A tyre holds a car weighing a tonne and a half on four patches the size of a postcard. Everything it's made of serves to keep those four postcards stuck to the road.
What's inside
A tyre isn't a block of rubber. It's an assembly of layers, each with a job.
- The tread: the rubber that touches the ground, with grooves that clear water away.
- The belt: two layers of crossed steel cords under the tread. They stiffen it and keep it flat.
- The carcass: textile cords (polyester, rayon) embedded in rubber, running from one edge to the other. It's the tyre's frame.
- The sidewalls: the sides, flexible, soaking up shocks. They're thin: this is the fragile part.
- The beads: two rings of braided steel wire that press the tyre onto the rim and stop it jumping off.
- The inner liner: a layer of butyl rubber, almost airtight. It's what replaces the inner tube in a "tubeless" tyre.
Radial or bias-ply
In a radial tyre, the carcass cords run straight from one bead to the other, like spokes, and the steel belt holds the tread. The sidewall flexes, the tread stays flat: better grip, less heat, longer life. Michelin patented it in 1946 and sold it from the end of the 1940s; today it's on almost every car.
In a bias-ply tyre, older, the cords cross diagonally. It's still used on some farm machinery, trailers and classic motorbikes.
The rubber
- Natural rubber, tapped from the rubber tree, and synthetic rubbers made from oil.
- Carbon black: this is what makes tyres black. It strengthens them and protects them from ultraviolet.
- Silica, in recent tyres, which improves wet grip and lowers rolling resistance.
- Sulphur: heated with the rubber, it links its molecules together. That's vulcanisation, discovered by Charles Goodyear in 1839. Without it, rubber gets sticky in the heat and brittle in the cold.
Making one
The layers are built up flat on a drum, forming a soft "green" tyre. It then goes into a heated press, around 150 to 180 °C: a mould stamps its tread pattern and markings while the rubber vulcanises, in about ten minutes for a car tyre.
Reading a sidewall
Example: 205/55 R16 91V
| Marking | Meaning |
|---|---|
| 205 | Width, in millimetres |
| 55 | Sidewall height, as a percentage of the width |
| R | Radial |
| 16 | Rim diameter, in inches |
| 91 | Load index: 615 kg (1,356 lb) per tyre |
| V | Speed rating: 240 km/h (149 mph) maximum |
The date of manufacture: a four-digit code in an oval, after the letters DOT. 2319 means the 23rd week of 2019.
Winter tyres carry the symbol of a three-peaked mountain with a snowflake (3PMSF). The M+S marking alone means "mud and snow", without guaranteeing real performance on snow.
What wears it and what kills it
- Under-inflation: the sidewall flexes too much on every turn, heats up and eventually bursts. It's the leading cause of tyre destruction. The recommended pressure is on a label in the door frame or the fuel flap, and is measured cold.
- Age: even with little use, rubber hardens and cracks. Cracks on the sidewalls or between the tread blocks: it's time to replace it, whatever its depth. Generally count 6 to 10 years at most.
- Wear: European law requires at least 1.6 mm of tread. Wear indicators, small bars at the bottom of the grooves, come level at that depth.
- Driving on a flat, even for a few hundred metres, destroys the inside of the sidewall. See Fixing a tyre.
Tips from people who fit them every day
- A one-euro coin slipped into a groove: if its gold rim shows, there's more than about 3 mm left. Less, and the tyre is tired on wet roads. (In the US, the same test uses a penny and Lincoln's head.)
- A tyre that loses a little air every week often has a nail in the tread, or a corroded rim leaking at the bead. Soapy water bubbles where it leaks.
- Aircraft tyres are inflated with nitrogen: no moisture, no oxygen, no fire if they overheat.
- An old tyre is a resource: sandal soles (Vietnamese soldiers walked on them), earth-filled retaining walls, protection, bumpers. Burned, it gives off toxic black smoke and oils that pollute the soil: never use it as fuel.