Posted by Video-calibrator on July 09, 2018

Serious question, one I've pondered for a while now: You've bought the car of your dreams - a golden oldie from 1977, a dependable 5 year old family sedan, or next year's top of the line SUV. The vehicle tire placard, on the driver's B-pillar or in the glove box, recommends 32psi cold, front & rear. You've driven it that way for a few days, and want to experiment with a higher or lower pressure. So you inflate all the tires to 33psi cold, drive it that way for one week, then deflate them to 32psi cold, and drive them that way for a few days. Now, those pressures are PSI, as in, pressure per square INCH. Which suggests that we are not simply removing or adding a 'couple' pounds of air to the tires. In fact, we are adding many many MANY pounds of air to those tires. The question then, is: How many pounds? Has the interior surface of the average modern tire, say, a 60-series 16", ever been calculated? And does that total area, in square inches, include or exclude that part of the vehicle rim or wheel which faces/mates with the tire? If we know that figure, I think it would lend a much more accurate idea of approximately how much we really are changing the air pressure inside the tires, and thus rolling resistance, load capacity, handling, and stopping ability. Thoughts anyone?

Answers / Replies
  • Posted by Video-calibrator on July 10, 2018

    Correction: Should be 'down to 31psi' in above post! Wasn't thinking! smh

  • Posted by Jb on August 08, 2018

    Quicker than the math involved, would be to fill it with water, measuring the volume as you went. Moreso, your question begs of the engineer, the effects of pressure on the casing, the compound, belts, etc. having a summary effect on the spring rate of the tire (interacts with body weight) and ultimately on the contact patch (interacts with the road surface). MUCH has been studied and quantified about the effects of just a little change in air pressure (ask race teams!)

  • Posted by Jb on August 08, 2018

    Side note:. Tire pressures haven't changed much over the years; yet, the weight, construction and handling systems of vehicles certainly have!

  • Posted by Sam on March 18, 2019

    What you're really doing is changing the size of the contact patch and the overall stiffness of the wheel/tire assembly. The pressure pushing on the entire sidewall, the rim, etc, it exists in equilibrium. That pressure pushing in all directions is what holds the rim in the center of the tire. There's no point in calculating it, though, as I can't think of any reasonable use for the number you'd come up with. But. The reason your air pressure is important, is what happens on the pavement. Say you're driving a 4000 pound car with perfect weight distribution, so each tire is holding up 1000 pounds. At 40 psi, then, you'll have a contact patch of 1000 lbs / 40 psi = 25 sq in. That's where the weight is exactly balanced by the air pressure. As pressure goes up, contact patch gets smaller. As pressure goes down, you get lots of contact patch. Isn't that good? No, it's a trade off, a flat tire has tons of contact area... you want enough pressure to hold the rim up and keep it steady and stable. As you add pressure your sidewall stiffens, handling improves, the ride gets firmer, and the contact patch gets smaller. If you overinflate your tires, you end up with only the middle of the tire touching the road. This is why there's a recommended pressure, and it's why you go by what the car maker says rather than the max pressure on the sidewall of the tire: the tire makers don't know how much your car weighs, and without that information they can't calculate the correct inflation pressure. They can only tell you the maximum that their tire can handle.

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