Hi all,
I'm a little late coming to this thread, but having been invited to weigh in, I'm gonna throw out a long post here. Skip it if you want, or see it live at Cirque 2009.
I want to first say that from everything I've read and learned by experience, evenness of spoke tension is the most important parameter in wheel building.
Let me explain why. Jobst Brandt posited that the overall strength of a wheel is directly proportional to the aggregate tension on the spokes. Park Tool and Barnett's Bicycle Institute both say that production wheels are acceptable with spoke tension that is plus or minus 20 percent of the average. Ric Hjertberg, founder of Wheelsmith, warns that over-tensioned wheels are at least as fragile as under-tensioned wheels. One of the failures is cracking at the spoke holes in the rim, suggesting that there is an absolute maximum tension for any spoke.
So, when a wheel has its tightest spokes at the maximum allowable for the rim, and the tension is at plus or minus 20 percent of average, then the average is going to be at roughly 80 percent of maximum. If one keeps the maximum tension at the same number, as one reduces the variation among the individual spokes, then the average tension becomes higher, and the aggregate tension along with it. By reducing the maximum variation in spoke tension to less than 10 percent, the increase in the strength of the wheel is about 20 percent (90 percent of maximum tension vs. 80 percent of maximum tension). If anybody has an issue with this logic, I would be glad to hear it. But if you want to dispute it, please do so with some numbers.
Here's a primer on how I get there:
Once I have spokes laced into the hub, but still rim-rattling loose, I correct the bend at the elbow so that each spoke is forced into pointing directly at its spoke hole in the rim.
Then, I snug up the spoke tension just enough to stop the rim rattling. At this point, I will set the truing stand so that the side to side pointers are 4 to 6 mm wider than the rim, and add tension to the wheel by chasing the high spots. My goal is that when I am at 60 percent or so of final spoke tension, the wheel is within a millimeter of being perfectly round, and well within the side to side pointers. I try not to tighten any spoke more than half a turn at a time, and as I get closer to round, I may go around the wheel and add some tension to every spoke. During the process, I use a dishing tool to make sure I'm within my plus/minus 2mm tolerance of centered on the hub.
At that point, I will use a tensiometer to tighten each spoke to exactly the same tension, or actually as close as a Park TM-1 will allow. I do this without regard to roundness or trueness, just as a gauge of how well the rim was made. At the end of this process, spoke tension will be 75 to 80 percent of final value.
I then move the side to side pointers to about 1.5 mm from the rim on each side, correct the side to side trueness and then check and re-check the dish until I have a reference point that is within 0.2 mm or so of exactly centered.
I then turn my concentration again to the roundness of the wheel. I use the tensiometer to balance relative tension with the relative out-of-roundness. I sortof use the rule that the percentage difference in spoke tension should be about 20 times the percentage of out-of-round. On a 700 C wheel, 0.1 percent out of round is about one third of a mm, and I try to achieve this with a tension differential in the range of 2 percent. Every rim has some manufactured imperfections, so not every irregularity in roundness will be correctable within this ideal, and some modern rims are better judged by the machined-in wear line than the outer edge of the rim itself. That becomes a balancing act between cosmetic imperfection of round and true and evenness of tension.
In reality, if the final product varies by plus or minus a millimeter side-to-side and radially, the imperfections will be imperceptable when you ride.
I hope this helps those novices out there build better wheels. I believe that properly chosen handbuilt wheels are nearly as important as frame fit in determining overall ride satisfaction on a bicycle. Thanks for reading this far.
Earle Young, Madison, Wisc. Offering expert wheelbuilding service for classic and modern bikes. http://www.earleyoung.com