Cyclo-cross has 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 prioritize cyclo-cross races over other disciplines. Cyclo-cross bicycles are similar to road racing bicycles: lightweight, with somewhat narrow tires and drop handlebars. They are typically differentiated by their greater tire clearances, lower gearing, stronger frames, cantilever brakes or disc brakes and more upright riding position. They also share characteristics with mountain bicycles in that they use 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.