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!

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