Traction Control

Motorcycles have just a few square inches of contact to the road surface via its tires. Road surfaces can be wet, dry, sandy, hot, cold, freshly paved, cracked, uneven, slippery, or covered in leaves, fluids, or foreign materials that change the amount of traction our tires can utilize. Tires can be old or new (with traces of slippery mold release agents), improperly inflated, or have uneven wear. Traction control helps to mitigate loss of traction from all of these factors, as well as excessive application of throttle or other input errors on the part of the rider.

No matter what the circumstances, the common denominator among all of these factors is a difference between rear wheel speed and front wheel speed.

So how do we get an accurate sense of wheel speeds? We repurpose the ABS sensors that are already monitoring them. We’re specifically concerned with the rear wheel speed in relation to the front. When the rear wheel is unable to maintain traction through its contact patch, it slides, and the rotation of the rear wheel no longer matches the rotation of the front wheel. The sensors relate this information to the bike’s ECU. Since the ECU controls fueling and engine speed, it already knows the amount of power the bike is delivering to the rear wheel. In some cases, it also registers throttle position, gear position and even lean angle.

With the combination of wheel speed sensor data and engine speed data, the ECU can make a determination on available traction, and trigger one or more responses (which varies between motorcycle manufacturers).

At least one of three things will happen as the ECU attempts to prevent rear wheel slip :

1) retard the ignition timing,

2) skip fuel injection on a single cylinder, thereby intentionally creating a misfire, or

3) electronically adjust the throttle (assuming the bike is fitted with an electronic throttle).

Even with traction control on his bike Casey Stoner is still bucked off of the bike like a bull does to a cowboy.

An explanation from Motorcycle.com:

When the tire’s contact patch is moving at roughly 115% of the speed of the bike, the slip-vs-force curve of the typical tire reaches its peak and turns negative. Any higher slip results in less force. Less force to propel the motorcycle is okay, but less force to react to the side load induced by cornering is definitely not okay.

As the tire fails to push back against the cornering load, the motorcycle begins to rotate on its yaw axis. Here you’ve got three options. You can request the same amount of torque and the motorcycle will continue its rotation, terminating in a low-side crash. You can abruptly chop the throttle and as the contact patch force returns, the yaw rotation will convert to an abrupt roll rotation and a resulting high-side crash. Or you can modulate a delicate and nuanced intermediate throttle position that allows the tire to climb gradually and controllably back to the slip peak.

Sources: Wikipedia, Rideapart, Motorcycle.com