The drive unit swing arm is applied on the Heisenberg XF1 eBike. The small logo with the wording “Concept by BMW i” indicates the origin. Click to enlarge.

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 excellent propulsion and handling characteristics.

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

The HNF Heisenberg development team designed the new suspension technology for the XF1 with the kinematics 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, is perfectly aligned between both toothed belt discs and is able to effortlessly transfer even high peak loads from the mid-motor. Click to enlarge.

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.

Benefits of the BMW i 4-joint drive unit swing arm include:

• 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

The Heisenberg bike is also equipped with motor and 400 Wh Li-ion pack from Bosch. Price is €8,345 (US$9,200). Click to enlarge.

HNF GmbH, established in Biesenthal near Berlin in 2014, develops next-generation eBikes with a direct service and sales concept offered under the brand name HNF Heisenberg. The founders are Michael Hecken, Karlheinz Nicolai and Benjamin Börries.

eBike pioneers Hecken and Nicolai had already established Grace GmbH in 2008 and were commissioned by Daimler Benz AG to develop the smart bike. HNF Heisenberg—their new future-oriented brand for an increasingly technophile public—will begin with a model series ranging from the S-Pedelec city bike to the top-of-the-range full-suspension mountain bike.