Kart racing (as the word is so spelled by enthusiasts) or karting is a variant of open-wheeler motor sport with simple, small four-wheeled vehicles called karts, go-karts, or gearbox/shifter karts depending on the design. By definition a kart must have no suspension (relying on chassis flex), and no differential (solid back axle). They are usually raced on scaled-down tracks, but are sometimes driven as entertainment or as a hobby by non-professionals. Karting is commonly perceived as the stepping stone to the higher and more expensive ranks of motorsports. Karts were initially created in the United States in the 1950s post-war period by airmen as a way to pass spare time. Art Ingels is generally accepted to be the father of karting. He built the first kart in Southern California in 1956. Karting has rapidly spread to other countries, and it currently has a large following in Europe.