Rear Anti-lock Braking Systems (ABS) typically control both rear wheels using a single valve and a single speed sensor, often located in the rear axle. In essence, they function similarly to the rear end of a three-channel ABS system.
Key Components and Functionality
- Single Hydraulic Valve: The system uses one valve to modulate brake pressure to both rear wheels simultaneously. This is unlike front ABS systems, which generally have separate valves for each wheel.
- Single Speed Sensor: A single speed sensor, usually positioned in the rear axle, monitors the rotational speed of the rear wheels. This sensor detects if either of the rear wheels begins to lock up.
- Combined Monitoring: The rear wheels are monitored collectively. The ABS only activates when both rear wheels show signs of imminent lock-up.
Operation
- Normal Braking: During normal braking, the system does not interfere. Brake pressure is applied to the rear wheels as usual.
- Lock-Up Detection: If the speed sensor detects that both rear wheels are decelerating rapidly and approaching a lock-up, the ABS module is triggered.
- Pressure Modulation: The single valve then modulates (reduces and reapplies) the brake pressure to both rear wheels. This prevents the wheels from locking, allowing the driver to maintain steering control.
- Cycling: The ABS system cycles the brake pressure rapidly, creating a pulsing sensation in the brake pedal. This cycling continues until the threat of lock-up is gone.
Limitations
Because rear ABS uses a single valve and monitors both wheels together, it's less sophisticated than systems that control each wheel independently. If only one rear wheel is about to lock up (e.g., due to uneven road surfaces), the ABS might not activate because the combined reading from the speed sensor doesn't indicate an imminent lock-up of both wheels.
Analogy
Think of it like a seesaw. The ABS will only activate if both sides (rear wheels) are going down at the same rate, indicating a potential lock-up. If only one side is going down, the seesaw isn't "tipping" enough to trigger the system.
