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Re: [Phys-l] Another tire question

When you put the tire on the rim, the bead squeezes on the rim. When
you put air in the tire the bead squeezes less hard on the rim because
the sidewalls are pulling outward on the bead. When you mount the rim
plus inflated tire on a car and lower the car to the pavement, compared
with how hard it was squeezing just before you lowered the car to the
pavement, the bead squeezes less hard on the top of the rim because the
in-plane (in the plane of the circle formed by the bead) component of
the tension in the upper sidewall increases, and the bead squeezes
harder on the bottom of the rim because the in-plane component of the
tension in the lower sidewall decreases. The in-plane component of the
net force exerted on the rim by the bead is thus upward.

See my post of Thu, 8 Nov 2007 12:03:55 -0500 for a more detailed
discussion of this that omitted the increased tension in the upward
sidewall pointed out by Michael Edmiston.

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Richard Tarara
Sent: Sunday, November 11, 2007 5:12 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Another tire question

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.



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