wrote:
On Wed, 13 Oct 2004 10:05:58 +1000, big Pete
wrote:


Wrote:

On Tue, 12 Oct 2004 23:41:14 +1000, big Pete
wrote:


David L. Johnson Wrote:

You probably have them already, but if not I recommend butted

spokes.

They make a more reliable wheel.


I am a big guy (245 pounds last time I checked) will butted spokes

make

that big of a strength difference in the wheel? I did not buy the

rim

and spokes yet. I am thinking of a Sun double walled rim with eyelets
that I can get for a good price. And the spokes are 14 gauge

stainless

steal (non butted) as stated in the my first post. I will Also try to
make a dish stick. Looks like I am able to make it from the

suggestions

you guys have given me.

Thank you all

Pete

Dear Pete,

Both thick 14 gauge spokes and thin butted spokes with 14
gauge ends and a 15 gauge middle are more than strong enough
to bear the loads--they break at the ends, not in the
middle.

Double-butted spokes keep the ends thick to try to reduce
such breaks.

Their advantage is that, being thinner for most of their
length, they stretch more and therefore are less likely to
lose all tension (a bad thing) as they roll under the hub.

When the rim flattens ever so slightly against the pavement,
the spoke above it loses tension. A thick straight spoke
didn't stretch as far as a thin-center-section spoke, so the
thick spoke loses all tension and rattles, going out of true
and breaking more often and while the thin spoke can
contract that much and still have tension, which makes it
likely to last longer.

Think of them as rubber bands at the same tension, neither
of which is going to snap in the middle, but neither of
which we want to go slack. The thinner rubber band enjoys a
greater range of motion.

Carl Fogel

Dear Carl,

You are going to have to enlighten me a bit more. I think I agree with
your analogy. OK so the thinner band has a better range of motion. But
that would imply that they will ware out faster because of the bigger
range of motion they are going though.


Dear Pete,

You raise a good point, but the long thinner middle section
of spokes doing the stretching simply doesn't wear out.

That is, spokes don't break in the middle. They fatigue,
crack, and fail at the elbow about nine times out of ten.
The tenth spoke one breaks at the nipple. That's why the
spokes are left thick (double-butted) at each end.

Even a thin mid-section stainless steel spoke is so strong
in tension that it isn't going to fail due to the normal
stress cycle of losing and regaining tension as it rolls
under the axle.

As others have suggested in this thread, this is roughly
what Jobst Brandt points out in "The Bicycle Wheel" when he
recommends double-butted spokes. Paradoxically, a thinner
mid-section spoke produces a more reliable wheel, not
because it is stronger than a thicker spoke (it isn't), but
because it is more than strong enough and its increased
elasticity lets it function better at what spokes need to
do, which is to maintain tension throughout the load range
without going slack.

The metallurgical side of this is that ferrous metals
(steels) have the charming characteristic of being able to
cycle indefinitely if the stress is low enough--they just
won't fatigue and break. (The elbows and nipples break
because stress is concentrated there and encouraged by
various details like the bending, the threading, and so
forth.) So the engineers can design a steel structure to
last forever in terms of stress cycles if the stress is kept
low enough.

need more precision here carl. if by "ferrous metals" you mean mild
steel, then yes, it has an endurance limit below which it won't fatigue.
but if by "ferrous metals" you're including stainless steels, then
that's an incorrect statement. endurance limits are a function of
carbon content, and carbon is usually kept to a minimum in most
stainless steels to prevent harmful formation of chromium carbide.


Non-ferrous metals like aluminum, titanium, and magnesium do
not share this ruggedness. Given enough stress cycles, they
fatigue, no matter how low the stress is. The engineers have
to design non-ferrous structures to reduce the amount of
stress in each cycle, reduce the cycles, or both--and still
they can calculate when non-ferrous things should bust.

see above.


Some theory holds that steels eventually do fatigue even at
low stress cycling, but so much later that it's indefinite
for practical purposes. But no theory that I know of
predicts that a double-butted spoke will wear out in the
middle--it will break at the elbow or nipple, but not as
soon as a straight, thick spoke that is more prone to losing
all tension if it rolls under a heavily loaded axle.

For heavier loads, the usual solution is more spokes, not
thicker spokes--36 spokes might increase to 48 spokes for
many tandems and for truly heavy riders like Chalo Colina,
who has about 135 pounds on lightweights like you and 185
pounds on delicate creatures like me.

i'm down to 200, & i'm still riding 16 spoke wheels to no detriment.


Carl Fogel