| >>>>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
| >>>>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
| >>>"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
| > 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
| I didn't prased my OP post clearly. I know that that part of the ABS and
| couputer sytem will report a difference in the revolutions of the wheels
| after integrating the revolutions over some time period long enough to
| let you make a few consecutive turns in the same direction without
| trigering a warning.
| What I was incredulous about was the part of the puzzle's answer saying
| the tire with low air pressure would be rotating "a heck of a lot faster".
| The specific wording of the answer, by Ray, of Bob and Ray's "Car Talk"
| show was:
| RAY: But when a tire loses air pressure and its diameter gets smaller,
| when the car is going down the road, in order for that tire to keep up
| with all the others and not get left behind, it has to turn faster. And
| your car does have something that is constantly monitoring the speed of
| all the wheels and comparing them to one another.
| What most modern cars have is ABS-- antilock brakes. And there's a
| sensor at every wheel that's reading how fast each of the wheels is
| turning. So, if it notes that the right front wheel is going a heck of a
| lot faster than the other wheels, it can either assume that you're
| making a lot of left hand turns or driving around a circle...or that
| your right front tire is going flat.
| It sounded to me like Ray somehow tricked himself into thinking that the
| increase in rotations per unit distance would be in direct proportion to
| the decreased rolling radius, and I don't believe that could be the
| case, for the reasons I already stated.
| Jeffry Wisnia
| (W1BSV + Brass Rat '57 EE)
| "Truth exists; only falsehood has to be invented."
If the tire is low, the axle is therefore lower to the ground. That
means the effective radius is shorter. Since the radius is shorter, the
effective circumference must be smaller. Following the progression of basic
geometry, more revolutions are required to move the same distance.
When a tire is low, the contact patch is not necessarily larger, once
you discount the lack of equal pressure in the middle of the contact patch.
The circumference is still the same, it's just not round, so there's a
bubble in the middle of the contact patch. Anyone who has seen a flat (and
mounted) tire sitting for a long time will see it clearly when it's rolled
over. Since the tire's still rolling, that excess slack as it passes
through the patch "humps up," and you will see the sides of the tread worn
more than the middle, since the pressure is so much lower in the middle.
Since a tire with normal pressure has a given diameter, it follows that a
tire with lower pressure will have a slightly smaller diameter, although the
bulk of the movement is taken up by the sidewall's expansion (due to the way
the wires route.) There's obviously a lot of flexing, and you can see the
sidewall flexing and wrinkling in a very low tire being driven slowly. This
kind of flexing in rubber, strung with steel belts, gets really hot and the
rubber starts to break down, even pulverizing itself. At some point, the
flexing becomes so much that the bead wrinkles and breaks. At that time the
tire deflates rather violently and at that point how smart or stupid you are
determines the rest, and who lives and who dies. The stresses on a tire
when it's way low are incredible and I thank God for steel belted radials
every time I have a flat!
Hairy story: I was in a company Astrovan on a freeway in Dallas rush
hour, inches from the zipper barrier, doing ~70 when a dumb bitch in front
of me blew a tire. In the minivan, bolted to the floor, was 1000+ pounds of
scale test weights (500 was too little for the way I liked to calibrate
scales) and lots of tools, so when she slammed on her brakes, I about shit
my pants 'cause traffic was asshole to belly button and FLYING. I stopped
short of her by two feet and was surprised that no one hit me from behind.
As I'm chewing her out for being so phenomenally stupid, I was removing her
spare from the back seat (???) and changing it as fast as I could. I could
barely touch the old rim, it was so damn hot! The sidewall was all but
gone, and when the wheel flipped to the ground in front of me there was a
pile of steaming rubber powder on the ground some four inches high and six
or so inches around. I'm sure some of this was from the sidewall that
ground away when she stomped on the brakes, but I can't see the tire
providing any stopping effort given its condition, so I'm confident most of
it was rubber that crumbled before the tire blew and was trapped inside the
tread by centrifugal force. The tread, of course, was hotter than fuck and
intact. About the time I got her tire back on I started to "come down" and
the transportation truck showed up to (more kindly) explain to her how to
brake safely after a blowout. I looked back at the million car traffic jam
and there, a few cars back, were a couple folks exchanging information and
sour looks. The difference between a royal fuckup of massive proportions
was merely milliseconds at that speed and it could have all been averted if
that $%^&*! had used her brain instead of her foot. Thinking back on it,
I'm sure my normal scatterbrained self would have just run over her, but for
some reason, at that very moment, I had my head screwed on right. Still get
that intense feeling when I think about it.