Using standardized wheels with precision ball bearings, modern gravity-powered racers start at a ramp on top of a hill, attaining speeds of up to 35 miles per hour. Rally races and qualifying races in cities around the world use advanced timing systems that measure the time difference between the competing cars to the thousandth of a second to determine the winner of a heat. Each heat of a race lasts less than 30 seconds. Most races are double elimination races in which a racer that loses a heat can work their way through the Challenger's Bracket in an attempt to win the overall race. The annual World Championship race in Akron, however, is a single elimination race which uses overhead photography, triggered by a timing system, to determine the winner of each heat. Approximately 500 racers compete in two or three heats to determine a World Champion in each divisions.