The Tire Doctor Responds:
It’s not 95 pounds, but 95 pounds per square inch of air pressure that’s doing the work.
The modern pneumatic tire looks pretty simple, but it does some really amazing things,
as we’re about to see.

Why is that?
Because once you see that the tire is just the container, lots of other things become clear.

For example?
Try this experiment: Check the air pressure on a tire, then raise it with a jack until it no longer touches the ground. Check pressure again.
     You’ll find that the pressure doesn’t change when you remove the load.  If you had 95 psi when it was on the ground, it will still have 95 psi when jacked up.

How can that be?
Because the tire is somewhat flexible, like a balloon.  When you press it against the ground, the flexibility of the tire keeps the air pressure constant. The shape
of the container changes, but not the pressure.

Then why do we have to increase pressure
when we increase load?
To control the amount of change in the shape of the tire. Imagine that you have two balloons ­ identical in size and shape ­ and you’ve inflated them to identical pressures.
     Put them both on a glass-topped table. Place a book on top of one balloon, and two books on the other. If you crawl under the table and look up, you’ll see that the area of the balloons flattened against the glass is different.
     It should come as no surprise that the balloon with two books has a bigger “footprint.”

How much bigger?
If you measure the flattened areas, you’ll find that the one supporting two books has about twice the footprint area of the one supporting one book.
     The other big difference is that the balloon supporting two books is a lot more flattened. And that’s one of the reasons you have to increase tire pressure when you increase load.

To reduce the “squash”?
Absolutely. Remember, as a tire goes round and round, it is constantly cycling between the squashed, or “loaded” shape, and the un-squashed, or “unloaded” shape.
     The bigger the difference between the loaded and unloaded shapes, the more flexing takes place, and the more heat is generated. And heat is one of the biggest enemies of tires. In fact, more tires probably are damaged by heat than by road hazards.

So why not just put more air in every tire?
For one thing, tires play a part in cushioning the load against road shocks.
     Second, and even more important, the tire has to be built to take the increased pressure. Every tire has a maximum usable pressure ­ and a particular load it can handle at that pressure. 
     If you need to increase load beyond that maximum, you must change to a tire suitable for the higher pressure.
     Third, the amount of inflation in a tire also affects the footprint size and shape. And that is critical to traction and wear.

In what way?
Let’s go back to our balloons. This time, put one book on each balloon, but put half the pressure in one balloon.  We know from experience that the balloon with half the pressure will be more flattened, and the footprint will be a lot larger.

How much larger?
Just as when we used two books, the footprint will have about twice the area. We find there’s a relationship between the inflation pressure, load and footprint area.

What is the relationship?
As an approximation, the number of square inches of footprint area is about equal to what you get when you divide the load by the inflation pressure. (It’s not perfect, but it’s close enough for our purposes here.)

Wouldn’t you want a big footprint for traction?
Not necessarily. A spiked heel digs in a lot deeper than a tennis shoe ­ on a soft surface. That’s because the load is distributed over such a small area. But if you’re on a hard, smooth floor, you’re less likely to slip with a tennis shoe.
     If there’s just a thin layer of wet or slick material, a higher inflation pressure and smaller footprint may help the tread bite through and grab a solid surface below.
     On the other hand, on a thick, but soft surface, like deep mud, sand or mushy soil, a lower inflation pressure can distribute pressure over a larger area, so that the tire almost “floats” ­ like a snowshoe.
     And if the surface is hard and dry, a low inflation pressure and large footprint produce lots of contact and traction, which is why Indy cars run on “slicks.”
     Since inflation pressure controls the footprint size, we can see that the right pressure is not only important for controlling tire heat, but is vital for proper traction.

Somehow, it always comes back to inflation pressure, doesn’t it?
Absolutely. Many things affect traction, load capacity, tire wear and casing durability. And proper inflation pressure produces a tremendous benefit in all these areas ­ for just a very small investment in effort.

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