Editing Caster (section)

===Swivel casters===
[[Image:Caster-wheels-chair.jpg|thumb|left|upright|Two wheeled swivel casters on a [[desk chair]] (no fork)]]
Like the simpler rigid caster, a swivel caster incorporates a wheel mounted to a fork, but an additional [[swivel joint]] above the fork allows the fork to freely rotate about 360°, thus enabling the wheel to roll in any direction.  This makes it possible to easily move the vehicle in any direction without changing its orientation. Swivel casters are sometimes attached to handles so that an operator can manually set their orientation.  The improved swivel caster was invented in 1920 by Seibert Chesnutt, US Patent 1341630, which was easily manufactured by stamping, and incorporated ball bearings for longer life. Basic swivel casters were in evidence in Charles Darwin's famous "office chair" as early as the 1840s.

Additionally, a swivel caster typically must include a small amount of offset distance between the center axis of the vertical shaft and the center axis of the caster wheel. When the caster is moved and the wheel is not facing the correct direction, the offset will cause the wheel assembly to rotate around the axis of the vertical shaft to follow behind the direction of movement. If there is no offset, the wheel will not rotate if not facing the correct direction, either preventing motion or dragging across the ground.

When in motion along a straight line, a swivel caster will tend to automatically align to, and rotate parallel to the direction of travel. This can be seen on a [[shopping cart]] when the front casters align parallel to the rear casters when traveling down an aisle. A consequence of this is that the vehicle naturally tends to travel in a straight direction. Precise steering is not required because the casters tend to maintain straight motion. This is also true during vehicle turns. The caster rotates perpendicular to the turning radius and provides a smooth turn. This can be seen on a shopping cart as the front wheels rotate at different velocities, with different turning radius depending on how tight a turn is made.

The angle of, and distance between the wheel axles and swivel joint can be adjusted for different types of caster performance.<ref>Siegwart, R. and Nourbakhsh, I. "Introduction to Autonomous Mobile Robots", MIT Press, Cambridge, MA, 2004. 321 p. {{ISBN|0-262-19502-X}}</ref>