In swivel castors the wheel and fork tum a full 360 degrees, whilst the top or mounting plate remains in a fixed position. The horizontal distance between the centerline of the swivel and the centreline of the wheel is called the offset. When pressure is applied through the mounting plate the offset causes the castor to swivel in line so that the wheel always trails in the opposite direction to which the equipment is being moved. Rolling resistance determines the, effort necessary to move. the equipment from rest and to keep it rolling. If a wheel, so hard that it will not flatten under load, rolls on an absolutely flat hard surface, perfect rolling is possible.