How Motorcycle ABS Works

ABS, or the Anti-Lock Braking System, is a word we get to hear on an almost daily basis, but there are rather many fellows out there who don't know exactly what this system does and why they should mind it. The purpose of this short guide is to put some light on how the ABS works in motorcycles.
ABS is a great safety feature for motorcycles 1 photo
A brief history

Apparently, being invented by Gabriel Voisin in the late 20s to solve some braking problems for airplanes, the anti-lock brakes have been used in the automotive industry first. Almost 50 years have passed from the invention of the most primitive designs to the first fully-operational modern-era ABS brakes.

It's of rather little importance who did what and when, suffice to say that the first ABS brakes began to be developed in the mid-80s, with the first ABS-equipped motorcycle presumably being BMW's 1988 K100.

The ABS technology came a very long way until the present day, and despite the principle behind its operations remaining basically the same, numerous upgrades in performance and design have been made. Without any trace of doubt, we can say that the actual ABS systems equipping modern motorcycles are the most advanced and the safest brakes ever to be present on a two-wheeler.
What does ABS do?
The necessity for a safety system such as the ABS became obvious under hard braking conditions and when having to slow down on slippery surfaces, say wet asphalt. One of the biggest problems of braking hard when riding a motorcycle is the wheel or wheels lock-up.

As the rider detects a dangerous obstacle and squeezes the brakes, applying excessive force may cause the wheel to stop spinning and this leads to losing the grip. With no firm contact between the tire's contact patch and the asphalt or concrete, the bike becomes unstable and a crash is imminent.

The role of the ABS is to detect the wheel slip fractions of a second before it would normally occur because of the braking force and adjust or modulate the said force, allowing the wheel to keep turning back within the limits of grip.

Now, some would say that a system that counteracts the braking force is silly. It may look so at first, but the technology is really working to the rider's advantage.
The big problem with wheels slipping on the riding surface is that losing control of the whole motorcycle usually occurs in fractions of a second and restoring traction while keeping the bike balanced is only a result of luck, or extreme training, as is the case of professional stunt riders who drift.

Studies have showed that it's way better to prevent the wheels from slipping due to excessive braking force than it is to compensate for losing control, if any such compensation was truly possible.

So what the ABS does is actually limiting the braking force the rider exerts by either squeezing the lever or pressing the foot pedal and keep the wheel spinning. Once the imminence of the locking (and therefore skidding) is avoided, the system re-applies the maximum braking force until the next skid is anticipated. By limiting the max force of the braking maneuver, the ABS systems practically allow riders to use the greatest stopping force possible without locking the wheels.
How does the ABS know wheels would lock?
Good question! While the first ABS brakes relied on purely mechanical components and were terribly imprecise and often showed a very big lag, modern-day electronic technology has made things simpler.

Basically, ABS includes 4 major components: the sensor array, the control unit, the pump and the valves which physically regulate the braking force.

Sensors. While the early ABS technology proved to be unreliable due to a plethora of reading and interpretation errors, the modern systems are equipped with extremely precise sensors, redundant architecture and more safety/ failsafe systems.
Video thumbnail
The main sensor on a typical ABS is the speed sensor. Motorcycles equipped with ABS are easy to recognize by a special design of the brake discs: slotted design close to the center of the rotor tells even to the untrained eye, that the specific model is an ABS version. Those slots, called pulser rings act like measuring units for the sensors: the more times the sensors can read one another in a given period of time, the greater the speed.

Speed is measured constantly for feeding real-time data to the ABS ECU. While some bikes only have one-wheel ABS, most modern systems have dual-channel ECUs, meaning they receive info from both wheels.

A notable thing must be added: these sensors are detecting the actual speed of the wheels themselves, and not the absolute speed of the motorcycle in relation to the ground: it's exactly this variable that allows the ABS ECU to help detect whether wheel slip will occur in certain situations, as you'll see ahead.

Alongside wheel speed sensors, modern ABS also comprises gyroscopes and handlebar sensors for detecting the leaning angles of the bike. Knowing the bike's lean angle helps present-day ABS provide extended functionality when turning.

ECU. The ECU or Electronic Control Unit is the brain of the ABS: it receives info from all the sensors, analyzes the data, compares the results with the specific values and when needed, uses special algorithms to regulate the braking force.

This miniature computer is specialized in such operations and higher-spec ECUs can be constantly updated and can “learn” a lot of scenarios or maps to be used in certain situations.

Even street-oriented bikes come with different ABS mappings, maximizing the riding performance and providing safe braking in various scenarios. Thanks to the digital technology, these ABS mappings can be recalled and cycled through with just a press of a button and they become operational in milliseconds.

The ECU receives multiple readouts from all the bike sensors it is connected to and whose info it can interpret: the higher the frequency of these readouts and the comparison computations, the higher the efficiency of the ABS.
Video thumbnail
In case the ECU detects a scenario that matches to what the real world would see as a locking wheel followed by the inevitable skid, this computer sends a command to the pump and valves adjusting the braking force as necessary.

The pump and the valves. These are the physical elements used by the ABS to control the braking force. Since the ABS is regulating the pressure in the brake lines, it needs a pump to work both ways, that is, decreasing and increasing the pressure to normal specifications.

While the pump acts like any casual electric pump using a master cylinder and a piston, the operation of the valves is equally simple. When the ABS kicks in, it means the braking pressure the rider applies to the discs is too big, and the ECU calculates how much it should lower it to prevent the wheels from losing grip.

The amount of “release” is sent as electronic data to the solenoid valves which are moved in the right position to decrease the pressure pushing the caliper pistons, easing the stopping force. As the wheel slip potential is eliminated, the ECU sends another command to the pump and moves the valves in another position, allowing the pressure of the initial braking maneuver to be restored and basically re-applying the hard brake.

This process only takes fractions of a second and it will be repeated until the bike stops. When the ABS works, riders will feel slight vibrations in the lever or pedal, as the pressure in the line is constantly modulated.

While some say that the same braking can be obtained by a professional trained rider, this is only partially true, as the human brain cannot have the processing and precision of the digital system in assessing the various riding environments.
How does it help, is it dangerous?
There still is a debate whether the ABS takes away the pleasure of riding or not, with most of the riders saying it actually doesn't. Considering that the ABS only works when the rider brakes very hard and there is a real risk of a wheel locking and skidding, it is completely inert the rest of the time.

While riding off-road on surfaces covered in sand or gravel, ABS might engage and cause a bit of trouble, as the ECU might erroneously interpret the data form the sensors, lean angle and the like. With riding on such surfaces usually involving riding on a shifting ground, only the latest ABS would be able to understand the “bigger picture.”

It is said that the next BMW R1200GS Adventure will be equipped with an off-road dedicated ABS mapping, but other bikes such as the KTM 1190 Adventure and more are using the technology. As for the other bikes, engaging and disengaging the ABS is done via push-button, with optical warning lights.
Video thumbnail
Bikes with ABS brakes benefit from two things: more stability during hard brakes and shorter stopping distance.

Myth. A common misconception among riders is that once they throw a leg over a bike equipped with ABS, all their problems are solved in an almost magical way. It's sad to admit it, but it's mostly such fellows who are getting into serious trouble because they are less aware while riding, relying on the false hope that the ABS brakes will compensate for their poor riding.

ABS is, by all means, a system developed to aid the rider in tight situations and it was never designed to take over basic safety precautions and maneuvers.
By allowing the rider to apply the absolute maximum braking force possible repeatedly without losing grip, the bike will obviously stop faster.

Seasoned riders who know their bikes well can also predict when they are about to “lose” one of the wheels, and will ease the braking, reapplying the (almost) full force immediately, also avoiding wheel lock and slip. Well, this is exactly what ABS does, but thousands of times faster. This explanation should have cleared things.

More ABS magic
Modern ABS brakes in new machinery also come to the aid of the riders and contribute to greater safety. They are now interlinked with multiple other systems present on the bike and work together, providing their interpreted data and receiving feedback from other ECUs to offer safer launches, safer turning and easier braking.

Launch Control. When riding from a complete stop, the transmission has to deliver quite a lot of power to defeat inertia and get the bike moving. In quite a lot of such scenarios, a slightly excessive throttle can cause the rear wheel to slip and skid the bike out of control.

By predicting the slip, ABS sends the critical data to the bike and either brakes by itself or limits the fuel delivery, regardless of the throttle position. The bike will launch with the max power possible without any dangerous wheel slip.

Stability Control. A feature commonly met in powerful bikes, the SC helps the rider automatically ease the throttle and avoid skidding even while riding at high speed. Together with the info received from the leaning sensors, wheel sensors, throttle position and the bike's speed relative to the ground, the motorcycle's ECU array can detect whether the rider will be in trouble even before he or she manages to assess the danger.
Over- and understeering are happening all the time, but in some cases, they can lead to very bad results, such as the infamous highside crashes (when oversteering) or going wide, off the road, into guard rails and roadside vehicles/obstacles (when understeering).

While the trajectories through a turn are a complex combination of multiple factors, such as bike speed, leaning angle, road condition, tire type and condition and last but not least, rider experience, mistakes and errors occur at every few turns. The role of the ABS is to compensate the wheels' speed and maintain the best grip on a predictable track.

Finally, distributed ABS brakes now equip motorcycles, and they are so advanced so as to control the braking force for both the front and the rear wheel being only given a single command, i.e. pressing the brake pedal.

Working the same way all the way around, a corresponding rear brake force modulation is produced as the rider squeezes the front brake lever. All these new functions that ABS systems have today are also contributing to better bike stability when braking and swerving past obstacles in critical situations.

Given the major difference that stopping 1 or 2 meters earlier can make when a vehicle cuts a rider off or in any similar emergency scenario, buying a bike equipped with ABS seems a good investment in personal safety and property.

In Europe, ABS will become mandatory for any new bike greater than 125cc, manufactured and sold from 2016.

For more on braking on a motorcycle, please check out these 3 Autoevolution articles.


Would you like AUTOEVOLUTION to send you notifications?

You will only receive our top stories