total lock caster, has an additional rotational lock on the vertical shaft so that neither shaft swiveling nor wheel rotation can occur, thus providing very rigid support. It is possible to use these two locks together or separately. If the vertical shaft is locked but the wheel can still turn, the caster becomes a directional caster, but one which may be locked to roll in one direction along any horizontal axis. In some cases it is useful to be able to brake or lock all casters at the same time, without having to walk around to individually engage a mechanism on each one. This may be accomplished using a central lock mechanism engaged by a rigid ring encircling each swivel caster, slightly above the wheel, that lowers and presses down on the wheel, preventing both wheel and swivel rotation. An alternative method is the central lock caster, which has a rotating cam in the center of each vertical caster shaft, leading down to a braking mechanism in the bottom of each caster. Kingpinless casters A Kingpinless caster has an inner raceway, an outer raceway which is attached to the vehicle, and ball bearings between the raceways. This mechanism has no kingpin, hence the name kingpinless. The absence of a kingpin eliminates most causes of swivel caster failure and reduces or eliminates shimmy . They offer capacity and durability comparable to units having sealed precision ball or tapered bearings, and are a practical alternative to traditional swivel casters in high-impact situations. Caster flutter One major disadvantage of casters is flutter. A common example of caster flutter is on a supermarket shopping cart, when one caster rapidly swings side-to-side. This oscillation, which is also known as shimmy, occurs naturally at certain speeds, and is similar to speed wobble that occurs in other wheeled vehicles. The speed at which caster flutter occurs is based on the weight borne by the caster and the distance between the wheel axle and steering axis. This distance is known as trailing distance, and increasing this distance can eliminate flutter at moderate speeds. Generally, flutter occurs at high speeds. What makes flutter dangerous is that it can cause a vehicle to suddenly move in an unwanted direction. Flutter occurs when the caster is not in full contact with the ground and therefore its orientation is uncontrollable. As the caster regains full contact with the ground, it can be