The Motorcycle Anti-lock Brake System (ABS) prevents the wheels of a powered two wheeler from locking during braking situations. Based on information from wheel speed sensors the ABS unit adjusts the pressure of the brake fluid in order to keep traction and avoid fall downs (e.g. maintain deceleration). Motorcycle ABS helps the rider to maintain stability during braking and to decrease the stopping distance. It provides traction even on low friction surfaces. While older ABS models are derived from cars, recent ABS are the result of research, oriented on the specifics of motorcycles in case of size, weight and functionality. National and international organizations evaluate Motorcycle ABS as an important factor to increase safety and reduce motorcycle accident numbers. The European Commission passed legislation in 2012 that made the fitment with ABS for all new motorcycles above 125cc to be mandatory from 1 January 2016.
Wheel speed sensors mounted on front and rear wheel, constantly measure the rotational speed of each wheel and deliver this information to an Electronic Control Unit (ECU). The ECU detects on the one hand if the deceleration of one wheel exceeds a fixed threshold and on the other hand whether the brake slip, calculated based on information of both wheels, rises above a certain percentage and enters an unstable zone. These are indicators for a high possibility of a locking wheel. To countermeasure these irregularities the ECU signals the hydraulic unit to hold or to release pressure. After signals show the return to the stable zone, pressure is increased again. Past models used a piston for the control of the fluid pressure. Most recent models regulate the pressure by rapidly opening and closings solenoid valves. While the basic principle and architecture has been carried over from passenger car ABS, typical motorcycle characteristics have to be considered during the development and application processes. One characteristic is the change of the dynamic wheel load during braking. Compared to cars, the wheel load changes are more drastic, which can lead to a wheel lift up and a fall over. This can be intensified by a soft suspension. Some systems are equipped with a rear wheel lift off mitigation functionality. When the indicators of a possible rear lift off are detected, the system releases brake pressure on the front wheel to counter this behavior.Another difference is that in case of the motorcycle the front wheel is much more important for stability than the rear wheel. If the front wheel locks up between 0.2-0.7s, it loses gyrostatic forces and the motorcycle starts to oscillate because the increased influence of side forces operating on the wheel contact line. The motorcycle becomes unstable and falls.
Anti-lock Braking System (ABS)
Piston Systems The pressure release in this System is realized through movement of a spring-tensioned piston. When pressure should be released, a linear motor pulls back the plunger piston and opens up more space for the fluid. The System was used for example in the ABS I (1988) and ABS II (1993) of BMW. The ABS II differed in size and an electronically controlled friction clutch was mounted on the shaft instead of a plunger. Further displacement sensors record the travel distance of the piston to allow the control unit a more precise regulation. Honda also uses this system of pressure modulation for Big Sports and Touring Bikes.
Valve and Pump Systems The main parts which are part of the pressure modulation system are solenoid inlet and outlet valves, a pump, motor and accumulators/reservoirs. The number of the valves differs from model to model due to additional functionalities and the number of brake channels. Based on the input of the ECU, coils operate the in- and outlet valves. During pressure release the brake fluid is stored in accumulators. In this open system approach the fluid is then brought back in the brake circuit via a pump operated by a motor which is felt through pulsation on the brake lever.
Combined Braking System (CBS)
Different from cars, planes or trains, motorcycle rear and front wheels are controlled separately. If the rider only brakes with one wheel, this braked wheel tends to lock up faster than if both brakes had been applied. A Combined Braking System therefore distributes the brake force also to the non-braked wheel to lower the possibility of a lock up, increase deceleration and reduce suspension pitch.
With a single [rear] CBS the brake pressure applied on the rear brake (pedal) is simultaneously distributed to the front wheel. A delay valve cuts the hydraulic pressure to assure that only when strong braking is applied, pressure is also created at the front wheel. Honda installed its first Single CBS on the GL1200 in 1982.
Larger models with two front discs use a dual CBS System. The system was first installed in a Honda CBR1000F in 1993. Here, applied brake pressure at the front is also applied to the rear wheel and vice versa. If the front lever is applied, pressure is built up at 4 of the 6 pots in the 2 calipers at the front. A secondary master cylinder at the front wheel distributes remaining pressure to the rear wheel through a proportional control valve and acts on 2 of the 3 calipers. If strong brake force is applied at the rear wheel force is also distributed to 2 of the 6 pots of the front wheel. More modern dual CBS use front and rear calipers (and all pots) according to a preset load ratio of front to rear.
Reference : https://en.wikipedia.org/wiki/Anti-lock_braking_system
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