Dirt track racing is a form of motorsport held on clay or dirt surfaced oval race tracks often used for thoroughbred horse racing. Dirt track racing started in the United States before World War I and became widespread during the 1920s and 1930s using both automobiles and motorcycles. Two different types of race cars dominate — open wheel racers in the Northeast and West and stock cars in the Midwest and South. While open wheel race cars are purpose-built racing vehicles, stock cars (also known as fendered cars) can be either purpose-built race cars or street vehicles that have been modified to varying degrees. There are hundreds of local and regional racetracks throughout the nation. The sport is also popular in Australia, New Zealand, Canada, South Africa and the United Kingdom. Racetrack A dirt track's racing surface may be composed of any soil, although most seasoned dirt racers probably consider a moist, properly-prepared clay oval their favorite dirt racing surface. Preparing a dirt track correctly for racing is both a traditional learned craft as well as a long and complex job requiring hours of work. Typical machines used in track preparation include a grader, a cultivator or rototiller, 2 types of rollers and a watering truck, although the process varies at different dirt tracks around the world. After an event, a track's rutted surface is first graded and smoothed back into the contours of the track's layout. then further smooth and tamp loose soil shoveled into any soft spots. The freshly-smoothed track surface is then aerated with a cultivator or rototiller to loosen the now-compacted upper layer and assist with greater water penetration for a more moist track on race day. Watering is repeated as necessary according to climate, location and soil composition. Using offset boom nozzles to on water trucks will avoid mashing aerated soil. A dirt track should be rolled and watered as required on race morning with a final light watering, just prior to wheel-packing. Outdoor dirt tracks are typically aerated 12" deep. North America Nearly all tracks are oval and less than in length with most being ½ mile (804 m) or less. The most common increments in the U.S. are ½ mile, ⅜ mile (603 m), ⅓ mile (536 m), ¼ mile (402 m), and ⅛ mile (201 m). With the longer tracks, the race cars achieve higher speeds up to 160 mph (257kmh) and