MotoGP riders practice starts at the end of every practice session, and it appears that they can use much more throttle than you'd think for a 250-plus-horsepower machine. Electronics are no doubt used to keep wheelspin and wheelies in check, leaving the rider's main job to modulate the clutch enough that the engine does not bog.Working with the basic concept of measuring wheel speeds and comparing them, we can consider the other aspects of a motorcycle's dynamics that must be taken into account and how to accommodate them. One important point is that riders use a spinning back tire to help steer, so a certain amount of slip must be enabled. Some wheelspin can actually increase traction when the bike is upright, but when the bike is leaned over then very little slip is desired. Conveniently, gyro sensors, a common part of any data acquisition system, can detect how far a bike is leaned over, and this value can be used as part of the control algorithm. Similarly, the throttle position sensor can be used--if the rider is using very little throttle, he is probably in the middle of a corner and wants as little wheelspin as possible. Use more throttle, and more spin can be allowed.
More effective systems will take into account every sensor possible: gear position, gyro, rpm, throttle position, accelerometers (going uphill or downhill?) and even suspension travel (what if the front wheel is off the ground?). All the data is crunched using algorithms and look-up tables in the ECU, which then comes up with a number for the amount of slip required. The other side of the equation is controlling the power. Similar to the car applications, various methods can be used, including fuel or ignition tampering, or even control of the throttle in a fly-by-wire system. Heuristic systems can even adapt over time, learning how much traction is available at various lean angles and speeds and adjusting themselves accordingly. It really comes down to how savvy a software engineer is on your team more than anything.
With the right electronics and a good software engineer, there is a lot of traction to be found in the wet. Alex Barros demonstrates at Shanghai.One major obstacle with traction control in four-wheel drive cars is how to detect the car's speed when all the wheels are under power and could be spinning. MotoGP bikes have a similar problem. On many straights, the front wheel is rarely on the ground and the front-wheel speed sensor may not be accurate. One solution for autos is a small radar gun pointing at the ground. Another is GPS, which is accurate enough these days to measure small changes in speed very quickly. Most, however, poll the various gyros and accelerometers and calculate the car's ground speed based on the sensor readings, and this is surely used on the MotoGP bikes in conjunction with the wheel speed sensors.
Many four-wheel series allow traction control simply because policing any rules outlawing it is nearly impossible. The required sensors can be easily hidden on a car and, in some cases, wheel speed is not even measured. Some add-on units using only engine rpm as a guide are as small as a disposable lighter and can be easily removed after a race. Disallowing the use of front-wheel speed sensors, as the AMA has done in an effort to stop traction control being used, is not much of a deterrent if the system is part of (and tucked away inside of) the ECU.
As ABS (and the wheel speed sensors and computing power that comes along with it) becomes standard equipment on more production motorcycles, traction control and other electronic trickery won't be much further behind. -SR
One way a traction control system can reduce power is by electronically closing the throttle. In this Honda setup, a servo motor and gears can advance or retard the throttle plates compared to the rider's inputs. A similar arrangement on the original Yamaha M1 gave throttle control for two cylinders to the ECU, and the most recent iteration has the ECU controlling all the throttle plates. | 
More pick-up points for a wheel speed sensor to detect gives a higher resolution to the data, enabling a traction control system to react faster. The solid disc used here is a magnetic ring element, into which a strip of small magnets is embedded for more data points and accuracy than a toothed ring. The Yamaha M1s have been seen with sensors on each side of the wheel for redundancy. Also note in this picture the small opening in the fender with gradations, used to quickly measure maximum fork travel using an O-ring on the tube. | |