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04-07-20 | 02:55 PM

#86

LV2TNDM

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From: Northern CA

Bikes: Cannondale tandems: '92 Road, '97 Mtn. Mongoose 10.9 Ti, Kelly Deluxe, Tommaso Chorus, Cdale MT2000, Schwinn Deluxe Cruiser, Torker Unicycle, among others.

Quote:

Originally Posted by davidad

ndex Home About BlogFrom: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Weight of a bike
Date: 22 Aug 2000 00:38:48 GMT

Alan Hosker writes:

> Is the weight of the bike suspended on the spokes above the hub or
> is the weight pushing down on the spokes below the hub? Intuition
> tells me that it is hanging down from the upper spokes but I read
> once that wheelbuilding experts believe that it is pushing down on
> the spokes below. I find this hard to understand. What do you
> think?

First you must define your terms. Obviously you can't stand any load
on a wire as long and thin as a spoke, that is unless you pretension
it. That is why a bicycle wheel is tensioned and tensioned fairly
highly, almost to the yield strength of the rim. So in fact the hub
is hanging from all the spoke in one sense, since they all pull on it
in all directions. How a wheel responds to loads is not a belief but
rather a straight forward structural analysis and, as you point out,
not obvious. That is why the bicycle wheel remained misunderstood
until the mid 1960's... and then the analysis was rejected by people
who should have known better. It is still rejected by many engineers,
people who refuse to accept all results that conflict with their own
perception.

However, structurally the pretension can be analytically ignored
because it is only there to prevent the wires from buckling in
compression. Other than its tension the bicycle wheel is the same as
any other wheel such as a wooden wagon wheel. The spoke under the hub
is shortened in compression by a load on the axle, while the upper
spokes remain unaffected by the load. If you watch a dynamic display
of a structural analysis program on your screen, there is no way of
distinguishing between a wire spoked wheel and one with thick
compression members. That leaves it up to the observer to come to
terms with the description that "the wheel stands on the bottom
spokes".

You can test this by plucking a front wheel spoke (near the nipple)
before and after you put weight on the handlebars. You'll find that
the only spokes to change are those about the tire contact patch on
the floor. These spokes are compressed and lose tension. If the load
is great enough, they will become slack and the wheel can collapse
sideways. In any case, a wheel can only bear loads that do not
consistently slacken the preload.

> I apologize if this has been asked before.

It gets asked regularly but most of those who ask also give a
dissertation on how it REALLY works, except that they are always
wrong... otherwise they wouldn't have asked in a contentious manner.
That is what usually makes this such a testy question. As you see,
you got straight forward answers, in contrast to the last times this
came up, where the name calling began in the question.

Jobst Brandt <jbrandt@hpl.hp.com>

Your wife is a wise woman.

Brandt says, among other things, "structurally the pretension can be analytically ignored..." and "These spokes are compressed and lose tension."

OK, not that I'm pretending I know more than Brandt here (insert your witty quip here), but if you "ignore" the pretensioning of the wheel, then you're simply ignoring the most important part of the wheel, its tension. It's the single most important aspect of a bicycle's spoked wheel.

And It's simply impossible to literally COMPRESS a spoke as Brandt describes. Yes, it loses tension when the axle sees loads compressing the wheel. But to consider the resultant forces on the bottom spokes as compression is, in my opinion, a misnomer. They remain under tension, and very high tension.

I think what Brandt is doing is, by ignoring the pretension, effectively re-setting the tension values in the wheel to 0.00. Then, when a load is applied to the wheel, the bottom spoke tension drop slightly, yet into a negative tension value. Negative tension is compression, ergo compression.

Given this, there is no further need for argument or citing further explanation by Brandt. I understand what he is saying.

I will take the stance that this is a silly explanation of wheel dynamics (again, me being stupid again). But since there is no ACTUAL compression on the spokes (otherwise, they would protrude through the bottom of the rim), I refuse to call it compression. I guess I'm just difficult (wife quip please).

But thanks for bringing this up. I was obviously wrong as it relates to Brandt's discussion in the book and on the topic. I understand the theoretical approach Brandt is taking.

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