A factor that influences the directional stability of a bike is wheelbase, the horizontal distance between the ground contact points of the front and rear wheels. For a given displacement of the front wheel, due to some disturbance, the angle of the resulting path from the original is inversely proportional to wheelbase.[9] Also, the radius of curvature for a given steer angle and lean angle is proportional to the wheelbase.[9] Finally, the wheelbase increases when the bike is leaned and steered. In the extreme, when the lean angle is 90?, and the bike is steered in the direction of that lean, the wheelbase is increased by the radius of the front and rear wheels. Wheelbase is the horizontal distance between the centers (or the ground contact points) of the front and rear wheels. Wheelbase is a function of rear frame length, steering axis angle, and fork offset. It is similar to the term wheelbase used for automobiles and trains. Wheelbase has a major influence on the longitudinal stability of a bike, along with the height of the center of mass of the combined bike and rider. Short bikes are much more likely to perform wheelies and stoppies. [edit]Steering axis angle Telescopic forks o a BMW motorcycle reveal the head angle or rake Example of a chopper with an unusually large rake The steering axis angle, also called caster angle, is the angle that the steering axis makes with the horizontal or vertical, depending on convention. The steering axis is the axis about which the steering mechanism (fork, handlebars, front wheel, etc.) pivots. The steering axis angle usually matches the angle of the head tube. In bicycles, the steering axis angle is called the head angle and is measured clock-wise from the horizontal when viewed from the right side. A 90 head angle would be vertical. For example, Lemond[1] offers: a 2007 Filmore, designed for the track, with a head angle that varies from 72.5 to 74 depending on frame size a 2006 Tete de Course, designed for road racing, with a head angle that varies from 71.25 to 74, depending on frame size. In motorcycles, the steering axis angle is called the rake and is measured counter-clock-wise from the vertical when viewed from the right side. A 0 rake would be vertical. For example, Moto Guzzi[2] offers: a 2007 Breva V 1100 with a rake of 2530 (25.5 degrees) a 2007 Nevada Classic 750 with a rake of 27.5 (27.5 degrees)