Halfpipe
A halfpipe is a wooden structure used in dry gravity extreme sports such as skateboarding, freestyle BMX, or roller skating. Appearance wise, it resembles a cross section of a swimming-pool, and in its most basic form, it consists of two concave ramps (or quarterpipes), topped by rails and decks, facing each other across a transition. The attraction of the halfpipe lies in the fact that a skilled skater can play on it for an extended period of time, attaining extreme velocity, while expending relatively little effort. Large (high amplitude) halfpipes make possible many of the aerial moves in extreme vertical skateboarding.
For winter sports such as snowboarding, a halfpipe can be dug out of the ground. In such cases the plane of transition will be oriented downhill at a slight percentage of grade, allowing a rider to take advantage of gravity to maintain his/her speed. In the absence of snow, halfpipes of this type can be used by dirtboarders, motorcyclists, and mountainbikers during the "off season."
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