wrote:
On Fri, 30 Jul 2004 19:44:49 -0700, jim beam
wrote:


wrote:

snip

Spoke-squeezing is an intriguingly mysterious subject to
research. I remain agnostic, wavering one way and the other,
but haven't seen any experimental data or analyses involving
bicycle spokes. If you have the 3rd edition, perhaps you
could peek at the Wiedemer stuff and give me your thoughts
on it?

you may also want to consider this question:

q: elevator safety certification requires loading the cab to double
it's "safe working load". this is to test the wire ropes that suspend
it. the reason is that fracture mechanics predict that this process
will typically reveal by failure any latent flaws. but, if we extend
spoke squeezing theory, wouldn't this overload procedure also prevent
fatigue of elevator cables?

a: no. elevator cables still fatigue and need regular testing,
inspection & replacement.

the bottom line is that there is no quantification or testing of this
spoke squeeze theory. squeezing "as hard as you can" is no more
scientific than building with spoke tension "as high as the rim can
bear". i would suggest to you that the reason academics "change the
subject and get back to "serious" work" is because this theory is mere
speculation - it's author has shown no basis in statistical fact, and
most definitely not by metallurgical analysis.


Dear Jim,

Aaaargh!

I really wish that you could have thought of a different
example, since I didn't want to hear that elevator cables
need replacing.

I draw comfort from a vague memory that they have some kind
of safety brake, according to Ambrose Bierce and to some
famous demonstration in which Otis cut the cable while
standing on top of an elevator.

as indeed i do too!


Two questions occur to me, both illustrating the depths of
my ignorance.

First, how much do elevator cables resemble spokes?

obviously, rope is multistrand, a spoke is single strand, but the
materials & applications are the same. it's only the practical issues
of price, of needing a spoke that resists torque sufficiently to be able
to tighten a nipple and indeed, ability to thread a nipple in the first
place that lead to the use of single strand.

Are they
made of stainless steel?

sometimes. but typically not unless environmental conditions demand it.
doesn't make much of a difference - neither stainless nor typical
non-stainless steel rope have an endurance limit so they'll both fatigue.

Does it matter that they bend
around pulleys in a constant side-to-side flexing different
than spokes?

yes, and those pulleys cause wear and bending stresses, but that's why
you use multi-strand in the first place. also, one strand breaking in a
rope of 100 leaves 99 others - pretty comforting. and there's a small
degree of freedom to move between strands which reduces cross sectional
stress considerably.

Do spokes and cables go through similar cycles
of tension, partial release, and back to normal tension?

yes, absolutely.


Second, do spokes in well-built (by whatever means) wheels
require constant inspection and replacement?

if you read the instructions that come with all these expensive
pre-built wheels yes! but, that's only a cursory visual inspection.
it's just like the contrasts between the safety & inspection regimes for
cars vs planes, wheel spokes are not usually considered a high fatality
risk, so there's no reason to subject them to a rigorous expensive
certification procedure.


I understand that spokes are different from cables. I'm just
wondering how big the differences are and how much they
matter.

See you in the stairwell,

Carl Fogel