# Car Tire Deformation & Pressure

How a car tire's layers and trapped air carry the load, and how changing pressure reshapes the contact patch, heat and ride\.

By Slash

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/SHGhDoGYBZIG)

## Transcript

### 1. A tire is a bag of air

This tire carries a quarter of a car\. The black rubber is not what holds the car up\. The air squeezed inside does that job\.

The squash is drawn bigger than life so you can see it\.

### 2. Layers inside

Cut open, a tire is built in layers\. Grippy tread on the outside, then steel belts, then fabric cords that run from rim edge to rim edge\. An airtight liner keeps the air in, and steel wire hoops grip the rim\.

Watch each layer light up in turn\.

### 3. The air carries the load

Air pushes outward on every bit of the inside, shown by the blue arrows\. The red arrow is the car's weight pressing down\. The cords stretch tight like a drum, and the air takes the weight\.

Normal pressure is about 32 to 35 psi: over twice the push of the air around us\.

### 4. The flat spot

Where the tire meets the road it flattens into a contact patch\. Less air means a bigger patch is needed to hold the same weight, so the tire squashes more\. Slide the pressure and watch the side bulge and the inked footprint\.

At normal pressure, each patch is only about the size of your hand\.

### 5. Let air out, pump it in

Tap the valve stem on the wheel to let some air hiss out\. Tap the pump handle to push air back in\. The gauge needle shows the pressure\.

Green on the gauge is the right range for this car\.

### 6. Rolling makes heat

As the wheel rolls, every bit of rubber squashes flat and springs back, many times a second\. That bending warms the rubber\. A soft tire bends much more, so it runs hotter and wastes fuel\.

Each psi too low costs about 0\.2 percent more fuel\.

### 7. Too hard is not better

Now the road has a bump\. A hard, overfilled tire is round and stiff, so its patch shrinks and every bump jolts the car\. A soft one squirms and overheats\. The right pressure is in between\.

Never pump to the maximum printed on the tire wall\.

### 8. Check it cold, every month

The right number is on a sticker inside the driver's door\. Check it before driving, while the tires are cold, because warm air reads higher\. Tires slowly lose air, so check once a month\.

A tire 25 percent below its number counts as badly underinflated\.