How It Works: Anti-Lock Braking System (ABS)

Anti-lock braking systems (ABS) have been around since the 80-90s but only recently have they been universally embraced. There’s an innate suspicion among motorcyclists of anything that might appear to undermine our hard-won skills and lessen our input, and this makes us slow adopters of innovation – especially compared to our contemporaries on four wheels.

What Is It?

How It Works : Anti-Lock Braking System (ABS)

Anti-Lock Braking System (ABS) - The clever tech that stops your wheels locking up

While ABS has been a welcome standard on cars for decades, it’s only now becoming accepted as a standard fitment on bikes, along with a host of rider aids to make a purist blanch while simultaneously keeping him out of the ditch.

ABS exists simply to prevent the wheels locking up under braking, and to avoid the consequent loss of control caused by rubber no longer gripping tarmac – such as when you grab too much lever or stamp too hard on the pedal, or on uneven, damp or loose surfaces. In emergency situations, where your braking touch and finesse might have deserted you, ABS will hopefully help in pulling a bike up safely in a short distance.

After all, it has less to do with how quickly you can stop and more to do with the distance you can come to a halt in. Furthermore, if the front wheel isn’t locked up and sliding, it can still be used to steer clear of disaster. Its advocates also point to the contribution ABS can make to lateral stability, thereby controlling dive.

BMW were the first to bring ABS to the motorcycling public with the K100 in 1988. Offered as an optional extra the system, created in conjunction with FAG Kugelfischer, added an additional 11kg to the bike. Yamaha followed BMW’s lead with the 1991 FJ1200A and Honda offered an ABS option for the ST1100 in 1992.

How It Works?

BMW’s first ABS 1 system uses magnetic field sensors front and rear to measure the rate of rotation of a serrated disc bolted to each wheel hub. The system becomes active just as the bike gets above standstill, the ABS measuring wheel rotation speed. If the ABS electronic control unit (ECU) system senses that one wheel is turning slower than the other, it knows that wheel is being braked, or being braked harder than the other, and a lock-up may be imminent.

BMWK100 ABS system This is where ABS began back in 1988, way before the technology received the benefits of years of R&D. Known as the ABS 1 system, it weighed a hefty 11kg and kicked in when the difference between the two wheels’ rotational speed exceeded about 30 per cent. By the time BMW brought out ABS II in 1993, the system weighed 5.96kg.

When the differential between the rotational speeds exceeds a pre-determined amount, typically around 30 per cent, the ABS system acts. A main controller containing an electronically operated plunger relief valve is connected in line with the front and rear wheel hydraulic brake circuits. When a wheel’s rotation is significantly slowed by applied brake pressure, the ECU directs the plunger in the controller to move, diverting pressurised brake fluid from the main hydraulic circuit into the space created by the retreating plunger. This in turn reduces the amount of pressure pushing on the caliper pistons and the grip of the pads on the discs.

In BMW’s ABS 1 the controller’s plunger valve opens and closes around seven times a second, a process called ‘modulating’ which leads to the controllers often being referred to as modulators. The wheel that is turning slowest is repeatedly spared the brake caliper’s grasp and is free to rotate. The ABS continues to operate in use until the locking wheel’s rotational speed is close enough to that of the other one.

BMW quickly refined its anti-lock braking offering with the introduction of ABS II in 1993. This new edition took advantage of developments in digital electronics to make a smarter and more refined ABS ECU interacting with a more sophisticated ABS plunger system. In the five years between the two systems, BMW had got its weight down to 5.96kg (13.14lb) against ABS I’s 11.1kg (24.5lb) mass.

In Practice

Yamaha FJ1200A ABS system - Four main components make up the Yamaha FJ1200A’s electronic ABS. The hub-mounted sensors transmit electronic data to the Electronic Control Unit (ECU), and at the point at which wheel lock-up is imminent the ECU tells the relay box to activate the hydraulic unit (HU). The role of this is to adjust brake fluid pressure, which prevents a lock-up.

ABS was slow to arrive on sportsbikes, only really starting to be deployed after the end of the Practical Sportbikes era. That’s because the technology was insufficiently evolved to deal with the relentless extreme braking and acceleration forces that caused the chassis to dip and lift far more than something heavier and more stable.

Today all flagship sportsbikes have it, made possible by technological advances with clever hydraulic valves taking over from crude plungers and pressure sensors controlling the distribution of hydraulic forces to the brakes. Legislation has caught up with braking too, and all new bikes over 125cc sold in the EU must now be fitted with ABS.

The old prejudices about ABS probably still apply to Practical Sportbike era bikes in that where it is found it tends to be crude and even intrusive. Yet today’s systems evolved out of those early efforts to become almost unrecognisable. Should you happen to encounter modern ABS and still find it not to your taste, then many systems have switchable presets to suit the rider. Or failing that, you could just switch it off...

Labels: How It Works, Knowledge