Tire PSI question

[TEK]

Wasnt sure where to put this but here goes--- If you have an unloaded vehicle with tires at 32psi and put a load on it, will the psi go up? Co-worker Bill said that he read in a car mag that psi remains the same no matter what your load is. I contend that if you load it, psi goes up,even if only fractionally. Can somebody enlighten me? Obi-won?Anyone?Thanks........Tim:cool:

[MAC702]

Wasnt sure where to put this but here goes--- If you have an unloaded vehicle with tires at 32psi and put a load on it, will the psi go up? Co-worker Bill said that he read in a car mag that psi remains the same no matter what your load is. I contend that if you load it, psi goes up,even if only fractionally. Can somebody enlighten me? Obi-won?Anyone?Thanks........Tim:cool:
Ever see an overloaded truck blow a tire? What do you think caused it to fail?

[TEK]

Ever see an overloaded truck blow a tire? What do you think caused it to fail?

Yes I have. And that is pretty much my stance on this. However, Bill claims if you keep the tire within its load specs it wont blow and the psi stays the same:confused:

[dda52]

Ever see an overloaded truck blow a tire? What do you think caused it to fail?

All the crap we have to drive over, tire imperfections, bad castings, and junk like that....and maybe an exceeded work limit.:rolleyes:

But, when I check my tires, loaded with 12+ tons on them, they read the same pressure as when I was unloaded. DOT checks my tire pressure, loaded and unloaded. If the pressure was over the prescribed 110 lbs, ticket city.:( If it is under the minimum pressure for the weight on it, ticket city....same applies when unloaded. Heat will increase the pressures slightly, but not to extremes. Too low a pressure is actually more detrimental than too high. A low tire will build up twice the heat as an over pressured one.

The only reason I know this is because I get checked all the time...and I have yet to get a tire ticket.;)

[TEK]

Well,I rekon thats pretty definative. Thanks guys. I stand corrected.

[MAC702]

Interesting. So when the tire blew on that half-ton pickup that was loaded with 40 bags of cement, it was caused by a nail? I'm sticking to my original story.

Here's my reasoning. We assume temperature remains constant for our purposes here. That means pressure is inversely proportional to volume. No other relationships matter, and it is also completely unavoidable.

Pressure will be INSIGNIFICANTLY raised as the vehicle is loaded because the tires will be SLIGHTLY compressed. I don't see how they have a choice. But, as mentioned, I will agree that within the load rating of the tires, it will be very little, perhaps not measurable with normal instruments.

But, when the tires are significantly overloaded, they go beyond their ability to maintain their volume, and pressures WILL RISE SIGNIFICANTLY from that point on, with tire failure imminent.

[MAC702]

Rereading this thread makes me realize that the original question did specify keeping the tire in its load rating. So while I agree that it might be immeasurable, I maintain that the pressure will rise on an absolute scale.

[dda52]

Could be...but testing that theory could get me a $2500 ticket, so I'll pass. I do know this...when I was doing some ranch work...I was routinely loading up at 6,000-8,000# overweight. The tire weight rating was being exceeded by a good bit. I was exceeding tire axle weights by 30% easily, but well within spec for my air suspension.:rolleyes: My pressures were the same as normally loaded. I did some airing up on a leaking tire and checked a few that looked flat, but really weren't.:eek: :rolleyes: Maybe I just wasn't heavy enough.:rolleyes:

[Sandy]

The blowouts we often encounter during heavy loads are mainly are caused by heat build up which in turn causes extreme softening of the tire which in turn causes bla bla bla, a cascading event that is called a blowout. The heat build up itself will raise the tire pressure somewhat but too little too late. The heat build up is caused by the extreme deflection. Deflection being all the physical shape changes the tires have to go through as they move down the road. When heavily loaded what was originally a fairly round tire, centered around the axle/spindle, is now squished (technical term). As it moves forward the tread is compressed and the sidewalls bulged then they must rapidly return to their original shape as the tire moves forward only to go through that whole process again in an instant. I ain't gonna do the math but I'd guess that every inch of the tire gets deformed and returned to shape maybe 10 to 20 times a second at normal speeds.

Jackin the pressure way up for heavy loads helps keep the tire "rounder" ( another technical term) and reduces the deflection. :)

[Blacksmith]

1) Its the extreme flexing and resultant heat build up that blows out overloaded tires.

2) I guess there are millions of vehicles with overinflated tires on the road since just about every tire jockey checks the pressure on new tires while it's on the tire machine (zero load) and then installs it and set the vehicle down on it. I'm afraid to go out now, I could be killed in a massive simultaneous tire explosion.

[MAC702]

1) Its the extreme flexing and resultant heat build up that blows out overloaded tires.
I'll buy that.

[professur]

Well, Oddly enough, up here in Canada, I change my tires alot. In fact, I jsut took off the winter shoes for the summer ones. I've got them all mounted on their own rims, so it's just a bolt on job. but before I go through the effort of putting them on, I check the tires. I did the full tour of 4 tires, and set them all at the spec 32psi. Then I put them on the car. Any guesses as to what the tire pressure is now, with a ton of steel weighing them down? Yup, 32psi.

It all has to do with load, and footprint. As you load a tire, it flattens out and increases it's footprint. It's designed to do this, and that keeps the pressure constant. Over load it, and the edge of the footprint becomes too sharp, and heats up, causing tire failure.

When setting the bead of a tire, it's not uncommon for them to put 60psi in a tire. Then they back it down to spec. If you managed to load a car or pickup to the point where pressure caused the blowout, *shakes head*

Although, I will say this. I have seen this once. I work for the phone company up here. And with the price of copper, spool theft isn't uncommon. Usually they try to take spool ends. But you always get one genius that tries to steal a full one. And they usually try to steal it off a loading dock. Drop a 10 ton spool onto the back of a half ton pickup ... and yeah, then, you'll blow tires. And lots more.

[nice bike]

i think that if you put a load on the tire that it would gain in preasure
but im not so sure

-Greg

[JD in SoCal]

The pressure in the tire won't noticeably increase with load, because the tire will deform and expand enough to ease the increase in pressure. If you could confine the entire tire, so that it could not expand, and add load to it, you would see an increase in the air pressure inside.

Imagine a water balloon that you push (gently) against a table top with your flat hand. It will deform enough (squish out) so that the pressure inside will not change significantly. Now put it into a tube with a tight-fitting plunger in it and a cap on the other end. If you push on the plunger, you would get a corresponding increase in the pressure inside the balloon. Tires are not confined and will expand enough that the air pressure won't change noticeably.

In practice, air temperature will change the psi inside a tire, and elevation will also (the psi will go up as you go up the mountain), but the load on the vehicle won't change the psi reading enough to measure.

JD

[Terry Lingle]

If the volume of the tire cavity decreases the pressure will go up but at normal inflations and loads the volume does not change only the shape changes . the belts in the tire ensure that the internal volume remains unchanged so there will be little if any pressure change with load change. Terry

[JD in SoCal]

If the volume of the tire cavity decreases the pressure will go up but at normal inflations and loads the volume does not change only the shape changes . the belts in the tire ensure that the internal volume remains unchanged so there will be little if any pressure change with load change. Terry
Thanks for saying that more clearly than I could. And you only used half as many words.


JD

[MAC702]

It was a lot easier to believe Terry, when he said it, too... :p

Actually, you guys are convincing me, all of you. But do you think there is a sharp pressure spike just before bursting, or will it matter if it bursted while sitting there, versus bursting from a load just being dropped on it?

[calweld]

Too low a pressure is actually more detrimental than too high. A low tire will build up twice the heat as an over pressured one.

dda is correct -- low pressures cause heat, since there is more friction with the road and more flexing of the tire itself, which in turn breaks down the cord and causes separation of the belts and the tread, which is probably the cause of the vast majority of blowouts. Overpressurizing a tire (within reason) never causes it to blow out, in fact back in the '80's I was working for a trucking outfit, anytime we knew a truck would be overloaded (of course, I never :o ), we always filled them 10 - 20 psi over the rated pressure.

[Terry Lingle]

It was a lot easier to believe Terry, when he said it, too... :p

Actually, you guys are convincing me, all of you. But do you think there is a sharp pressure spike just before bursting, of will it matter if it bursted while sitting there, versus bursting from a load just being dropped on it?

A corrrectly inflated tire will not show a significant pressure spike from any shock load unless the load is large enough to significantly reduce the internal volume. The rim would be destroyed before that could happen .
Tires can be destroyed while driving over loaded. Usually by the heat generated or stress flexing breaking steel belts. The tire may not burst immediatly but it is destroyed to the point where it is ready to fail . If it is sitting with the failing part of the tire at maximum flex location just droping the load into the truck is "the straw that broke the .........."
In reality that truck owner should not move too quickly or all those horseshoes up his butt will fall out and tear him a new orfice.

As a further note you should hear a failing tire before it gets that bad . If you are paying attention and hear thump thump ..... STOP put a glove on and carefully check your tires for bulges or belt failures . I saw a guy do this without the glove once - he found about 200 razor sharp belt ends sticking out of his failed sidewall . Stitches were required.
I changed his tire while his wife took him to emergency.
Terry

[Thomas Harris]

Squash a balloon. The sides protruding are now bearing almost ALL the pressure, but the sides have bulged enough to maintain close to the same volume/temp, so the pressure is still the same. The same pressure on the strained, bulging sides may be enough to burst the balloon. The same thing would happen to a tire. Steel belts can flex some, although not as much a rubber. Consider also, that the tire is much more rigid and resists "squashing" much more than a balloon, it needs this to allow loads.

[MAC702]

I've never stopped thinking about this and have recently had two thoughts that finally flipped the switch in my head and made it all make complete sense, even though I was convinced y'all were right.

First I thought about my spare tire. At zero load, sitting under my bed, it's got 80 psi. Yet I know that if I put it on the fully loaded truck, it would still read 80 psi.

So I did a lot of thinking and finally I realized where my error was. I was picturing the tire under compression from the load. But in reality, the tire is part of the suspension system. The weight is on the wheel, which is SUSPENDED inside the tire. At the same time that the wheel is compressing the bottom of the tire (which intuitively would increase pressure) it is also pulling down, or wanting to increase the volume of the top of the tire.

And even though these movements are very small, I now understand how they cancel each other out.

And it makes perfect sense now why the tire keeps the same pressure, and it's the squashing of the side wall that causes the failures.

Excellent question and a great discussion. I'm sorry it took me so long to "get it."

But now it makes me question the "squashed balloon" analogy.

[Terry Lingle]

The difference between a baloon and a tire is that the balloon lacks the restaining belts . Without the belts the both the volume and the pressure change with temperature or adding air
When you blow a balloon up the volume increases but so does the pressure meaning that the firs cubic inch of air causes a larger change in volume than any later cubic inch because the pressure is rising.
As long as you do not add air or change the temperature the volume of a balloon will be almost independant of shape. Hope this helps Mac.

regards Terry

[tigger1]

The pressure in the tire won't noticeably increase with load, because the tire will deform and expand enough to ease the increase in pressure. If you could confine the entire tire, so that it could not expand, and add load to it, you would see an increase in the air pressure inside.

Imagine a water balloon that you push (gently) against a table top with your flat hand. It will deform enough (squish out) so that the pressure inside will not change significantly. Now put it into a tube with a tight-fitting plunger in it and a cap on the other end. If you push on the plunger, you would get a corresponding increase in the pressure inside the balloon. Tires are not confined and will expand enough that the air pressure won't change noticeably.

In practice, air temperature will change the psi inside a tire, and elevation will also (the psi will go up as you go up the mountain), but the load on the vehicle won't change the psi reading enough to measure.

JD

Will it be the pressure that goes up in the mountains or just the volumn? Fill up your thermos bottle with coffee at sea level and go up to about 10K feet.......if you open it quickly, you'll get a big suprise........the volumn of coffee will of increased because the air pressure at 10K feet is a lot less than sea level where it was filled.

Increase altitude slowly and open the thermos slowly.........no problems!!!!

[Terry Lingle]

what happens when you go to a higher altitude is that the external pressure decreases tyhe internal pressure remains constant. Typical pressure gauges measure the pressure relative to the ambient pressure so your tire starts out at about 14.7 psi absolute at sea level . you need an additional 35 psi for proper inflation so you put in 35 reerencet to ambient . this gauge works correctly regardles of altitude . Terry

[MAC702]

Tire guages measure "gauge pressure" so it will show an increase at higher elevations, if it's enough to cause a significant drop in atmospheric pressure.

Coffee as a liquid will undergo no appreciable volume change from pressure. You were experiencing something else.

[TEK]

Coffee as a liquid will undergo no appreciable volume change from pressure. You were experiencing something else.

Air DOES compress and when released suddenly may carry the coffee with it, somewhat like a carbonated drink would act when opened.

[MAC702]

Air DOES compress and when released suddenly may carry the coffee with it, somewhat like a carbonated drink would act when opened.
I agree. That is one of the possibilities, and probably the likely one. But the coffee's volume itself would not increase measureably.

[TEK]

. But the coffee's volume itself would not increase measureably.

Agreed. Liquids do not compress. I do not know if they expand in a low pressure environment(sp?)

[coalsmoke]

Agreed. Liquids do not compress. I do not know if they expand in a low pressure environment(sp?)
Seeing how were getting to the small details, liquids can compress, just most not very much. Water can even be compressed, but it will take more than an elephant standing on a glass of water to do so;)

[MAC702]

Agreed. Liquids do not compress. I do not know if they expand in a low pressure environment(sp?)
Like 'Smoke says, they technically WILL compress, which is why I was careful to use the word "measureably" in our context.

However, another point to consider is that if they expanded in a vacuum, then they must compress under pressure. Vacuum is simply absence (or lowering) of pressure, especially if you are starting at atmospheric pressure as your reference point.

[ace4059]

Man, That question got everyone fired up and lots of responses.
Heres another question. If I have 32 psi at sealevel and I drive to the mountians and am at 17,000 feet, did I loose any pressure since the air is thinner in the mountians. Or if I am in the mountians at 17,000 feet with 32 psi in my tires and I drive to sea level did I gain presure in my tires due to the fact that the air is greater or more dense and is pushing more on my tires.

IMHO I think that atmospheric presure and weight do make a difference in tire pressure. But it may only may make it increase or decrease by .001 psi. Whith is not enough to tell or read with a tire presure guage. This is kinda like the water in a swimming pool. Say you mark the level of the water on the side of the pool. Then you jump in. Did the water level rise. Yes it did because your body displaced some of the water. Now you look at the mark on the pool. But the water is at the same level on the mark. This is because even though your body displaced the water and caused it to rise, the rise of the water is not enough to be noticed.

[Terry Lingle]

since for all practical purposes the amount of air and the enclosing volume do not change the internal pressure will not change. The gauge pressure will however reflect the change in external pressure which you might mistake for a change in internal pressure.
The tire will be incorrectly inflated after the 17,000 ft change in altitude because the desired inflation pressure is referenced to the ambient pressure. I will ignore temperature changes for this discussion.
Terry

[JD in SoCal]

since for all practical purposes the amount of air and the enclosing volume do not change the internal pressure will not change. The gauge pressure will however reflect the change in external pressure which you might mistake for a change in internal pressure.
The tire will be incorrectly inflated after the 17,000 ft change in altitude because the desired inflation pressure is referenced to the ambient pressure. I will ignore temperature changes for this discussion.
Terry
In other words, the gauge will read the difference between the pressure inside the tire and the ambient pressure (outside the tire). If your tire pressure is set correctly at sea level and you drive to higher elevation, the tire pressure will read higher, which accurately indicates that the pressure differential (inside to outside the tire) is greater. The tire pressure should ideally be adjusted to the correct pressure at any given elevation.

Regarding tire temperatures and pressure, the temperature will also significantly affect the tire pressure. As long as we are debating this, one should be aware that temp is a factor that will swing the pressure by at least several psi. When precise tire pressure is really important (race cars), tire temp is a factor that is closely monitored. In street use, it may not matter a lot most of the time. However, if you are driving on a trip for example, the temp could swing 50 degrees or more in a day. Don't think that the pressure that was correct in the morning in a cold hotel parking lot will still be correct when you drive through the desert that afternoon.

JD

[Terry Lingle]

For most tires the manufacturer specifies the inflation pressure cold. The tire design has alarge enough optimum inflation pressure zone that changes in tire temperature and elevation do not pose significant problems.

Raceing tires are a completely different subject . Altitude changes are usually pretty minimal but temperature changes can be critical. Terry

[JD in SoCal]

The tire design has alarge enough optimum inflation pressure zone that changes in tire temperature and elevation do not pose significant problems.
Come on now, Terry. You're letting reality get in the way of a good conversation. :D Most humans ignore their tire pressure and couldn't care less what affects that pressure or how it is affected. A higher level of curiousity about how things work is the reason for most of the conversations on this board.

That being said, you are 100% correct, of course.



Raceing tires are a completely different subject . Altitude changes are usually pretty minimal but temperature changes can be critical. Terry
I ran an SCCA road race car for a while in my past, and a big part of the fun is figuring out all the tiny factors that add up to making you go fast (or not). Many factors that we can all ignore on road cars and other machines are important to understand and address in racing. It makes a guy use his brain effectively if he wants to be successful, and also can generate some knowledge that hopefully a guy can apply to other subjects in life.

JD

[whateg0]

Raceing tires are a completely different subject . Altitude changes are usually pretty minimal but temperature changes can be critical. Terry

Which is why those better funded teams (not ours) use nitrogen, which is much less susceptible to changes in temperature. The rest of us rely solely on bleeder valves, which stick and let all the air out of your tire and then you... Sorry, I digress. BTW, I believe it is the moisture in the air which causes the volume to change so drastically.

Dave

[MJL]

As a guy who works in a tire factory, this has been an entertaining thread.

[walker]

And also a very old thread, as they stopped using stone tires some time ago.:D

[MJL]

Tire guages measure "gauge pressure" so it will show an increase at higher elevations, if it's enough to cause a significant drop in atmospheric pressure.

Coffee as a liquid will undergo no appreciable volume change from pressure. You were experiencing something else.

He probably shook the bottle as he opened it.

[cboerema]

first things first. what color is the car? just havin fun, but on a finer note.The company i was working for had the owners kid as shop boy. Now one day the boss calls me and says would you stop in the shop and help the kid put one of the steer tires from the crane in the pickup so the kid could take it to get repaired, sure no prob. when i get to shop i find the kid letting the air out of the slow leaker.i ask him why is he lettin the air out cause theyll just have to put it back in to find the leak. {high school graduate] his response was it had 85 pounds of air in it and wanted to lighten it up. ??????????? I was astounded as i never gradiated school!