Cornering ABS combines basic ABS with a lean angle sensor or IMU, and a composite brake function that can distribute brake pressure electronically to the front or rear brake as necessary. Cornering ABS combines basic ABS with a lean angle sensor or IMU, and a composite brake function that can distribute brake pressure electronically to the front or rear brake as necessary. Courtesy of Bosch

Trevitt's Blog: Cornering ABS and using the rear brake

Written by  Andrew Trevitt on Wednesday, 11 January 2017 14:53

One rider aid that is available on an increasing number of new motorcycles is cornering ABS, sometimes incorporated into a stability control package. In its basic form, a cornering ABS function takes into account the motorcycle's lean angle to adjust brake pressure, increasing the ABS effectiveness beyond a simple straight-line stop. However, as I outlined in a previous Inside Motorcycles article ("Smarter ABS," Dec. 2015), these systems offer much more than just improved ABS function.

One basic handling characteristic of practically all motorcycles is the tendency to stand up when the front brake is applied in a corner. Due to the front-end geometry and the relationship between the front tire's contact patch and the steering axis, using the front brake when the motorcycle is leaned over causes the steering to turn further into the corner, inducing countersteer that stands the bike up and causes it to run wide. This, of course, is exactly what we don't want when we encounter a fallen tree limb or some similar hazard in the middle of the corner.

The various cornering ABS functions, sometimes part of a stability control or cornering management feature, counter this by sensing that the motorcycle's lean angle is decreasing as the front brake is applied, and transferring brake pressure from the front brake to the rear. This reduces the induced countersteer but retains overall braking pressure, so the motorcycle still brakes as the rider wants but doesn't stand up.

Noted tuner Kaz Yoshima uses the analogy of driving a car with a trailer when it comes to using a motorcycle's rear brake. Just as using the trailer brakes alone can stop a trailer from uncontrollably swaying side to side, using the rear brake on a motorcycle can add stability when entering a corner.

On a road racing machine, riders brake so hard that the rear wheel is often in the air, and the rear brake has minimal effect through the majority of the braking zone. As the rider releases the front brake and arcs into the corner, load does transfer to the rear and more rear brake can be used during this brief transition. That said, this typically can be managed using engine braking, either through electronic controls or an adjustable slipper clutch, and many riders do not use the rear brake at all on track.

The situation is much different for street riders, however. Data shows that even at a "spirited" pace for most riders, braking forces on the street are considerably less than those seen on the track. This means that there is typically much more load on the rear tire under braking, even in a straight line and especially entering a corner. This additional load can tolerate significantly more braking than the engine alone can provide, and now the rear brake is more effective. Using additional rear brake and less front brake will reduce the chance of the front tire locking up, and at the same time - using the trailer analogy - will add stability to the situation.

Take a step back from the sport bike realm, and the effect is even more noticeable. Standard bikes, sport touring bikes, and especially cruisers and touring bikes don't have the front-end bias of a sport bike, leaving plenty of load on the rear tire that can be put to good use for braking.

The takeaway here is that while riders on the track may use little or even no rear brake, on the street it is a much more effective tool for not only increasing safety, but also influencing the handling of the machine. Cornering ABS and stability control functions use this to look after the safety aspect should you get into trouble some day, but using the same concept, pro-active use of the rear brake has both safety and performance benefits.

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