Anti-lock braking system for motorcycles

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The ABS sensor of a BMW K 1100 LT

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A toothed-wheel ABS sensor. These are the front brake discs on a BMW R1150GS. The toothed ABS ring indicates that this bike was manufactured before November 2002.

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Another toothed-wheel ABS sensor. This is on a BMW K75 motorcycle.

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.

History

In 1988, BMW introduced an electronic/hydraulic ABS for motorcycles, ten years after Daimler Benz and Bosch released the first four-wheel vehicle ABS for series production. Motorcycles of BMW K100 series were optionally equipped with the ABS, which added 11 kg to the bike. It was developed together with FAG Kugelfischer and regulated the pressure in the braking circuits via a plunger piston. Japanese manufacturers followed with an ABS option by 1992 on the Honda ST1100 and the Yamaha FJ1200.

Continental presented its first Motorcycle Integral ABS (MIB) in 2006. It has been developed in cooperation with BMW and weighed 2.3 kg. While the first generation of motorcycle ABS weighed around 11 kg. The current generation (2011) presented by Bosch in 2009 weighs 0.7 kg (ABS base) and 1.6 kg (ABS enhanced) with integral braking.

System and functions

Basic principle

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 closing 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, at a motorcycle rear and front wheel are controlled separately. If the rider only brakes with the front or rear wheel, the braked wheel tends to lock up faster as if both brakes would have been applied. A Combined Braking System distributes the brake force also to the non-braked wheel to lower the possibility of a lock up, increase deceleration and reduce suspension pitch.

At a single 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.

At larger sports models with two front discs Honda installs a dual CBS System. The system was first installed in a CBR1000F in 1993. Here, applied brake pressure at the front is contributed 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.

CBS and ABS

CBS helps to reduce the danger of wheel locks and fall downs. But in certain situations it is possible that CBS causes a fall down. If brake pressure is distributed from the rear wheel to the front wheel and the friction of the surfaces changes suddenly (puddle, ice on the street) the front wheel might lock even so only the rear brake has been applied. This would lead to a loss of stability and a fall down. A possibility to avoid this is the combination of CBS and ABS on a motorcycle. Different approaches are possible to realize this combination: Without active pressure Build up Single Version: A third additional channel links the rear wheel circuit through a delay valve to the front brake. At strong brake maneuvers at the only at the rear wheel or both wheels brake fluid is distributed to both brake circuits but through the measurement of the wheel speed pressure is adjusted according to wheel speed and brake slip.

The dual version combines Hondas Dual CBS with a secondary master cylinder and a proportional control valve with the Piston ABS, where a modulator regulates the pressure for each With Active Pressure Build up In 2009, Honda introduced the electronic controlled combined ABS for its high performance sports bikes which utilizes brake by wire technology. The brake input of the rider is measured by pressure sensors and the information is provided to an ECU. Together with the information of the wheel speed sensors the ECU calculates the optimal distribution of pressure to prevent lockups and to provide best possible deceleration. Based on this output a motor for each wheel operates a pump which builds up and regulates the brake pressure on the wheel. This system offers a high reaction time because of the brake to wire functionality.

The MIB (Motorcycle integral Braking system) from Continental Teves and the eCBS (electronic CBS) in the Motorcycle ABS enhanced from Bosch are results of another approach. These systems are based on the pump and valve approach. Through additional valves, stronger pumps and a more powerful motor the system can actively build up pressure. The input pressure of the rider is measured with pressure sensors at the lever and pedal. The pump can build up additional pressure to adjust it to the riding conditions. A partial integral System is designed for working in one direction: front→rear or rear→front. On the other hand, a fully integral system can work in both ways: front→rear and rear→front.

Because these systems are electronically controlled and are able to build up pressure actively, they offer the opportunity to adjust the motorcycle braking behavior to the rider. CBS and ABS can be switched of by experienced riders and also different regulation modes with higher and lower thresholds can be chosen. Like the rain or slick mode in the BMW S1000RR.

Perception and legislation

A report of the European Transport Safety council showed that riding a motorcycle is 20 times more dangerous than driving a car the same distance. And the accidents situation in Germany from 1990 to 2011 showed that that total traffic fatalities decreased drastically (11 000 to 4 009) but motorcycle fatalities remained constant.

The Insurance Institute for Highway Safety (IIHS) conducted a study on the effectiveness of ABS for motorcycles and came to the conclusion that motorcycles above 250cc without ABS are 37 percent more likely to be involved in fatal crashes and a study of the Swedish Road Administration came to the conclusion that 48 percent of all severe and fatal motorcycle accidents above 125cc could be avoided due to motorcycle ABS

These studies caused the EU commission to initiate a legislative process in 2010 that was passed in 2012 and led to ABS for motorcycles above 125 cc becoming mandatory from 2016 onwards. Organizations like the Federation International de l’Automobile and the Institute of advanced Motorists (IAM) demanded the implementation of this legislation already for 2015. On the other hand, some motorcycle riders are protesting against a compulsory ABS for all bikes because they call for a possibility to switch the system off, for e.g. off road usage. In 2011 the United Nations (UN) started the Decade of Action for Road Safety. The main goal is to save 5 million lives until 2020 through global cooperation. One part of their global plan is to: Encourage universal deployment of crash avoidance technologies with proven effectiveness such as Electronic Stability Control and Anti-Lock Braking Systems in motorcycles.

See also

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