Re: [Phys-l] Another tire question

["Richard Tarara" <rbtarara@sprynet.com>]

Sorry to those getting 'tired' of this thread, but I'm still not sure--and I
think we have two camps--the wheel being pulled from above or pushed from
below.

I'm trying to think very Newtonian about this, and unless the bead is
literally glued to the wheel rim (or trapped) I can't see a mechanism for it
to 'pull up' on the rim.  I don't think the goop that Michael refers to is
absolutely necessary, or always applied--any know for sure.  I guess the
model I'm thinking of now is putting a rubber band around the wheel (a big
rubber band), grabbing it at the topmost point and lifting.  This puts the
band under tension, more at the top or equal all around (not sure), but with
a 'stretchy' band certainly the wheel is supported by the band from the
bottom.  This is how I'm visualizing the bead(s)--tensioned from the top but
pushing up from beneath.   I can't see how the tension in the bead sans glue
can pull upwards (isn't the tension along the curved path of the bead due to
stretching the bead to fit over the wheel and radially outwards due to the
sidewalls?)  Below the wheel the upwards force comes from reaction force of
the wheel's weight pushing down on the bead.  What is the actual upwards
force of bead on wheel above?

Rick


----- Original Message ----- 
From: "Michael Edmiston" <edmiston@bluffton.edu>
half becoming less, when the car's weight is
> added to the wheel.
>
> The thing I still struggle with is the bead.  It is not clear to me
> whether
> we should consider the bead/rim as a rigid unit, or if there is some
> movement of the bead with respect to the rim.  I have watched tire people
> remove tires from wheels.  It is not easy to break the bead from the rim.
> Remember that the air pressure is pushing axially outward on the sidewall
> and bead, pushing the bead into the rim sideways.  When they mount the
> tire
> they spread gluey goop on this joint.  The bead generally becomes stuck to
> the rim so hard that when you want to remove the tire you have to work to
> break the bead/rim seal.  They have motorized equipment to stretch the
> bead
> off the rim once the seal is broken.  They also have hydraulic equipment
> to
> push on the bead to get it separated from the rim.
>
> I once observed a person removing one of my tires from the rim.  The air
> pressure was all released, the tire machine was pressing down on the
> sidewall trying to break the bead/rim seal, and not succeeding.  While the
> tire machine was still trying to break the bead seal, the serviceman
> grabbed
> a very large rubber mallet and starting hitting the sidewall near the rim
> with what looked like all his might.  It took several hammer blows before
> the bead separated from the rim.
>
> In that case, the bead was stuck so hard, without air  pressure, that I
> wondered if he was going to free my wheel from the tire.  When the tire is
> pressurized, the bead is pushed sideways into the rim and that would make
> it
> even more difficult to break the bead/rim seal.
>
> Based on that experience I can certainly imagine that the rim can "hang"
> from the upper bead.  By I can also imagine that if the rubber/metal
> interface on the top stretches some, then that upward pull from the
> sidewall
> will "propagate" some distance circumferentially along the bead and down
> around the wheel.  A few inches?  All the way to the bottom of the wheel?
> I
> don't know.