Trochoid describe a family of curves. (See: Curve Family Index) Trochoid is defined as the trace of a point fixed on a circle that rolls along a line. Sometimes the name trochoid is used to mean hypotrochoid and epitrochoid. (curve traced by rolling circle on another circle) More generally, trochoid is any curve that is the locus of a point fixed to a curve A, while A rolls on another curve B without slipping.
If Q lies outside the rolling circle, it is sometimes called prolate (extended) cycloid.
If Q lies inside the rolling circle, it is curtate (contracted) cycloid.
Cyloid (blue), extended cycloid (green), contracted cycloid (red).This animation shows the trochoid with parameters r:=1 and h increase from 0 to 3. The radial of the curve is also ploted as black dots. When r==h, it is a cycloid and its radial is a circle.
Varing Trochoid.
