On Jan 19, 4:29 am, Ben C <
[email protected]> wrote:
>
>
> Just because something is _possible_ doesn't mean it's necessarily what
> happens or what is significant.
>
> I shouldn't need to tell you that.
>
> There are plenty of reasons why spokes can fail: poor quality, poor
> surface finish and bad spoke line are top of my list as the _most
> likely_. But I don't have the evidence either so I make no further claim
> than that.
Ben, for those with background in mechanical design, most of what
Jobst says regarding stress relief in spokes is easily understood, and
pretty obviously correct. It's well known that cold bending induces
residual stresses. It's well known that residual stresses add
(algebraically) to the overall stress level a part sees. It's well
known that residual stresses can be difficult to detect, and are often
overlooked. It's well known that neglecting their contribution can
lead to fatigue (or other) failures.
To illustrate the importance of these points: Strain gage companies
sell special strain gages that are adhesively applied to a site on a
workpiece, then balanced to zero, then re-read after a hole has been
drilled in the workpiece. The new reading is used to compute the
residual stresses. Obviously, this method is somewhat destructive and
quite tedious. The fact that its used at all is evidence of the
importance, and difficulty, of assessing residual stresses.
Mechanical stress relief is used in other fields. It's not nearly as
well known as thermal stress relief, and in fact I once had a
conversation with a respected metallurgical engineer (_not_ jim beam)
who had never heard of it. But from what I understand, the method
works quite well.
Of course, this doesn't mean that material, surface finish, etc. have
no bearing. And I seriously doubt that Jobst has ever claimed that.
But bringing microscopic high-tension-stress areas beyond yield, then
relaxing them, leaving a fatigue-resistant compressive stress in their
place, is certainly likely to help. And despite the fact that lots
of people are familiar with the concepts, many of us didn't think in
detail about their application to spokes and wheels until Jobst's
explanations. I give him credit for that.
> You have to stop making this personal.
It's obvious to me that for at least a couple individuals, it is
_very_ personal. While some of it may be triggered by Jobst's
sometimes abrupt manner, their rudeness and abuse goes to extremes,
and comes without any substantial contributions to our knowledge.
> What you write is very
> interesting and informative, but you can't expect people to just swallow
> everything.
I don't think one should "swallow" absolutely everything Jobst
claims. But I think he's got a good record for stating things that
make other mechanical engineers say "Hmm. I never thought of that
point, but he's absolutely right." And conversely, I think some
people are over-skeptical largely because they lack the background to
evaluate his claims.
- Frank Krygowski
