In article ,
says...
On Thu, 18 Aug 2005 13:09:28 -0400, Ned Simmons
wrote:

In article ,
says...

The "NPR "Car Talk" show's "Puzzler" a couple of weeks ago gave an
answer stating that some car's computer "knew" a front tire was low on
air because the ABS system noted that wheel was rotating "a heck of a
lot faster" than the other wheels when the car was driven.

I didn't buy that one.

Sure, the rolling radius of a low tire is less than that of a fully
inflated one, but the overall circumference, particularly on a steel
belted tire, remains the same. Barring slippage, that circumference must
lay its whole length on the road once per revolution, just like the
circumference of a full tire does.

But if the circumference remains constant as the rolling
radius decreases there has to be slippage. Underinflated
tires run hot, and some of that heat surely comes from
excess flexing of the tire, but I imagine a large
proportion is a result of the rubber scrubbing against the
pavement.

"a heck of a lot faster" may be exaggeration, unless the
tire is seriously under inflated, but I'm sure the effect
is measurable under controlled conditions even with small
changes in pressure. I guess the question is how sensitive
can the system really be without causing nuisance alarms?

Ned Simmons

Picture a spoked wheel with string instead of spokes, and the strings
1/2" too long. Just because the axle is closer to the road doesn't
mean the tire is slipping,

I don't think it's the fact that the axle is closer to the
road that's causing the tire to slip relative to the
pavement. When the tire deforms the radial distance from
the axle to the ground across the length of the contact
patch is not constant. So either the linear velocity or the
angular velocity of the rubber on the road has to vary - in
other words, something's got to give. The sidewall probably
absorbs most of the difference when the tire is properly
inflated, but can only do so much. Keep in mind that
underinflated tires wear more rapidly, which implies at
least some scrubbing.

Your example of a loosely strung wheel with a rigid (I
assume) rim really isn't analogous since the rim only
contacts the road at a point.

or that the tire's radius has actually
changed.

If the axle is closer to the ground, hasn't the effective
radius of the wheel been reduced?

The heat is probably almost exclusively from the flexing,
primarily in the sidewall.

I'm skeptical, especially in a seriously underinflated
tire.

Ned Simmons