It's partway through the race. The leader is slowly losing his advantage, as the other cars gain, when from out of nowhere, the 36th-place car-some six laps down-runs down that dominant car and passes him on the outside like he was standing still. How? Tires. That backmarker has just come out of the pits with fresh rubber. The leader's tires are 60 laps old. New rubber can make a dramatic difference in a vehicle's performance, as evidenced on the race track, where the new tires can turn a zero into a hero.
The same holds true on the street, where the right tires can bring an older car back to life, tighten up a sloppy riding and handling SUV, quell apprehension on a wet, curvy backroad, and silently perform years of faithful service. The wrong tires, on the other hand, can make you think the shocks and bushings have failed, the alignment is out of spec, the steering is demon-possessed, a band of screaming bobcats have taken up residence in the wheelwells, and the water on the road is a foot deep. There's much more to tires than the meets the eye.
Designing circles
Tires are the single most important component on a vehicle, one that seemingly requires an advanced engineering degree to fully understand. And yet, despite advanced computer simulations, designing and developing a tire remains as much art as science and involves as much compromise as an international peace accord. With a tire, improvement in one performance characteristic almost always comes at the expense of another. For example: Reducing rolling resistance (for better fuel mileage) or increasing tread life usually decreases grip, wet or dry. The opposite is true too: A race car wearing race tires can corner at 1.5g without aero aids, but gas mileage is of almost no concern and the rubber is worn out within 150 miles.
Sometimes, the interactions don't work the way tire designers' computers predict. This is due partially to the tire literally being cooked (or "cured") in the manufacturing process: Polymers, sulphur, carbon black, oils, waxes, resin, and structural components evolve and develop new properties. Some of this evolution continues over the tire's life, especially if the tire is overheated.
With the latest tire production methods, which lay individual cables of material rather than sheets of fabric or steel, there are more individual components in a tire than in a car engine. An engine can be torn down to measure and test every component for design and material compliance. Once reassembled, it's as good as new. Disassemble a tire, and even experts have a devil of a time accurately measuring the basic components, much less figuring out which compounds evolved into what. (Imagine cutting apart a cake in an effort to determine whether free-range or coop-raised eggs were used.)