Tire Pressure

On the 7/23/2010 show, someone called in and said that her boyfriend (husband) makes her get out of the car when he checks the tire pressure.

When I get new tires, the tire guy sets the pressure of the tires OFF the car and it doesn’t change when the tires are mounted on the car.

Tom and Ray said that having the passenger leave the car would make an extremely small (probably unmeasurable) change in the tire pressure. I contend that it makes ABSOLUTELY NO difference in the tire pressure since one quarter of the weight of the car makes absolutely no difference in the tire pressure. WHY WOULD IT? The tire pressure is the amount of pressure on the INSIDE of the tire.

A loaded tire will have a slightly larger air pressure than a tire off the car. The reason is simple: The tire when loaded with weight will deform slightly, and reduce the interior volume, raising the pressure.

What does this mean to your checking the tire pressure? Absolutely nothing. The difference in pressure between a normally-loaded tire and an unloaded tire is too small to see with most consumer grade tire pressure gauges and can only be seen with laboratory-style super-accurate gauges. Telling the woman you love to get out of the car to check the tire pressure? A good way to become a lonely, single man.

I have to defend Tom and Ray, because any and all pressure exerted on any system, hydraulic, pneumatic, electrical, will result in some loss of energy. I think their point was that, although the loss is negligible, it exists, and that is true.
You are assuming that a tire is a completely closed system, which of course it is not. There in fact are no closed systems in physics, including black holes. I don’t think your tire qualifies.
But I think that Tom and Ray’s point may apply to under-inflation. If you leave a three-hundred passenger in the rear of a car and try to inflate the rear tires to their proper pressure, you will under-inflate the tires, because the person in the back seat adds to tire pressure.

First a physics lesson:

Pressure vessels - the engineering term for anything that gets pressurized, especially with a gas - want to deform in the shape of a circle/cylinder/sphere, the most efficient shape for containing pressure. That’s because these shapes have the most internal area/volume compared to the perimeter/surface area.

Ever notice how airplanes are wings with a cylinder for the body. How about submarines? The same principle at work.

That applies to tires in a number of ways. First is that the inflated but unloaded cross section shape is controlled by the steel belts, but the pressure is trying to turn that cross section into a circle. When you load the tire, you flatten one part of that - so the interior volume of the tire (the volume being contained by the tire and rim) goes down. - AND - The pressure goes up. It’s not much, but it is measureable if you have a sensitive enough pressure gauge - one measuring to the nearest 0.1 psi will do.

Don’t believe it? There are several pressure gauges the are affordable (under $100) that will do the job. Perform the unloaded vs unloaded test yourself!

It makes a very tiny difference unmeasurable without very sensitive equipment, like a manometer.

Air does compress, and as it’s compressed its pressure increases. Additional load on a tire, which is restrained by the belts from expanding, will increase pressure. But unless she weighs 5,000 pounds you’ll never be able to measure the increase.

To illustrate the point, let’s envision putting a loaded 80,000 pound tractor trailer on the wheels and tires from a Civic.
What would happen? The tires would explode.
Why? Because the pressure inside would increase beyond the ability of the carcass to hold it.

Enough already ! ENOUGH ! aaaarrrrgggghhhhhhh !

A related post here, http://grassrootsmotorsports.com/forum/grm/tire-pressure-hoist-vs-on-the-ground/3231/page1/ near the bottom of that page describes inflating an aircraft tire unmounted, then mounting and loading the plane with 25000 lbs. No change in tire pressure measured, to within 1 psi.
My thought here is that tires don’t stretch. The material, starting to stretch, will quickly become a blowout. Consider if the tire were entirely stiff, say made of steel.
Clearly the pressure does not change with load.
This would mean a blowout is not due to pressure increase, rather to deformation causing the tire material to begin to stretch.
Where is the weight bearing? The side wall is carrying the weight, not the interior air! If the tire material were steel, it would carry the load until the metal bent.

This post is along the same line as people that think having the lights on affects their gas mileage, well the power has to come from somewhere.

I read that web page - interesting discussion.

However when I read CapriRacer’s reply (and knowing he spent his career designing tires), and think about how his reply and some of the others align with what I learned in my physics courses long ago, I have to believe pressure does indeed change with load. The change is small but it does change.

These type of questions reflect the dumbing down of thinking… using a common sense approach:
weight of car divided by 4= ? weight of person divided by four= ? persons weight/tire by comparison is trivial.

I don’t know why they didn’t get a difference, but they should have.

When I did it on a car, I got between 0.1 psi and 0.5 psi.

Richalt, the tire stretches, if it stretches at all, because of increased pressure. It blows for the same reason.

A tire is a suspension device. The wheels of the car are actually holding the car up, suspended in the tire carcass, the fibers that it’s suspended from kept under tension by the air pressure in the carcass. As load weight increases, the tire deforms, but since its volume is kept constant by cords in the carcass, the pressure increases instead of the tire expanding (like a balloon would). Eventually, as illustrated in the 80,000 pound load example in my earlier post, if you continue to load a tire, the pressure will exceed the tensile strength of the tire and the tire will blow.

You are correct that the sidewall of the tire is carrying the weight, but it’s doing so by the tensile strength of the fibers above the wheel rather than by the compressive strength of the fibers below the wheel. As obvious in a flat tire, the sidewalls have little ability to hold up the wheel without the air pressure keeping them under tension.

If anything it probably reflects the dumbing down of the primary and secondary educational systems. Rather than teaching math, physics, history, economics, our system of government, and world affairs it’s teaching politically-corrected quasi-history, ebonics, and self-esteem…the last of which cannot be taught, but is rather a consequence of accomplishing against challenges that the schools will no longer subject the students to because they’re afraid it’ll damage the students’ self esteem. If you think that last comment is a circular statement that makes no sense, you are correct.

Let’s say the passenger weighs 150# and the car weighs 3700#. The added weight per tire is 1%. That might be measurable, but it is inconsequential.


I am going to take issue with you on the topic of education today.
While I also believe that standards have been watered-down over the years–almost entirely at the request of parents who don’t want their children to be “stressed”–math, physics, history, economics, government, and world affairs are still being taught.

While some inner-city schools undoubtedly tolerate the use of ebonics, Ebonics is not taught in any school system with which I am familiar. Could you give us a valid reference indicating the names of school systems which currently teach ebonics? Trust me–there aren’t likely to be any. There is a major difference between the teaching of something and reluctantly tolerating the existence of something that exists outside the control of the school.

Self-esteem seems to be a high priority in schools nowadays, but once again this is almost entirely at the behest of parents. Since their children played sports in elementary-school leagues that gave trophies to everyone, these parents want to perpetuate the fallacy that everyone is wonderful at everything that they do. Of course this concept is bizarre, but educators did not create this mess.

Dedicated teachers (and I put myself into that category) have long decried the lowering of standards, but unfortunately our voices have been overpowered by parents who want high grades for their children first, and actual education second. Over twenty years ago, a parent said to me, “I don’t care if he learns anything, I just want him to get an A”.

While that type of attitude was somewhat rare twenty or more years ago, I saw this type of attitude more and more often during the late '90s and the '00s, and this is one of the factors that led to my retirement. The lowering of educational standards is a crying shame, but until parents begin to support higher educational standards, I don’t see this situation changing.

As to quasi-history, the only place that I am aware of where this is true is the state of Texas, where they are revising history to fit some kind of ultraconservative agenda. In that state, the state Board of Education recently voted to remove references to Thomas Jefferson from history books because his ideals and principles were…“un-American”.

That is correct. The Texas State Board of Education considers the author of The Virginia Statute of Religious Freedom and The Declaration of Independence to be unworthy of mention in history books. It is rumored that their next candidate for elimination is Teddy Roosevelt, due to his “un-American” policies of promoting conservation and “Trust Busting”.

Among Jefferson’s allegedly “un-American” sentiments was the concept of Separation of Church and State. US Conservatives’ belief that government and religion should be intertwined is a sentiment that they share with…The Taliban and the governments of Saudi Arabia, Pakistan, and Iran, among others. Once you open the door to undue influence from religious groups, you have to allow ALL religions to have this influence. You know the old saying–Be careful what you wish for.

There are surely problems in American education, but the things that you cited are largely untrue.

My kids got a good education in grade school. They even got college credit for courses they took in HS after taking advanced placement tests. That didn’t exist when I, and mountainbike, were in school. Things have changed in my neck of the woods, and I think for the better.

Good answer; the lady in question likely weighs about 135 lbs or so and if the car weighs 3700 lbs, that’s 3.6% divided by 4 is indeed 0.912% That times 30psi would be 0.276 psi provided THE TIRE DID NOT STRETCH! Very hard to measure with an off the shelf tire gauge, and irrelevant to the safety of the car.

jtsanders raises a good point.
While educational standards–overall–having been lowered, most schools now have the option for the brightest students to take Advanced Placement and/or International Baccalaureate courses while in high school. Because of this opportunity, many of the brightest students actually start college nowadays with enough college credits to equal a semester or more of course work.

Many school systems have–in effect–two sets of standards nowadays.
For the parents who want a really rigorous education, and whose kids are very bright, we have very rigorous AP/IB courses with very high standards. And then, for those kids who are not high achievers and/or whose parents don’t particularly care about how much their kids learn, we have courses with watered-down standards.

However, Ebonics is not part of the curriculum, even in these watered-down courses.

VDC, you’ve written a thoughtful and articulate response with many truths in it.

As to ebonics, you are correct in that I misworded the thought, however the teachers in Southern California were REQUIRED to accept ebonics and to attend training to actually learn to better understand ebonics. Students graduating from that school system were having difficulty moving forward into positions in other areas of the country because they were never required to learn proper English. I recommend that you Google “Ebonics in the California Schools”. You’ll get tons of information, including the famous “Oakland Resolution”.

Mathematics and physics in particular have been watered down to the point that students can graduate from most public school systems with an extremely tenuous grasp of even basic algebra and with no physics whatsoever.

I still contend that much of the history being taught is, based on the books my kids got when they were in school, biased.

I’ll agree that the opportunity to get a good education exists for those students that are self motivated, but my frustration is that it isn’t required in order to graduate.

While there exist parents that don’t care about the quality of eduaction their sons and daughters get, I don’t blame the parents. I blame politics, including federal mandates and the local politics of the school board. Every parent and every teacher I know feels as you do, that the lowering of educational standards is appalling. And every teacher I know is hamstrung by bad school board policies designed to protect the schools against lawsuits and/or loss of federal funds and by budget cuts that force oversized classes and force too large a portion of the education to be done by temporary personnel. Colleges now, especially community colleges, are utilizing way, way too many adjuncts. While adjuncts care and do the best they can, they don’t have the long term investment in the programs. And a heavy reliance on adjunct often leaves program development efforts lacking. And if they get an opportunity for something better, they leave, with an adverse impact on the continuity of the learning through the program.

And I’m very comfortable in stating that way too many secondary school graduates have no grasp of economics or any idea of how our system of government is structured.

In summary, while primary and secondary systems provide more opportunities for the self motivated students than existed when I was young, they also allow way too many students to graduate with extremely poor skills in English (including reading comprehension and writing), economics, our government, and especially in math and physics.