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, or that the tire's radius has actually
changed. The heat is probably almost exclusively from the flexing,
primarily in the sidewall.

Pete Keillor