In recent years, the development of performance motors has primarily focused on increasing torque, while drives optimized for installation space and weight were mainly introduced as support motors for sporty bike types such as gravel bikes and light e-mountain bikes. ZF's development goal was to combine the torque of classic performance motors within the compact installation space of so-called light-assist motors. The core technology behind this is the strain wave gear, which enables a concentric design around the crank axle.
CentriX drive unit
The centerpiece of the ZF Bike Eco System is the newly developed CentriX drive unit. From the start, ZF pioneered new approaches to achieve the key development goals and USPs of the system, such as reduced weight, high torque density, and compactness. This was accomplished through a single-axis design, enabling seamless integration into the bicycle. The drive unit comprises the electric motor and a gear stage. The electric motor is a brushless DC motor, designed as an external rotor, which allows for greater leverage forces. As a result, the motor can generate higher forces from a standstill, delivering more torque than typical assistance power.
Gearbox stage
For the gearbox stage, ZF relies on an innovation in the form of a strain wave gear, which has been adapted to the requirements of an e-bike drive - unlike the more commonly used spur gears in this performance class. Strain wave gears have traditionally been used in robotics due to their high precision and backlash-free operation. These characteristics provide a very direct and responsive feel in the assisted force transmission of the e-bike. Additionally, the strain wave gear allows for a high transmission ratio in a compact space, making a significant contribution to the development of such a powerful drive unit in a small form factor.
Excessive torque
A torque limiter, designed as a friction-locking multi-plate separating clutch, protects the mechanical gear from excessive torque, making it a key feature for using this type of gearbox in an e-bike drive. The single-axis, coaxial design allows the drive to operate with an oil filling, ensuring consistent lubrication throughout its entire lifetime. Additionally, a co-rotating torque measurement system, based on strain gauge technology, was developed to measure the combined forces applied by the rider on both sides of the crank in the existing setup.
Compact design
The combination of an electric motor and strain wave gear on a two-part shaft is primarily responsible for the compact design of the drive unit. Its cylindrical form, with a diameter of just 88 millimeters, allows for easy integration into the bike and provides manufacturers with greater design flexibility, enabling them to make an e-bike resemble a conventional bicycle.
The drive unit is compatible with both derailleur and hub gears. Cranks with the ISIS standard are used, and either specific direct-mount solutions or a spider with a standard 104 mm bolt circle can be used for chainrings.
This article is sponsored by ZF.
