In the differential geometry of curves, a roulette is the general concept behind cycloids, epicycloids, hypocycloids, and involutes. Take two curves. Fix some point, called the generator or pole, in relation to the first curve. Roll the first curve along the second; the generator traces out a curve. Such a curve is called a roulette.

Working on the complex plane, let r and f be parametrisations of curves such that | r'(t) | = | f'(t) | for all t. The roulette of p as r is rolled on f is then

Roulettes in higher spaces can certainly be imagined but one needs to align more than just the tangents.

A Sturm roulette traces the center of a conic section as the section rolls on a line.[1] A Delaunay roulette traces a focus of a conic section as the section rolls on a line.[2]

Example

Fixed curve is a catenary, rolling curve is a line:

f(t) = t + icosh(t)   f'(t) = 1 + isinh(t)

r(t) = sinh(t)   r'(t) = cosh(t)

if p=-i the expression is real and the roulette is a horizontal line. In other words, a square wheel could run without bouncing in a road that was a matched series of catenary arcs.

External links

• Mathworld page on roulettes It needs pix!