Originally Posted by
LV2TNDM
Yes, I think he's considering it this way. As a whole, the wheel undergoes compression and its resistance to that compression is what holds the bicycle by the dropouts. I think it's an unnecessary simplification that serves no purpose. Of COURSE the wheel is opposing the compressive force of the dropouts on the hub. Otherwise the bike wouldn't function. The downward force of gravity is being opposed by an equal and opposite force from the wheels. But the way the wheel achieves this is not by compression. He explains earlier in the book about how a wagon wheel IS a compression wheel and acts completely differently than a tension bicycle wheel. (Unless, of course the wagon wheel's spokes are so well attached that the top spokes undergo tension forces as the bottom spokes undergo compressive forces, thereby sharing the overall load or force. Then it gets more complicated.)
But that doesn't happen in a bicycle wheel. Tension wheel spokes, as almost always configured, cannot tolerate individual compression loads. They can only go to zero tension and stay there. They cannot be compressed because the are not supported in that direction - they simply push out of the spoke holes or eyelets.
This, again, is why his discussion makes no sense to me. Again, I may just be incredibly stupid. My wife tends to think this
ndexHomeAboutBlogFrom:
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.