The pneumatic tire is a rotating air spring continuously compressed in the region of road contact during each revolution. The pressurized air inside the tire’s toroidal chamber is principally composed of a mechanical mixture of two diatomic gases, nitrogen (78%) and oxygen (21%), plus trace amounts of a whole host of other gases – some quite exotic. Often forgotten is the fact that an inflated tire is a pressure vessel with the compressed air governed reasonably well by the gas laws of physics. Accordingly, air under pressure possesses a large amount of stored energy which, if released instantaneously, can rupture the tire with explosive force. This burst of energy is potentially fatal to nearby humans. Not surprisingly, the energy flow rate, not energy per se, is the controlling factor with respect to possible harm (see Walter, July 2009, p12).
Pressurized tire air can also be thought of as a ‘working fluid’, a notion usually reserved for pneumatic hammers and impact wrenches. For example, Maurice Olley (see Tire Legends, April 2012, p52) studied the effect of inflation pressure on vehicle control and stability during the early 1930s. His analyses and subsequent on-road experiments showed that lowering inflation on the front tires of a passenger car tended to promote oversteer, while doing the same for the rear tires promoted understeer. His work with tire cornering stiffnesses, slip angles and wheel loads eventually evolved into what is known today as the cornering compliance concept of vehicle handling.
When the source of inflating air is not completely dry, water vapor is sometimes present in measurable amounts. During normal gas permeation, such ‘wet air’ can wick moisture along the steel cord of the belt plies, causing corrosion and break-down of the cord-rubber bond. The open steel cord construction developed during the 1980s (replacing the compact construction) tends to mitigate such wicking; the openness enables increased rubber penetration while encapsulating more of the reinforcing material with surrounding skim stock. Obviously end-of-life tires of any type with rusted cords are not suitable for retreading.
The air contained within the tire wheel assembly also has weight or, equivalently, mass. The higher the pressure, the denser and heavier the air. This added mass is slight, 150-200g for most automobile tires at operating pressures and temperatures. If filled with sulfur hexafluoride (SF6), the largest gas molecule suitable for inflation that is neither toxic or carcinogenic, the added mass would be 2.4 times as great as with atmospheric air – and with a significant cost increase. On the other hand, consider hydroflated rear farm tractor tires filled, as recommended for improved traction, with a water-based, calcium chloride solution; this adds 500-1,500 lb to each tire depending on size – at the expense of soil compaction and rutting.
Air pressure has a noticeable influence on many important tire and passenger car performance characteristics. For example, to ensure the safest possible driving conditions, the higher the tire pressure, the greater the speed at which hydroplaning is likely to occur. However, perhaps of the greatest importance today, is the impact of pressure on a tire’s rolling resistance, a loss mechanism that decreases inversely and asymptotically with increased inflation – to a point. Air is extremely efficient at storing energy – about 90% in an automobile tire compared with 10% in its cord-rubber structure. Even through storing most of the energy, air contributes little toward the rolling resistance of an operating tire compared with its confining hysteretic components.
Contrarily, the non-pneumatic components of an airless tire store and release energy with each revolution of the wheel. The compliant but solid structure is supported without the load bearing benefit of compressed air. Consequently, non-pneumatics inherently have greater energy losses than their inflated counterparts. By efficiently utilizing air, the pneumatic tire was groundbreaking in the road transportation sector at the beginning of the 20th century. While airless tires suffer from many drawbacks, they could be advantageous in niche applications in which the risk of a puncture is high and having a flat tire would adversely impact productivity. But unless there is a significant breakthrough in airless tire materials or design, it will be difficult to match the rolling resistance and fuel efficiency levels of a pneumatic.
