The patent for a drive unit swing arm, which is now entering production at the HNF eBike factory after development to series maturity in collaboration with BMW Research and Technology, originates from the first stage of BMW i market research and the early concept phase.

As there was no longer any immediate use for the patent due to the focus at BMW i on two-track electric vehicles, it has now been finally released for external usage.

Its application in reality can now be experienced on the eBike “Heisenberg XF1”, on which a small logo with the wording “Concept by BMW i“ indicates the prestigious origin of this innovative functional principle.

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The drive unit swing arm is a new kind of frame technology for full-suspension eBikes featuring a mid-motor. 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.

The BMW i patent for the drive unit swing arm principle facilitates for the first time the integration of mid-motor, gears and belt drive into an innovative suspension module, thereby dispensing with a belt tensioner.

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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.


From the summer, the HNF Heisenberg XF1 will benefit from the following advantages of the BMW i 4-joint drive unit swing arm:

  • Mid-motor and even axle load distribution
  • Maintenance-free belt drive with hub gear, without chain tensioner
  • Highly stiff frame
  • No pedal recoil
  • Grip and traction in all riding situations, as there is no stiffening of the rear arm swing
  • Possibility of implementing a modular drive system
  • Freedom in the design of the main frame