Cyclo-cross (sometimes cyclocross, CX, CCX, cyclo-X or cross) is a form of bicycle racing. Races typically take place in the autumn and winter (the international or “World Cup” season is October–February), and consist of many laps of a short (2.5–3.5 km or 1.5–2 mile) course featuring pavement, wooded trails, grass, steep hills and obstacles requiring the rider to quickly dismount, carry the bike while navigating the obstruction and remount. Races for senior categories are generally between 30 minutes and an hour long, with the distance varying depending on the ground conditions. The sport is strongest in the traditional road cycling countries such as Belgium (Flanders in particular), France and the Netherlands. Cyclo-cross has some obvious parallels with mountain bike racing, cross-country cycling and criterium racing. Many of the best cyclo-cross riders cross train in other cycling disciplines; however, cyclo-cross has reached such size and popularity that some racers are specialists, and many prioritise cyclo-cross races over other disciplines. Cyclo-cross bicycles are similar to road racing bicycles: lightweight, with narrow tires and drop handlebars. They are typically differentiated by their greater tyre clearances, lower gearing, stronger frames, cantilever brakes and more upright riding position. They also share characteristics with mountain bicycles in that they utilize knobby tread tires for traction and, increasingly, disc brakes. They have to be lightweight because competitors need to carry their bicycle to overcome barriers or slopes too steep to climb in the saddle. The sight of competitors struggling up a muddy slope with bicycles on their shoulders is the classic image of the sport, although unridable sections are generally a very small fraction of the race distance. Compared with other forms of cycle racing, tactics are fairly straightforward, and the emphasis is on the rider’s aerobic endurance and bike-handling skills. Drafting, where cyclists form a line with the lead cyclist pedalling harder while reducing the wind resistance for other riders, is of much less importance than in road racing where average speeds are much higher than in cyclo-cross. A cyclo-cross rider is allowed to change bicycles and receive mechanical assistance during a race. While the rider is on the course gumming up one bicycle with mud, his or her pit crew can work quickly to clean, repair and oil the spares. Having a mechanic in the “pits” is more common for professional cyclo-cross racers. The average cyclo-cross racer might have a family member or friend holding their spare bike.