Question:
I'm curious about something. The main criticism of machine built wheels
seems to be that they aren't properly stress-relieved. This seems like it
would be fairly easy to automate. Why isn't it? It is just cheapness or is
there something about the process that makes it more difficult?
Answer:
I never had any trouble with any Huffy wheels. They stayed true a very long time;
one rear wheel lasting 40k miles before breaking a spoke and going out of true,
and that was under daily heavy load carrying groceries home.
In other words, it's a canard; except they're all 26" wheels and perhaps larger
thinner ones are more pretzel-prone.
I think the main problem with machine-built wheels is that they are
almost always under-tensioned.
Jobst Brandt has reported of a machine that performs stress relieving.
You ought to be able to find his report by searching groups.google.com.
Stress-relieving can be done in a number of ways, either by hand or machine.
I've been to TREKs wheel facility and seen how they do it; the wheel is
placed on something shaped like the top of a trash can (supporting the rim)
with a lever that comes down and pushes on the hub area with a predetermined
(measured) amount of force. This occurs after the initial machine
tightening & truing, and then it goes back for the final work (not much,
surprisingly, is required).
One of the overlooked "features" of stress-relieving is that it straightens
out the spoke line from hub to rim. Frequently, the flange will hold the
spoke in such a way that the path of the spoke is perpendicular to the axle,
instead of pointing it towards the rim (slightly inward). Check out new
wheels on many bikes (and sometimes on handbuilt wheels from people who
haven't mastered the craft) and you'll often see the spoke making a slow
bend from the flange on its way to the rim. This is bad! Over time, the
spoke will seek the shortest path and become loose.
Stress-relieving usually applies enough force to bring the spokes into
alignment with the rims, eliminating a major source of trouble on cheap
wheels. I don't count on stress-relieving to do the job though; while
building the rims, I bend the spokes into alignment at the flange before
going further. I find it much easier to build and initially true the wheel
after having done so (because you have more slack in the spokes and, when
you draw up the nipples to a fixed point on the threads, the tension will be
more consistent).
