On 17 Dec 2006 09:55:03 -0800, "Ron Ruff" <[email hidden]>
Quoted message said:
jim beam said:next time, use the tensiometer and the "mavic method" of bedding the
spokes in. the wheel will remain true and tension constant thereafter.
Jim, what is the "mavic method"?
Dear Ron,
It's a little confusing. Here's a somewhat mistaken recent
endorsement:
"A better method of stress relief is to stand the wheel on its axle
and press down on the rim with hands diametrically opposite, once
between each pair of spokes."
--Jobst Brandt
http://groups.google.com/group/rec.bicycles.misc/msg/aaa1c64310024d4f
But, as I said, it's a little confusing.
Lay the wheel flat on a workbench, with the axle supported by a chunk
of 2x4.
Now lean hard on the wheel, pussing down with one hand on each side of
the rim.
Go around the wheel, doing this for all the upper spokes, then flip it
over, and do the spokes on other side.
When I tried this on a wheel whose spokes had been squeezed before it
was ridden about 1,200 miles, some spokes cracked noisily as they
unwound a little. Another poster who tried the method reported the
same noisy results.
Most people can put more tension on the spokes by leaning on a wheel
laid flat like a roulette wheel than by squeezing a pair of spokes
with one hand.
But more tension obviously doesn't allow a spoke to unwind. If
anything, more tension makes a spoke more reluctant to unwind.
The spokes unwind because the tension drops on the other spokes on the
wheel, more than tension would drop if you just squeezed a pair of
spokes together. It's the spokes going slack that are unwinding and
making the cracking noise.
So the Mavic lean-on-a-wheel-laid-lfat method does provide better
unwinding than squeezing spoke pairs. You can hear the cracking.
(At least that's what I assume the noise means. I had to lean hard to
make the cracking noise, and once the wheel made a cracking noise, it
never cracked again, no matter how hard I leaned on that section. Lean
on another section hard enough, and crack! Eventually, no more
cracking noises.)
But it's not stress relief. Other tests have shown that increasing
tension beyond normal levels doesn't provide any significant
mechanical stress relief. There are indeed demonstrable residual
stresses from bending spokes, but these stresses are demonstrably
relieved just by building the wheel up to normal tension.
A typical 60-pound hand-squeeze raises spoke tension only about 55~65
pounds. (The dramatic spoke bending is due to the slack gained when
the rim deforms into a faint N or Z shape, not wildly increased
tension.)
Leaning on the wheel the Mavic way with 100 pounds of force is much
easier and raises tension higher, to around 85~95 pounds for the top
spokes at each hand.
However, raising spoke tension from normal levels like this has no
apparent effect on residual stresses because they've already been
removed.
A spoke bent but not tensioned will show obvious stress relief when
the bend is heated with a propane torch. The ends of the bent spoke
move quite obviously as the bend opens up.
Tension a spoke with the same bend a little, heat it to relieve the
remaining stresses, and the movement is noticeably less.
Repeat the process, increasing the tension each time, and the spoke
bend eventually shows no significant change.
By the time that a bent spoke was tensioned to ~200 pounds, it no
longer showed any more stress relief by unbending when heated with a
propane torch than a straight, unused spoke, fresh from the factory.
There was no apparent difference between the spoke tensioned to ~200
pounds and a spoke tensioned to ~400 pounds. By the time that normal
tension was reached, there just weren't any residual stresses left to
relieve.
Here's a post about what happened to the upper and lower spokes around
a wheel laid flat when I loaded 100 pounds onto a board laid across
the wheel:
http://groups.google.com/group/rec.bicycles.tech/msg/5c0ce7ddb9094b02
The Mavic lean-on-the-wheel-laid-flat method allows slightly higher
tension than squeezing spoke pairs, but the real advantage is probably
that the lowering of tension in other spokes allows more unwinding.
There's no law against using both methods on the same wheel.
Cheers,
Carl Fogel
