Re: [Phys-l] Another tire question

["Michael Edmiston" <edmiston@bluffton.edu>]

Brian W. said...

> The lower spokes alone of a bike's wheel will surely not support
> my weight.     And yes, a wire-spoked wheel can be considered
> at a naive level to be suspended from the upper spokes.

Let's see how naïve that turns out to be.

> Moreover, the  nipples with which spokes are fixed to the rim
> are not capable of handling radially outwards directed force.

Excellent.  If any spoke switches from tension to compression it will push 
the nipple into the inner tube and puncture it.  Therefore, when working 
properly, no spokes are under compression.  (read it again... *no* spokes 
are under compression)

> But the strange thing is, both upper and lower spokes take a share
>  in supporting my weight.  How can that possibly be?

It can't be.  This is a false statement.  The lower spokes are pulling down, 
not pushing up.

> Opposing spokes are in tension.     When an upper spoke sees
> greater tension, a lower spoke is also seeing reduced tension.

Yes.  You said it.  The lower spokes are still in tension, although reduced. 
Were it not true, they would either buckle or would push their nipples into 
the inner tube.  They don't buckle; they don't push their nipples into the 
inner tube;  they are in tension, not compression.  So why do you say they 
are taking a share in supporting your weight?  Suppose you have four people 
around a card table and you want to lift the table.  You and the person 
opposite you are lifting, but the other two people are pushing down.  You're 
not succeeding in lifting the table, so you ask the two people pushing down 
to "get with the program and help."  They comply, but not by lifting; 
rather, they just push down less hard.  So... when they push down less hard 
such that you and your partner successfully lift the table.. you're going to 
tell me that the people pushing down are helping to support the weight of 
the table because they are pushing down less hard than before???  That seems 
odd wording to me.  I'm not sure if this is a semantic problem or a physics 
problem.  If you say the people pushing down are helping because they are 
carrying some of the weight, then we have a physics disagreement.  If you 
say the people pushing down are "helping lift the table" because they 
reduced their downward push, then we dealing with semantics.

It's the same for the car tire except the spokes are replaced by a 
continuous sidewall.  The sidewall is always in tension... never 
compression.  If the lower sidewall is in tension it is pulling down, not 
pushing up.  How is that helping to support the weight of wheel?


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu