Electric Bus Powertrain: Wheel-Side Drive Motor – Introduction and Analysis Electric buses have traditionally relied on centralized drive systems. Two main configurations have dominated the market: Direct-Drive Motor System In these systems, the electric motor directly replaces the internal combustion engine. This approach offers a simple and straightforward system architecture, with mature vehicle structural design technology that has made it the mainstream configuration for battery electric buses. Motor + Gearbox Systems This configuration, which combines an electric motor with an Automatic Manual Transmission (AMT) gearbox, delivers superior performance on steep slopes, meeting the demands of mountainous or scenic areas. Although it borrows extensively from conventional fuel vehicle technology and is relatively mature, including an automatic shifting mechanism can compromise reliability, leading to practical challenges in real-world operations. As electric bus powertrain systems evolve toward higher speeds, greater integration, and lightweight designs, distributed electric drive technology—exemplified by wheel-side drive motors—has begun to enter the practical stage. Compared with centralized drive systems, wheel-side drive configurations eliminate the need for transmission shafts, main reduction gears, and differentials. This results in a shorter power transmission chain, higher transmission efficiency, and a more compact structure. In addition, precise control of wheel-side motor speed and torque enables integrated functions such as vehicle propulsion, braking, differential action, and energy recovery. Owing to its high degree of freedom and functional expandability, the wheel-side drive system has become a focal point in the research and development of pure electric bus powertrain systems. In addition, precise control of wheel-side motor speed and torque enables integrated functions such as vehicle propulsion, braking, differential action, and energy recovery. Owing to its high degree of freedom and functional expandability, the wheel-side drive system has become a focal point in the research and development of pure electric bus powertrain systems. Electric Bus Powertrain: Wheel-Side Motor Drive Configurations Wheel-side motor solutions for battery electric buses bridge the gap between centralized and hub motor designs. They typically integrate a motor with a fixed-ratio reducer mounted on the chassis, directly driving the wheels via short axles. Two primary configurations exist: A. Fixed-Motor Configuration The fixed-motor configuration usually takes the form of an integrated wheel-side drive axle. In this design, the conventional axle housing and half-shafts are eliminated, and the drive motor is mounted adjacent to the wheel. Despite maintaining a rigid axle structure, this design can employ either steel leaf springs or a combination of air springs and coil-over shock absorbers. Brogen wheel-side drive system with fixed motors Key advantages: Reduced Weight and Space: By removing the axle housing, casing, and half-shafts, the overall structure is significantly lighter and more compact. High-Speed Motor Integration: With the use of high-speed motors paired with high reduction ratios (often through a planetary gear structure), the design minimizes motor volume and weight while increasing power density. Robust Performance: Our integrated wheel-side drive axle, for example, features two high-speed motors mounted on either side of the axle. With a two-stage reduction system, it delivers enhanced torque and is capable of handling high axle loads—ideal for heavy-duty, low-floor city buses. B. Swing-Motor Configuration In the swing-motor configuration, the traditional rigid axle is abandoned in favor of an independent air suspension system. Here, both the drive motors and reducers are mounted directly on the suspension, and the reducer can be designed as either a two-stage or planetary gear system. Brogen wheel-side drive system with swing motor and independent suspension Key advantages: Lower Unsprung Mass: The elimination of the rigid axle structure reduces unsprung mass. A well-designed suspension can effectively transfer the motor’s mass to the vehicle body, improving ride comfort and handling. Enhanced Cabin Design: This configuration enables increased interior space, a wider aisle, and lower floor heights—critical factors in the design of modern low-floor city buses. Benefits of Wheel-Side Drive Motors in Electric Buses The adoption of a wheel-side drive motor for an electric bus powertrain – where flexible electrical connections replace some mechanical linkages – offers significant benefits in electric bus design and performance. Below are the primary advantages: 1. Increased Cabin Space and Lower Floor Height Low-floor city buses are a growing trend. In these designs, the area from the front passenger door to the last axle forms a continuous, step-free zone. Lowering the interior floor not only reduces the number and height of steps – making boarding and movement inside the bus easier, safer, and more accessible for all passengers – but also increases headroom in key areas. For instance, the structural dimensions of an integrated wheel-side drive axle can reduce floor installation height by approximately 70 mm, achieving a floor clearance as low as 290 mm. This enhanced design meets both interior space and passenger safety requirements. 2. Vehicle Lightweighting Reducing the overall vehicle weight directly contributes to lower energy consumption. Research indicates that a 10% reduction in vehicle weight can lead to a 6% – 8% reduction in energy consumption. The wheel-side drive motor configuration achieves significant weight savings through: High-speed motor design: Increased motor speeds allow for lower torque requirements, which in turn reduces cost and weight. System integration: Consolidating components (motor, reducer, controller) into an integrated design reduces the need for additional attachments and cabling, thereby lowering both the weight and cost of the powertrain. 3. Enhanced Vehicle Dynamics Mainstream single-motor direct-drive configurations often struggle with steep climbs and mid-to-high-speed acceleration, while dual-motor systems face challenges in weight and cost. Additionally, motor+AMT configurations can suffer from power interruptions during gear shifts. In contrast, the wheel-side motor configuration: Dual-Motor Advantage: By deploying two motors, the power demand on each unit is reduced while maintaining overall system performance. Elimination of the Main Reducer: Replacing the main reducer with a high-reduction ratio gear system (using helical gears) not only simplifies the transmission patch but also improves strength, reduces manufacturing complexity, and lowers costs. 4. Improved Transmission Efficiency Eliminating the traditional main reducer and differential from the electric bus powertrain means that power is transmitted through a shorter chain – enhancing efficiency. The use of helical gears in the
