In geometry, a trochoid is a roulette formed by a circle rolling along a line. In other words, it is the curve traced out by a point fixed to a circle as it rolls along a straight line. If the point is on the circle, the trochoid is called common ; if the point is inside the circle, the trochoid is curtate; and if the point is outside the circle, the trochoid is prolate. The word "trochoid" was coined by Gilles de Roberval.
As a circle of radius a rolls without slipping along a lineL, the center C moves parallel to L, and every other point P in the rotating plane rigidly attached to the circle traces the curve called the trochoid. Let CP = b. Parametric equations of the trochoid for which L is the x-axis are where θ is the variable angle through which the circle rolls.
Curtate, common, prolate
If P lies inside the circle, on its circumference, or outside, the trochoid is described as being curtate, common, or prolate, respectively. A curtate trochoid is traced by a pedal when a normally geared bicycle is pedaled along a straight line. A prolate trochoid is traced by the tip of a paddle when a boat is driven with constant velocity by paddle wheels; this curve contains loops. A common trochoid, also called a cycloid, has cusps at the points where P touches the L.
General description
A more general approach would define a trochoid as the locus of a point orbiting at a constant rate around an axis located at, which axis is being translated in the x-y-plane at a constant rate in either a straight line, or a circular path around , The ratio of the rates of motion and whether the moving axis translates in a straight or circular path determines the shape of the trochoid. In the case of a straight path, one full rotation coincides with one period of a periodic locus. In the case of a circular path for the moving axis, the locus is periodic only if the ratio of these angular motions,, is a rational number, say, where & are coprime, in which case, one period consists of orbits around the moving axis and orbits of the moving axis around the point. The special cases of the epicycloid and hypocycloid, generated by tracing the locus of a point on the perimeter of a circle of radius while it is rolled on the perimeter of a stationary circle of radius, have the following properties: where is the radius of the orbit of the moving axis. The number of cusps given above also hold true for any epitrochoid and hypotrochoid, with "cusps" replaced by either "radial maxima" or "radial minima."
