At a glance, designing and building a traction control system is easy: Monitor front and rear wheel speeds, and cut power if the rear wheel goes faster than the front. And from a safety standpoint — preventing a crash — a basic system is quite adequate. Traction control for performance, however, is a different story. Racers have long used wheelspin to help the motorcycle finish a corner, and it’s well known that tires provide more grip if they are spinning a certain amount. These contradictory goals of safety and performance have led to increasingly advanced systems, with additional sensors, feedback and signal processing aiming to provide both benefits in a single system for sportbike use. Recent patents and patent applications published by the United States Patent and Trademark Office show how various manufacturers have developed their systems for better performance as well as safety.
If you are not familiar with traction control systems, here is the executive summary: Some systems, such as that used on the MV Agusta F4 or the Bazzaz Z-Fi aftermarket add-on box, work without comparing wheel speeds, but rather limit the engine or rear tire from accelerating at more than a certain rate. These rate-of-change or feedforward setups require few sensors and work quite well in known situations — such as at a given racetrack on a motorcycle with known modifications. More elaborate, wheel-speed-based systems use feedback to modulate engine power, providing more accuracy at detecting and maintaining traction. Most performance systems use wheel speed sensors and feedback, and some — as we shall see — use both rate-of-change and wheel-speed feedback for best results.
In a wheel-speed-based system, front and rear wheel speeds are compared to give a slip value — a measure of how much faster the rear wheel is travelling than the front — and the system cuts power to keep slip at a target value. On a motorcycle, however, traction control is complicated by the difficulty in measuring wheel speeds accurately. Using a traditional wheel speed sensor to measure how fast a wheel is rotating does not directly translate to ground speed; the tire’s circumference changes with lean angle, generating an error in a way similar to how your speedometer changes when you lean into a corner even though your speed hasn’t changed. Additionally, wear, load and growth at speed all have an effect on circumference and, in turn, measured speed.
Using Dunlop’s data for the...
Using Dunlop’s data for the tires used on the XR, a new slip signal can be generated, which is much more accurate. We have used the GPS signal here to determine slip (and GPS is certainly used in some advanced systems) as opposed to front wheel speed, but the concept is the same. This method is used in the BMW S 1000 RR traction control system.
One way to improve the accuracy of the slip signal is to take into account the wheels’ changing circumferences with lean angle. A BMW patent (Slip Control System for a Single-Track Motor Vehicle) discusses this method, which is employed in the S 1000 RR’s system. According to the patent, the front and rear profiles for a set of tires — or multiple sets — are programmed in the bike’s ECU. On the S 1000 RR, BMW’s HP Race Calibration Tool allows these parameters to be changed for different tires. Note that, in this case, the lean angle of the bike must be known to calculate the tires’ circumferences.
A series of Yamaha patents (Motorcycle, Device and Method for Controlling the Same and Device and Method for Detecting Slip Quantity of Motorcycle) detail a traction control system that manipulates the slip signal using a filter to correct for the changing tire circumferences. According to the patents, the system presumes that slow changes in the slip signal are due to the changing tire circumference as the motorcycle leans, since the motorcycle can only lean from side to side at a certain rate. Faster changes in the signal indicate a loss of traction. A filter can be used to remove the low-frequency component of the slip signal, leaving only the high-frequency slip-related component for a more accurate representation of slip. Filtering can be accomplished electronically or mathematically, although the patent specifically states that, in this case, it is done mathematically in the ECU. It’s also worth noting that some filtering models, such as moving-average or exponential smoothing, are used in forecasting trends — in other words, the Yamaha system with its filter would be capable of predicting slip to a certain extent.
According to a Yamaha patent,...
According to a Yamaha patent, the slip signal can be manipulated to remove the portion related to changing tire circumferences. This component has a low frequency compared to the traction-related portion and can be filtered out, leaving a more accurate slip signal. Compare the compensated slip signal here with the signal manipulated using wheel circumference data above.
The main benefit of this setup from a production standpoint is that a lean-angle sensor is not required, reducing cost and complexity. Further details in the patents show how other sensor data can be integrated to further improve accuracy: How quickly a motorcycle changes direction depends on speed, and this can be taken into account in the filtering process. In any reasonable scenario, the rate of lean is related to throttle position (the rider will not be suddenly changing lean angle when the throttle is wide open), and this data can also be used. Alternatively, rpm and/or torque can also be taken into account. While Yamaha does offer traction control on its 2012 Super Ténéré model, an R1 is used for reference in the patent and it’s a safe bet that the next generation of the company’s literbike will have some form of traction control based at least in part on this patent.
While accurately detecting slip is an important part of any traction control system, just as important is how power is controlled to keep wheelspin in check. Cut insufficient power to match the detected slip, and a crash could result. Cut power too much, and the motorcycle will slow down. With electronic ignition and fuel injection, power can be controlled by advancing or retarding ignition timing, or cutting spark or fuel from one or more cylinders. Furthermore, electronically controlled throttle valves (either primary or secondary) can be used to control power. As mentioned, for maximum performance on the racetrack a certain amount of slip is desired and the traction control system will use the wheel-speed feedback and power modulation to keep slip at the optimum value.