I. Why Low-Flatbed Trailers Need Rear-Axle Steering
Low-flatbed trailers, due to their low platform height, numerous axles, and large load capacity, commonly experience three major drawbacks when operating in narrow environments such as mountain roads, urban ramps, and construction site access roads: large turning radius, large tire sweep area, and severe tire sideslip. Rear-Axle Steering (RAS) significantly reduces the turning radius, decreases tire wear, and improves lateral stability by allowing the last 1-2 axles to actively or passively deflect.
II. Two Mainstream Rear-Axle Steering Solutions
Mechanical-Hydraulic Follow-up (Passive)
This method utilizes the steering tie rod or towing pin angle on the front axle, transmitting the steering angle to the rear axle via a gear-linkage mechanism. It is simple in structure and low in cost, but the steering angle is not adjustable with vehicle speed, making it suitable for heavy-duty transport below 80 km/h.
Electro-hydraulic Active (Active Type)
The ECU, angle sensor, and hydraulic pump/cylinder are mounted on the trailer frame to collect the tractor's steering angle and speed in real time, controlling the rear axle deflection according to the MAP (Modular Mapping) diagram. It can also operate independently during reversing, achieving "crab-like" or "lateral" maneuvers. The system weighs approximately 58 kg, has a maximum steering angle of ±20°, reduces the turning radius by more than 20%, and extends tire life by 15%.
III. Key Technical Parameters and Design Considerations
Steering Angle Range: Low speed (<30 km/h) rear wheel reverse steering angle 8°–12°; medium to high speed same-direction steering angle 2°–4°, ensuring yaw stability.
Control Strategy: Employs a dual-parameter MAP of "vehicle speed-front angle" to avoid high-speed malfunctions; the reversing mode is calibrated separately, limiting the vehicle speed to ≤10 km/h.
Structural Strength: The steering axle must utilize the existing BPW or SAF axle, with the addition of an integrated steering knuckle and thrust bearing, ensuring a fatigue life of ≥1 million cycles under a 55-ton axle load.
Safety Redundancy: The hydraulic locking cylinder automatically returns to center in case of power loss; the ECU includes dual MCU cross-monitoring, conforming to ISO 26262 ASIL-B.
IV. Actual Test Results
A three-axle low-bed semi-trailer (13 m) making a U-turn in a 12 m wide area: Without rear axle steering, the turn time was 17 m; with active rear axle steering, it was reduced to 13.6 m, a 20% reduction.
Tire Lateral Force Test: At a gross weight of 45 t and a speed of 40 km/h in a curve, the lateral force on the rear axle tires decreased by 28%, the tire tread temperature decreased by 15 ℃, and the expected lifespan was extended by 15%–20%.
Fuel Consumption: Under typical mountain conditions, fuel consumption was reduced by 1.8 L per 100 km, resulting in fuel cost savings of approximately 24,000 RMB per year for 200,000 km of operation.