The Olympic cycling sport of keirin was invented in Japan more than 70 years ago to raise money to rebuild the country after World War II. Now, fans bet billions of dollars a year on races, with the top riders earning huge sums. In each race, a pacemaker leads nine riders around huge concrete velodromes, then leaves the track with around a lap and a half to go - the cue for a frantic finish as the competitors reach speeds of up to 70 kph. Along the way they block and shove each other, clash heads and occasionally crash (the two Japanese characters used to write keirin mean 'battle' and 'wheel'). To prevent race fixing, the cyclists spend meets living in dorms, with no access to online technology. Their lives are ruled by ritual and fierce competition, from their rookie days at the Japan Keirin School near Mount Fuji to the annual Grand Prix final, whose winner takes home prize money of almost one million dollars. A small number of foreign riders are invited to compete in Japan every year and some, like Shane Perkins, have overcome culture shock to prosper in the home of keirin. The Olympic version - which has made stars of Chris Hoy, Jason Kenny and Victoria Pendleton - is being taken more seriously in Japan as Tokyo prepares to host the 2021 Olympics and Paralympics. Justin McCurry, the Guardian's Japan and Korea correspondent, explores a blue-collar Japan we rarely see and a uniquely fascinating sporting culture.