Between the two unstable regimes mentioned in the previous section, and influenced by all the factors described above that contribute to balance (trail, mass distribution, gyroscopic effects, etc.), there may be a range of forward speeds for a given bike design at which these effects steer an uncontrolled bike upright.[2] It has been proven that neither gyroscopic effects nor positive trail are sufficient by themselves or necessary for self-stability, although they certainly can enhance hands-free control.[1] However, even without self-stability a bike may be ridden by steering it to keep it over its wheels.[6] Note that the effects mentioned above that would combine to produce self-stability may be overwhelmed by additional factors such as headset friction and stiff control cables.[26] This video shows a riderless bicycle exhibiting self-stability. The headset is the set of components on a bicycle that provides a rotatable interface between the bicycle fork and the head tube of the bicycle frame itself. The short tube through which the steerer of the fork passes is called the head tube. A typical headset consists of two cups that are pressed into the top and bottom of the headtube. Inside the two cups are bearings which pr vide a low friction contact between the bearing cup and the steerer. Traditional bicycle head tubes and headsets are sized for a 1-inch-diameter (25 mm) steerer tube (also known as the fork column). Many frame and fork manufacturers are now building their parts around a steerer tube with a diameter of 1? inch. The larger diameter of the head tube and headset gives added stiffness to the steering portion of the bicycle. Common sizes 1" or 1 inch (25.4 mm). This may have a fork crown (The base of the fork steerer tube) of a number of different dimensions. Milling may be necessary to make some headsets fit. 26.4mm (ISO) 27.0mm (JIS) Other sizes are uncommon, but do exist.[1] 1.125" or 1? inch (28.575 mm) 1.25" or 1? inch (31.75 mm) 1.5" or 1? inch (38.1 mm), as used in the OnePointFive International Standard. Cannondale Headshok. Although a Headshok steerer is close to 1.5" it is actually 1.5625" or 1 9/16 inch (39.6875 mm). The Headtube dimensions for 1.5" and Headshok are very similar, differing only in the minimum press depth. 1.5 inch to 1? inch "tapered" headsets (2009 onwards). The lower bearing is 1.5 inches for increased stiffness and the upper is 1? in for reduced weight and to match existing stem interface.