 Class T uses tricycles, and Class TB uses tandem bikes. Finally, there's Class H, which uses hand cycles, a machine where the rider turns a crank with their arms as a way to propel themselves forward. But many traditional hand cycles are built for the road, with narrow tires and wide wheelbases to maintain stability. Occasionally, they even place riders kneeling forward. This puts their center of gravity higher, something that would make sharp turns difficult on a looser mountain bike trail.