On 31 Jul 2004 00:58:09 -0700, Benjamin Weiner
<
[email protected]> wrote:
[snip]
>
>Carl, did you see in the discussion at
>http://yarchive.net/bike/stress_relieve.html
>the article containing this quote and link
>
> "For some results of some actual residual stress measurements I did on
> 7050 aluminum plate before and after stretching see:
> http://www.lanl.gov/residual/alplate.pdf
> The residual stress was reduced by about a factor of 10 by the stress
> relief process."
>
Dear Benjamin,
Yes, I've seen the thread and looked at the link to the
test. While I can't recall where I read it, others pointed
out a few potential problems.
The test involved a comparatively massive metal slab, about
6 inches square and 3 inches thick, not a spoke about a
tenth of an inch thick. (My naive guess would be that this
size difference wouldn't matter because the flaws are
microscopic, but a materials engineer suggested that it
might.)
The test slab was cut out of a much larger piece that was
"rolled," so it was worked to some degree, but it may not
have been as heavily worked as a bicycle spoke. (This
sounded more plausible to me, but it wouldn't surprise me to
learn that the rolling of the slab worked it just as much or
more as the production of a spoke works a spoke.)
The metal tested was aluminum, not stainless steel. (I know
that the two metals are similar in that neither shows a
yield point on a stress-strain test, but are different in
that aluminum ain't stainless steel. I also know that
aluminum and steel have different fatigue characteristics
when used in frames and that I don't have the slightest
idea whether any of this matters in comparing the aluminum
slab to the stainless steel spoke.)
The question of how much "stretching" was used on the test
slab versus how much "squeeze" (same idea) is used by a
typical bicycle wheel builder was not addressed. The slab
was stretched 1.5% to 3% in the rolling direction, but how
much a spoke is stretched by the squeeze method is unclear.
(My own vague notion is that a good squeeze should stretch a
pair of 300mm spokes that much.)
What the stretching does to the material in terms of final
dimensions was also debated. My naive impression was that
both the stretched test plate and the squeezed spokes are
tensioned only elastically and snap back to their original
length when tension is released--that is, there's no
permanent elongation. However, others have suggested that
releasing any stress involves permanent changes in
dimensions, microscopic or not. Damned if I know the answer.
A final question concerned the quenching of the aluminum
plate (I think that you have to go to the later version of
the test to see this) and whether quenching aluminum creates
far more internal stress than would be found in spokes and
whether such a difference makes any comparison a matter of
apples and oranges. Again, I don't know enough about metals
to even hazard a guess--I have a hazy notion that spokes are
cold worked, not quenched, but whether this matters to
stress is a mystery to me.
None of these objections, as I recall, were meant to cast
any doubt on Mike Prime's aluminum plate test itself, which
was not undertaken to address the stainless steel spoke
question and seems like a reasonable test to bring up. They
were raised to point out that the two situations may differ
in significant ways (and were also raised probably for the
joy of quibbling, not that we ever see any of that on
rec.bicycles.tech).
The trouble is probably that the question of spoke fatigue
is trickier than we'd like it to be. Various explanations
exist, each with fierce partisans, but testing of the kind
that would stand up in a high school physics class seems to
be hard to find, partly because the testing is likely to be
much harder than expected, partly because the subject is
much more complicated than it might seem at first, and
partly because the question is not exactly crucial.
I often wonder whether my wavering on this matter resembles
what I had to endure with people debating who wrote
Shakespeare. I'm not sure who's right about spoke squeezing,
so I long for a site that patiently goes through the details
for the layman in the way that this site explains literary
matters:
http://shakespeareauthorship.com
Unfortunately, not everyone is as helpful as you are, so
questions here on rec.bicycles.tech often degenerate into
mere name-calling instead of explanations. It's not much
good trying to convince a skeptical audience either way on
spoke-squeezing if all that I have is that one person says
that the other is wrong and both claim that their spokes
last forever.
The last person whom I bored to tears about spokes raised an
interesting question. If spoke-squeezing works, either by
relieving stress or by other methods, and makes spokes
practically immortal, how soon should unsqueezed spokes
break? That is, when did I predict that the unsqueezed
spokes would fail?
My muttered "sooner" was dismissed as being a bit imprecise.
I'd mentioned that some spoke squeezers claim over 50,000
miles on failure-free spokes, so my tormentor kept asking me
when he should expect his unsqueezed spokes to fail--1,000
miles, 5,000, 10,000, or even 25,000 miles?
He relented when I promised to pose the question here, so
perhaps someone will speculate on how long the spokes on
what everyone agrees is an otherwise properly built wheel
should last if not squeezed.
Carl Fogel
