgh Bicycle's wheelbuilding course. While these guides told me what to do, they didn't tell me why. If you read this book before building your first wheel you'll approach it with confidence: you'll understand the materials and methods that make a strong, durable wheel rather than a trendy wheel. However, it is useful to ask your friends (or a newsgroup) about specific brands of hubs, spokes, and rims, as this isn't in the book. Also, those who aren't familiar with parts of bicycles may need a glossary to figure out what some of the words (like "flange") mean.
The book is plenty useful even if you've built a few wheels or have spent some time reading newsgroup or mailing list threads on wheels. There is a great deal of misinformation out there. For example, many mountain bikers feel that they need thick, straight-guage spokes to have durable wheels. This is not the case. Brandt explains why swaged spokes (which are thinner in the middle) make more durable wheels than straight-gauge spokes, and shows why under most circumstances it's better to have more spokes that are thinner rather than a smaller number that are thicker. Thus, for the ultimate in strength, make a wheel with 36 (or even more!) thin swaged spokes.
I've also read things like "radially spoked wheels give a harsher ride than a 3-cross pattern", and "you should use Loctite on the non-drive side of a rear wheel to prevent it from coming loose". Topics like this can be hotly discussed on newsgroups, and it can sometimes be hard to judge which side is right. This book will give you the understanding you need to sort out these arguments.
When I read this book, I'd made about half a dozen wheels, all of which are working fine. Now that I had some experience, I wanted hints on how to make my wheels even better. As I read the book, I was somewhat disappointed to find that it suggests doing things almost exactly the way I have been doing them, using exactly the same materials. Brandt recognizes this:
Cyclists who choose to build wheels often want something more than ordinary, but just building wheels conventionally is not a trivial task if a durable wheel is the goal. It may be disappointing to discover that "it's all been done before" and that conventional wheels are a result of a hundred years of refinement. The true contribution for the new wheel builder is to build conventional wheels exceptionally well.
The main practical thing I got out of it was an emphasis on stress relieving and correcting the spoke line: I realized that I hadn't been doing it as carefully as I could. Despite not having learned any radical new approaches to wheelbuilding, it was still worthwhile reading. In addition to seeing the importance of stress relieving I now have a good understanding of how a wheel works and why the general method I have been using is good (after having read about some of the alternatives). I am also much more aware of wheels than I used to be. I can now get a good idea of the quality of a wheel by looking at the parts its made from and plucking the spokes.
Other bike articles
Jobst Brandt often posts to rec.bicycles.tech, expounding on wheels and other subjects. You can find them via DejaNews. For example, here are his hundred most recent posts.