The HNF Heisenberg development team designed the new suspension technology for the XF1 in a very specific way. The kinematics are fully integrated above the mid-motor, connecting it securely to the frame. To ensure consistent spacing of the low-maintenance carbon drive belt between the spindle and the rear hub, the mid-motor and rear hub are integrated into the drive unit swing arm. The belt runs under tension and perfectly aligned between both toothed belt discs and is able to effortlessly transfer even high peak loads from the mid-motor.
In accordance with the principle of four-joint kinematics with a virtual rotary axis around the spindle, the 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.
One of the dynamics-related advantages of the drive unit swing arm is the fact that it functions without recoil from the pedal. Compared with other rear-mounted suspension concepts, which are specifically optimized for muscle-powered drive systems, 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. The suspension can respond sensitively at any time, ensuring excellent grip and high traction.
As the proportion of human driving force is relatively small, there is no unpleasant rocking movement when accelerating or breaking. Even in the event of extremely uneven terrain it is possible to pedal smoothly and efficiently.