Virtual pivot point (instant centre)
Description of the virtual pivot point – four-pivot-point system based on the Horst link
The virtual pivot point describes a point that moves through the compression process, also known as the instant centre of rotation. During the course of the compression, the rear axle performs a complex, compound rotation as well as a horizontal and vertical shift (rotational and translational movement). This path can be represented as a rotational movement as a function of time at every point of the rear axle via the virtual pivot point.
Determination of the virtual pivot point
In this process, the virtual pivot point is determined as follows. In a four-pivot-point system (based on Horst link) the virtual pivot point is given by the intersection of two straight lines. The first straight line here results from the two pivot points of the Horst link joint and the main rear swing arm bearing, the second straight line from the hinge point of the seat tube linkage and the fastening point for the linkage on the main frame.
Impact of the virtual pivot point on the rear swing arm when pedalling – anti squat effect
Depending on the position of the virtual pivot point in combination with the wheel trajectory curve, it may have a stabilizing (positive anti-squat) influence on the pulsating chain tension forces or further reinforce the deflecting “swing” effect (negative anti-squat). The cause of this are the large forces (up to 4,000 N chain tension force) transferred to the chain via the cranks during pedalling. In addition to the acceleration process, dynamic changes in wheel load also occur, which overlap with the forces from the chain tension. These dynamic wheel load changes result from the crank unit with its 180° offset mounting, which leads to a pulsating chain force during pedalling.
The aim of our development is to achieve a neutral “stabilizing” chassis. Here the virtual pivot point is constantly above the chain pull during the compression process; the result is a chassis that has minimum extension during acceleration and constantly provides optimum grip and propulsion.