Re: [Phys-l] Another tire question

[Rick Tarara <rtarara@saintmarys.edu>]

Brian (et. al.)

Let me rephrase Michael's argument so that maybe you can better appreciate 
it (and I can see if I really understand it).

As Michael says, your half-tire argument IS a different problem in that the 
air pressure pushes ONLY upwards on the bottom of the wheel--Pressure x 
cross-sectional area, giving a Net Force upwards on the wheel.  In the full 
wheel, there is an equal force downwards due to the air pressure acting on 
the top half of the wheel--Net Force = zero (discarding the TINY buoyancy 
force).  With the full, inflated tire, the air pressure does not directly 
provide any of the lifting force on the car although it is essential to the 
workings of the tire--this is, I believe, then one of Denker's zeroth order 
effects.

In your example below, I think you are reverting to my VERY STIFF model 
where we can talk about compressive forces in the sidewalls and at least 
some of the upwards force coming from such.  BUT...in the tire models being 
presented by Michael and others, the sidewalls cannot really sustain 
compressive forces only tension forces and therefore become like the spokes 
in the bike wheels.  Then ultimately, with no net force due to the air 
pressure, the tire itself must produce a net upwards force on the wheel and 
that can only be transmitted through tension which is going to be radially 
outwards from the wheel (trapped bead pulling outwards).  To have a net 
upwards force, that tension force must be larger on the top of the wheel 
than the bottom--hence the image of the wheel hanging from the top half of 
the tire.

Does that help any?

Rick

----- Original Message ----- 
From: "Brian Whatcott" <betwys1@sbcglobal.net>

> At 09:50 PM 11/7/2007,  Mike E., you wrote:
> > ..
> > > 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.
> ...
> > Michael D. Edmiston, Ph.D.
>
>
> Hmmm... I see I am not dislodging you from the upper half support
> position, Mike.   I'll offer one more extreme case to illustrate,
> then leave the topic on its merits.
>
> Let me select a tire of extraordinary wall strength, but great 
> flexibility.
> Let me inflate this tire so hard, that regular tests of deflection
> and load bearing cannot distinguish it from a wheel made entirely of 
> steel.
>
> You might think of this as the limit of  high inflation.
>  An engineer or physicist could hardly be faulted for measuring
> the load bearing contribution of the tire walls in the lower half,
> is that not the case?
>
> And at the other inflation limit, a flat tire demonstrably supports
> the entire load on the lower portion of the tire.
>
> You already concur that a (lower)  half tire can support weight, but
> you curiously spring to a "this is a balloon" explanation. Yes, it is!
>
> I rest.....
>
>
> Brian Whatcott    Altus OK    Eureka!