Cyclo-cross races take place during the winter, so mud can be a problem. To counter its effects, cyclo-cross bikes are fitted with cantilever brakes because callipers collect mud from the wheels and clog up. The bikes are fitted with fat, knobbly tyres for extra grip, and a wide range of gear ratios to cope with the difficult conditions likely to be found on the given circuit.
Cyclo-cross bikes also have to be light because during these relatively short events it is often quicker to run up steep hills carrying your bike on your shoulder than to ride up them. This is why cyclo-cross bikes rarely have sloping top tubes like some mountain and road bikes do – if they did it would make them difficult to carry.
Cyclo-cross races also differ from mountain bike disciplines in that competitors are allowed external mechanical assistance, and can even change bikes during the race if they wish. In fact top riders often do. In a muddy race they will swap a dirty bike for a clean one on each lap, leaving an assistant to clean the original one in time for the next swap.