A slick tire (also known as a "racing slick") is a type of tire that has no tread pattern, used mostly in auto racing. By eliminating any grooves cut into the tread, such tires provide the largest possible contact patch to the road, and maximize traction for any given tire dimension. Such tires are used on all four wheels for road or oval track racing, where steering and braking require maximum traction from each wheel, but are typically used on only the driven (powered) wheels in drag racing, where the only concern is maximum traction to put power to the ground.
Slick tires are not suitable for use on common road vehicles, which must be able to operate in all weather conditions. They are used in auto racing where competitors can choose different tires based on the weather conditions and can often change tires during a race. Slick tires provide far more traction than treaded tires on dry roads, but typically have less traction than treaded tires under wet conditions. Wet roads severely diminish the traction because of hydroplaning due to water trapped between the tire contact area and the road surface. Treaded tires are designed to remove water from the contact area, thereby maintaining traction even in wet conditions.
Since there is no tread pattern, slick tire tread does not deform too much under load. The reduced deformation allows the tire to be constructed of softer compounds without excessive overheating and blistering. The softer rubber gives greater adhesion to the road surface, but it also has a lower treadwear rating; i.e. it wears out much more quickly than the harder rubber tires used for driving on the streets. It is not uncommon for drivers in some autosports to wear out multiple sets of tires during a single day's driving.
In Formula One slick tires are no longer allowed, yet dry weather tires are still often referred to as 'slicks' as they have no appreciable tread pattern (having only circumferential grooves intended to reduce total traction) and similar behaviour in wet weather. They will be re-introduced into the sport in 2008.
Drag racing slicks
Drag racing slicks are typically very large, to deal with the enormous power delivery. For "closed wheel" cars, often the car must be modified merely to account for the size of the slick, raising the body on the rear springs for the height of narrower slicks, and/or replacing the rear wheel housings with very wide "tubs" and narrowing the rear axle to allow room for the wider varieties of tires. Open wheel dragsters are freed from any such constraint, and can go to enormous tire sizes. Some utilize very low pressures to maximize the tread contact area, producing the typical sidewall appearance which leads to their being termed "wrinklewall" slicks. Inner tubes are typically used, to ensure that the air does not suddenly leak catastrophically as the tire deforms under the stress of launching.
"Wrinklewall" slicks are now specifically designed for the special requirements of drag racing, being constructed in such a way as to allow the sidewall to be twisted by the torque applied at launch, softening the initial start and thus reducing the chances of breaking traction. As speed builds, the centrifugal force generated by the tire's rotation "unwraps" the sidewall, returning the stored energy to the car's acceleration. Additionally, it causes the tires to expand radially, increasing their diameter and effectively creating a taller gear ratio, allowing a higher top speed with the same transmission gearing.
Since completely slick tires are outlawed on most roads due to their inability to handle wet pavement, the "cheater slick" became a popular item in the hot rod world in the 1950s and 1960s; a typical slick type tire, but engraved with the absolute minimal amount of tread grooves required to satisfy legal requirements. Since then, however, tire development has progressed greatly, so that today's hot rod street cars typically use wide treaded tires which perform better than the slicks of the past; while the cheater slicks available today, both for nostalgic appearance of street cars and for competition use in classes where DOT approved street tires are required, have followed their own line of development, diverging from true slick tire construction to become a distinct tire design in themselves.
R compound tires
The development in cheater slick technology has affected the development of tires for racing series other than drag racing as well. When other forms of auto racing similarly instituted classes which require DOT approved street tires, some manufacturers similarly began to market tires which superficially resembled their high performance street tires, but with the least tread permissible and with very soft, sticky rubber, intended specifically for competition because the soft tread would wear too quickly for street use. These became known, loosely, as R compound tires. With additional years of progress, this class of tire has in its turn followed its own line of development, to the point where they have little in common with true street tires of the same brand. Ironically, this has led to new classes of racing which require not only DOT approval, but also a minimum treadwear rating, in an effort to eliminate the R compound tires from competition and require "true" street tires.