BMW and German eBike manufacturer HNF have developed an electric mountain bike, dubbed Heisenberg XF1, featuring a drive unit swing arm that was originally patented by BMW Research and Technology.
According to BMW, "the concept allows the drive train, which was previously firmly attached to the main frame, to float freely, eliminating the need for the conventional chain tensioner. This permits for the first time the combination of a rear suspension and the durable, maintenance-free carbon belt drive on full-suspension eBikes, resulting in outstanding propulsion and handling characteristics."
A new suspension technology, especially desgined for the XF1, sees the kinematics fully integrated above the mid-motor, connecting it securely to the frame. In accordance with the principle of four-joint kinematics with a virtual rotary axis around the spindle, the innovative BMW drive unit swing arm is guided in such a precise way as to prevent the relative position of the mid-motor spindle unit from deviating in relation to the main frame.
In conjunction with the shock linkage, this swing arm connectivity facilitates long spring travel of up to 150 mm on the rear wheel. Compared with other rear-mounted suspension concepts, there is no stiffening of the rear swing arm when the electric motor is providing a high degree of assistance such as during acceleration, constant travel at high speeds or on hills. As the proportion of human driving force is relatively small, a cyclist will feel no unpleasant rocking movement when accelerating or breaking. BMW also states that even in the event of extremely uneven terrain it is possible to pedal smoothly and efficiently.