OT - Rotations of a low tire?

I don't buy it either. I know a GM mechanic and he states there are sensors in the wheel that transmit tire pressure to the computer. His bitch is that it complicates simple things like rotating tires because now he has to connect the scan tool and tell the computer how he rotated the tires.

cs

This is indeed the "indirect" tire pressure rotation system. You're right, it actually requires that the pressure be so low that the outermost belts are sort-of buckled in, but this is what happens if you are 5 or more PSI down on a big vehicle.

I think SUV's are soon (already?) required to have a "direct" method (after the tire fiasco of a few years ago), an actual pressure sensor.

Tim.

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

Jeff

I wonder if it would be legit to consider the distance from the axel center to the pavement becomes smaller when the tire is underinflated. The tire is free to flex and scrub as it wishes. The important parameter would be the "rolling radius", wouldnt it?? The picture gets pretty clear if you'd allow the underinflated tire to get thrown off. Then it would really have to turn alot faster to keep up with the other wheel with the good tire.

Jerry

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

I like the free wheel with rubber band spokes. When you shine a strong beam of light onto the spokes on just one side of the wheel it heats them up, they shrink, the wheel goes out of balance, and it rotates, continuing to turn as long as the light is on.

"They Shrink when heated?", you ask.

Yep. I thought I knew about lots of things but I lived over 60 years before I learned that about rubber. It is composed of funny molecules that do the opposite of what I'd come to think of as normal, like shrinking when heated.

If you've never tried this one it might suprise you.

Stretch a rubber band between your hands, hold it stretched for a few seconds to let it come to near room temperature and then touch your upper lip to the center of the band and bring your hands together quickly.

Feel it get colder?

Jeff

Yup, learned that in P-chem about 33 years ago. Weird.

Pete Keillor

First, you have to realize that you can't pressurize a tire enough to not have some deflection when weight is placed on it. Even solid rubber forklift tires compress in the contact area.

Here is the data from a popular sized truck tire; overall diameter

40.84", loaded radius 19.20"; revolutions per mile 509; max inflation 110 psi.

Before belted tires, not just steel belted radials, but any belted tires, the tread of bias tires created a tread wave in front of the contact patch. And sometimes continued into the contact patch, depending on speed. This was caused by the arched tread having to assume the nearly flat profile of the road surface. The wave was simply tread rubber waiting to be compressed as it went through the weight bearing area. All this flexing heated up the sidewalls of the tire and the scuffing of the tread as the compressed rubber exited the compressed area caused the bias tires to not last long.

ABS brakes and indirect pressure monitoring systems have made it hard for hotrodders to put different size tires on the front and rear.

There is more to come as our government tries to protect us as we get dumber, read lazier. All 2007 model year vehicles under 10,000 GVW will have direct pressure monitoring. (The public will feel they are absolved of maintaining their tire pressure)

Each tire will have a pressure sensor mounted in the tire. Most are attached via the valve stem. Price? Between $175 and $300 each. Bumping a curb and breaking that non-replacable valve is going to be very expensive. Some high-end vehicles have them now. Corrosion is already a problem because of the brass, stainless, aluminum and steel components of the sensor, stem, core, nut, washer and wheel.

And what do we get for this expense? The law already passed states that the monitor must alert the driver when a tire is 25% below its recommended pressure. That in itself is absurd but the monitor has 20 minutes to determine if a tire is low and alert the driver. The law is pushed by auto makers and monitor peddlers. They want to be relieved of any responsibility such as the Ford Explorer rollovers. ( As an aside, the same Firestone tires were used on F150 pickups. Recall a rash of pickup rollovers? Me neither.)

Let's assume you have just checked the air in your tires at the corner self-serve-pay-for-air station. You were careful not to damage that valve, right? As you pull onto the freeway, you hit a piece of glass that cuts your tire. A cut that will deflate your tire to zero, not

75%, in 5 minutes. But it is 20 minutes before your high priced monitor comes alive. Another shredded tire, another irate motorist cursing the #&%* no-account tire. Or maybe, another casualty.

The Rubber Manufacturer's Association, several tire companies and many consumer groups have sued the feds to either require tighter monitoring (less pressure loss and quicker detection) or scrap the law until such is available. They feel a false sense of protection is more dangerous than no protection.

Changes you can expect. Who will check the air in your tires for free when exposed to the risk of $1200 of damage? Roadside assistance will no longer repair tires. They will mount your spare. But then who is responsible for reprogramming the computer to tell it where that spare is now. And what will it cost to repair the flat tire when simply taking the core out of the valve can cost $300? I'm guessing $30 to $50.

For this kind of money, I feel a system could be developed to inflate the tires while traveling. There could still be a caution light or whatever. Maybe with suspension height sensors to detect load and air regulators, proper pressure could be maintained constantly.

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.

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

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.

Jeff

My example was extreme for the point of illustration. Sure, the flexing in a radial tire is more complex. However, I've blown enough tires due to underinflation or overloading (boat trailer, radials) to have observed fairly intact tread with totally disintegrated sidewalls each time. These were 6-ply rated, with 50 psi pressure, and seem much more sensitive to load - inflation conditions than most passenger car tires.

Here's another extreme example. I've seen rubber treaded caterpillar tractors in the last few years, like the one on the right here:

Now you've got a tread with no sidewall, so it runs fine. Angular velocity vs. linear velocity loses its meaning. When a sidewall is forced to follow a tread trying to run like a track, it flexes more and more, and at least in my experience, fails. You make a good point that the tread will flex some. The two are of course one piece, at least until it all comes unstuck.

I've got to get to bed now, to catch a plane in the morning, and will be out of touch for a while.

I enjoyed the thought games.

Pete Keillor

"Jeff Wisnia" wrote in message news: snipped-for-privacy@comcast.com... | Ned Simmons wrote: | | > In article , | > snipped-for-privacy@chartermi.net says... | >

| >>On Thu, 18 Aug 2005 13:09:28 -0400, Ned Simmons | >>wrote: | >>

| >>

| >>>In article , | >>> snipped-for-privacy@conversent.net 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 | | | 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. | | Jeff | | -- | 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 f*ck 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.

6-ply rating means the sidewalls have 2-ply. Is ir any wonder that the 6-ply part lasts longer than the 2-ply part?

Nope.

| >> My example was extreme for the point of illustration. Sure, the | >> flexing in a radial tire is more complex. However, I've blown enough | >> tires due to underinflation or overloading (boat trailer, radials) to | >> have observed fairly intact tread with totally disintegrated sidewalls | >> each time. These were 6-ply rated, with 50 psi pressure, and seem | >> much more sensitive to load - inflation conditions than most passenger | >> car tires. | >

| >6-ply rating means the sidewalls have 2-ply. Is ir any wonder that the | >6-ply part lasts longer than the 2-ply part? | | Nope.

The tread bands are wrapped circumferentially and are the best wrapped part of the tire due to the least amount of flex needed and highest pressure. When a tire fails the tread is rarely part of it. When tires explode (car or aircraft tires, even) they blow out sideways, which is why you see a roll in cage at places that do semi truck tires.

The cage is to catch the split ring, not pieces of the tire.

Hummer has had a central tire inflation system for almost 10 years! All tires can be inflated or deflated at the push of a button. They can be inflated as a pair (front/rear) or all together. It also has alarms for high and low pressure.

This morning on the way to work a car came up beside making all kinds of funny tire noises. One front tire was almost running on the rim and the driver was oblivous to it all.

I'm not sure if this applies but on the newer Chrysler products they have a tire pressure monitoring system that works by having a transponder located in the valve stem. There is a sensor located in the fender.

If you are not familiar with transponders, they are common in several applications like injectable little pellets for dogs and cats that can be scanned to help a lost pet.

Newer cars also have them molded into the head of the key. if an unauthorized duplicate is made or the lock is forced the car will not start.

Perhaps the car in question had a tire pressure monitoring system.