Thrust roller bearing are generally used to support combined load mainly consisting of axial load and the radial load must not exceed 55% of axial load. This kind of bearing has a low friction factor, high rotational speed and aligning function. Thrust bearings come in several varieties. Ball thrust bearings, composed of ball bearings supported in a ring, can be used in low thrust applications where there is little radial load. Tapered roller bearings consist of small tapered rollers arranged so that their axes all converge at a point on the axis of the bearing. The length of the roller and the diameter of the wide and the narrow ends need to be carefully calculated to provide the correct taper so that each end of the roller rolls smoothly on the bearing face without skidding. These are the type most commonly used in automotive applications (to support the wheels of a motor car for example), where they are used in pairs to accommodate axial thrust in either direction, as well as radial loads. They can support rather larger thrust loads than the ball type due to the larger contact area, but are more expensive to manufacture.