Tires for road and other high-speed vehicles are inflated with air because of one basic fact about rubber: Flexing it generates heat, and the more rubber there is, and the more and faster you flex it, the hotter it gets.

Therefore most of the volume of modern tires is air, which can flex without generating heat. It wasn’t always so. Highway trucks did not operate in the US before 1926 because tires capable of carrying them at 40 mph did not exist. City trucks rolled on solid tires that turned to hot jam if run at other than stop-and-go speeds.

Inflation air is contained within a tire structure called the carcass, constructed of rubber-impregnated layers—plies—of high-strength fibers such as polyester, aramid, or even steel. Applied to the outside of this carcass is a band of wear-resistant rubber—the tread—which actually makes rolling contact with the road surface. The inner surface of the carcass is sealed by a thin layer of low-gas-permeability butyl rubber.

The carcass transmits the grip generated by the rubber tread to the wheel rim. The inflation air keeps the fibers of the carcass always in tension so they cannot buckle, and it is through this tension that tire forces are transmitted.

If a motorcycle tire were a perfectly rigid body—like a hard steel ball—it would touch the road at a single point. But because the tire's carcass is flexible and inflated with compressible air, under load the tread flattens in contact with the pavement to form what is called the tire's footprint. This is where the forces that drive the motorcycle and maintain its stability are generated.

As a tire rolls, its round tread flattens as it enters the footprint, bending in the process. It then unbends as a given element of tread leaves the footprint at its trailing edge. The lower the inflation pressure, the bigger the footprint, and the more the rubber must bend and unbend to enter and leave it. It is mainly this bending that generates heat in a rolling tire.

Valentino Rossi
In the previous Michelin era, Valentino Rossi—seen here at Mazda Raceway Laguna Seca in 2006—favored a stiffer rear tire construction than his factory Yamaha teammate, American Colin Edwards.Andrew Wheeler/AutoMotoPhoto

We can get an approximate idea of footprint area by dividing the load carried by the tire by its inflation pressure. Thus, if a tire carries a load of 300 pounds and is inflated to 30 psi, then footprint area would be approximately 10 square inches. Actual footprint area is less than this ideal because the tire carcass is not perfectly flexible, like a child’s balloon, but has some built-in stiffness of its own. As inflation pressure is increased, footprint area decreases and vice versa.

Several basic variables determine tire-operating temperature. They are pavement temperature, inflation pressure, load, and speed. No wonder there have been tire troubles over the years in the Daytona 200 motorcycle roadrace! Speed is very high and the 31-degree banking adds “centrifugal” force to the vehicle’s weight.

The highly publicized destructive failures of tires on certain SUVs came about because of unfortunate combinations of heavy load, high speed, underinflation, and summer temperatures.

Rubber, synthetic tire fibers, and the adhesive systems holding them together begin to lose strength and then structural integrity at higher-than-normal temperatures. The differing conditions of various tire applications require specific ranges of inflation pressures.

In motocross, a large tire footprint is necessary to give maximum grip on loose surfaces, so such tires are engineered to work at low pressures such as 14 psi. To keep tires from slipping on their rims at such pressures rim locks are typically required. Because a moto is roughly 45 minutes long, tires are inspected and their pressures checked frequently.

Tires for highway trucks are expected to carry 5,800 pounds each in continuous operation (18 wheels times 5,800 pounds equals 104,000 pounds). To keep their temperature under control, they are inflated to roughly 100 psi and are engineered to give long service at this pressure.

To provide the comfort and grip auto drivers require, car tires are inflated to much lower pressures, typically in the 28-40-psi range. I had the experience of two nonstop van trips to the West Coast, one on old-style multi-ply bias tires and the other on modern single-ply radials. You guessed it: The multi-ply “super-duty” bias tires overheated and one came apart. The thinner-structured radials, because the reduced amount of rubber in them generated less heat, made the round trip without incident.

Dunlop technicians were surprised to find that 40 percent of the bikes whose tire pressures they measured at a rally were substantially underinflated.

Large commercial aircraft are given the very smallest wheels and tires possible because any extra weight cuts into passenger and cargo revenue. Their main gear tires stand about 4 feet high and are inflated to 300 psi, with each tire carrying 50,000-60,000 pounds. An Airbus A380 with 20 main wheels gives us this arithmetic: 20 x 60,000 pounds = 1,200,000 pounds, the approximate maximum take-off weight for this type.

Although the take-off run ends at high speed—about 180 knots—it only lasts a bit over 30 seconds, after which the gear is retracted and the tires have the rest of the flight during which to cool. Even taxiing at take-off weight generates a lot of heat: Concorde required special taxiing restrictions to keep its tire temperatures at safe levels.

Michelin’s 2006 big rear MotoGP tire was inflated to an amazingly low 0.9-1.1 bar (13-16 psi) to give the large footprint Honda then needed for off-corner acceleration. The structure of that tire had to be engineered to operate at a safe temperature despite such low pressure. An idea of the conflicting requirements a tire must cope with came from two same-day Laguna Seca conversations, one with Colin Edwards Jr. and the other with Valentino Rossi. Edwards’ riding style called for late, hard braking and early turning, followed by hard acceleration. That acceleration required a big footprint, which requires a quite flexible tire casing.

Rossi’s style was different: He braked earlier and less hard, carrying more speed through corners at high lean angle. If he tried to use Edwards’ tire choice, his bike would, in his words, “jump sideways” as its supple carcass buckled under Rossi’s heavy cornering load. Therefore Rossi needed a stiffer tire carcass.

A loaded touring bike can easily scale over 1,000 pounds yet its tires are not much larger than those of bikes weighing half as much. Touring tires are kept within the range of safe operating temperature by use of higher inflation pressures in the 40-plus-psi range. Dunlop technicians were surprised to find that 40 percent of the bikes whose tire pressures they measured at a rally were substantially underinflated. Riders today tend to take tires for granted. Don’t! Keep a good tire pressure gauge, know your recommended pressures, and see to it that they are maintained.

There are limits to the inflation pressure a given tire’s structure can take. It is for this reason that motorcycle tire manufacturers ask that no more than 60 psi be used for bead seating (using pressure to seat the beads of a freshly mounted firmly against the inner faces of the rim flanges). I saw a tragic demonstration of the danger of higher pressures in 1976 at Daytona. Someone trying to seat the beads of a new tire kept adding more and more pressure until the tire’s bead-wires stretched and rose over the rim flange. The resulting explosion blew a chunk out of the man’s wrist.

Gene Romero was able to set pole at Daytona in 1970 while using higher-than-normal tire-inflation pressure to reduce rolling friction. That’s racing, where tire life may be as little as 60 miles and tire technicians are never far away.

A motorcycle’s stability depends upon the damping of the tire footprints. Inflating to higher-than-recommended pressure makes the tire footprints smaller, reducing this damping effect. Habitual readers of service bulletins will know that the first thing a motorbike manufacturer does when there is any reported stability problem is to call out a 2-3-psi reduction in tire-inflation pressure. That, it must be made clear, is an emergency measure only.

The tire manufacturer has engineered each tire to operate at a safe temperature in a stated range of inflation pressures, at recommended maximum load, and in a stated speed range. Why mess with it?