Keirin (競輪 / ケイリン?, [keiɽiɴ]) “racing wheels” is a form of motor-paced cycle racing in which track cyclists sprint for victory following a speed-controlled start behind a motorized or non-motorized pacer. It was developed in Japan around 1948 for gambling purposes and became an official event at the 2000 Olympics in Sydney, Australia.

Races are about 2 kilometres (1.2 mi) long: 8 laps on a 250 m (270 yd) track, 6 laps on a 333 m (364 yd) track, 5 laps on a 400 m (440 yd) track. Lots are drawn to determine starting positions for the sprint riders behind the pacer, which is usually a motorcycle, but can be a derny, electric bicycle or tandem bicycle. Riders must remain behind the pacer for a predetermined number of laps. Initially it makes circuits at about 25 km/h (16 mph), gradually increasing to about 50 km/h (31 mph). The pacer usually leaves the track approximately 600–700 m (660–770 yd) before the end. The winner’s finishing speed is around 70 km/h (43 mph).

Competition keirin races are often conducted over several rounds with one final. Sometimes eliminated cyclists get the opportunity to try again in the repechages.

 

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http://bg.keirin.jp/en/betting/velodromes/index.html