MotoMojo: Cornering ABS

MotoMojo: Cornering ABS

When a bike has to slow down or change direction rapidly, it is most susceptible to loss of traction and rider control. Hence, the vast majority of motorcycle mishaps occur while braking and/or cornering. As a result, the greatest challenge for technical safety improvement for motorcycles is to mitigate this problem by using technology which improves upon human control of braking in these situations. The most notable solution to this problem of wheel lockup during hard braking, which leads to skidding, loss of control, and crashes, has been anti-lock braking systems, commonly known as ABS.

Back in 1988, BMW’s motorcycle division introduced the first production electronic/hydraulic ABS on its K100 series. This was followed in 1992 by ABS as an option on Honda’s ST1100 and Yamaha’s FJ1200. Today many street bikes come with ABS either standard or as an option, but not all systems have the same features or capabilities.

Standard ABS

ABS prevents wheels from locking during braking by constantly measuring wheel speeds and comparing them with wheel speeds predicted by the system. Speed measurement is performed by individual speed sensors at each wheel.

During braking with ABS, if wheel speeds vary significantly from the system’s predicted wheel speed (for example, if one tire stops while the other is still turning rapidly), the ABS activates, reducing brake force at the wheel that slowed or stopped prematurely. This helps maintain optimum traction on-road and gives the greatest possible deceleration rate with control when braking in a straight line.

ABS control is separate for each wheel. ABS controllers on different models use various methods of modulating brake pressure with pumps, pistons, solenoids, and control valves, but the overall principles remain the same. When a wheel locks or skids, the brake pressure is reduced until it begins rolling again. As technology has advanced, ABS has become faster acting, more sensitive, and more effective. This has allowed newer systems to stop faster in virtually all road conditions than even a skilled racer can brake without ABS.

Older systems could often be “out braked” by a really good rider, at least under certain conditions when rapid stops were expected and anticipated. One of the advantages of all ABS is that often emergency braking is needed when something unexpected happens, such as when a corner is tighter than anticipated, or a driver turns across the rider’s path. In these situations, the rider may initially apply the brakes too hard in a reflexive “panic” situation and then may be able to ease off and begin modulating the brakes properly. Sometimes the rider can “save it” but, in other cases, one or both tires lose traction and the bike goes down. However, an ABS never panics; it starts working properly immediately and keeps working until the critical situation is over. Many sport and sport-touring motorcycles now have more advanced ABS, which even works well on the racetrack.

Harley-Davidson ABS demonstrator bike with outriggers during comparison runs.

The German In-Depth Accident Study (GIDAS) estimated that ABS can prevent one-fourth of all motorcycle crashes involving casualties. With regular ABS, riders have active protection in a straight line.

Cornering ABS

The next big breakthrough is cornering ABS. When a motorcycle is braking in a straight line with no lateral forces acting on it, virtually 100 percent of the tire’s grip is available for braking. As a motorcycle leans over, cornering forces require a larger percentage of the traction available between the tires’ tread and the road surface. Therefore, the portion of the tires’ grip that is available for braking is reduced as the lean angle increases. At maximum lean angle, there’s essentially no more grip available for braking, as added forces will cause the tire to slide out.

Cornering ABS has a complex and difficult job. It must determine not only wheel speeds, but also cornering forces and lean angles, and constantly and rapidly calculate what proportion of full braking force, called for by the rider, may be applied to the individual brakes. This requires accurate and fast-acting sensors, computing power, and actuators, along with complex software.

Presently two German companies, Bosch and Continental, are the pioneers in motorcycle cornering ABS and stability controls. Bosch’s top-of-the-line ABS 9 enhanced system features an electronic combined brake system (eCBS) function. With this, all the rider has to do is apply one brake, front or rear. Then, Bosch ABS 9 enhanced automatically activates the second brake to produce optimum brake force on both wheels.

The system calculates the traction the bike’s tires can sustain and then distributes the maximum brake force to each wheel based on the riding situation, ensuring the best braking performance under cornering conditions. When braking heavily in corners, the system also reduces a bike’s tendency to stand up, which often results in running out of the lane. The combined brake system is designed to deliver the optimal distribution of brake force between the wheels, which stabilizes the motorcycle during cornering.

Continental has introduced its optimized curve braking (oCB) system, which also monitors lean angle, pitch, roll, and lateral acceleration. oCB uses this information to send braking force to the tires, keeping the motorcycle from sliding out while braking and cornering. BMW’s S 1000 XR is the first production machine utilizing Continental’s oCB system.

Final Thoughts

ABS has many safety benefits and so much potential for preventing and/or reducing the severity of crashes that it is very likely that most, or all, motorcycle manufacturers will incorporate these, or similar systems, in the coming years. It’s also possible that safety legislation in North America may require their addition as well. The European Union has already made ABS mandatory in Europe for all motorcycles over 125cc. This regulation is effective as of 2016 for newly-developed model series and for all new motorcycles from 2017 onward.

Electronics and computing power have made it possible to stop, accelerate, and corner faster with greater safety and confidence than ever before. However, these systems cannot violate the laws of physics, so always keep that in mind when riding. Tires do not have unlimited grip.