Advanced Traction Control

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.

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.

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.

Using RPM, speed and throttle position, the Kawasaki system determines an initial slip threshold value at which the traction control system activates. This threshold is higher than the target value for optimum slip, so that the chance of falsely detecting a loss-of-traction event is minimized. The system immediately applies the rear brake, closes the secondary throttle butterflies and retards ignition timing to a set value. When slip falls below another threshold, indicating traction is returning, the rear brake is released. By advancing and retarding the ignition timing at a set rate, the system attempts to keep slip at a second threshold value — timing is retarded as slip goes above the threshold, and advanced as slip goes below the threshold. Once ignition timing is advanced to its normal state, the secondary butterflies take over in a similar manner, modulating power to keep slip at the second threshold. This order — brake, timing, and then throttle butterflies — represents responsiveness to power reduction, according to the patent, and cutting and then applying power in this manner keeps good drivability. The attached diagram from the patents graphically shows a traction event and the various responses.

A multitude of maps are used in the Kawasaki system to enhance performance. The slip thresholds change based on the gear selected, engine rpm and vehicle speed. Compensation for lean angle includes accounting for changing tire circumference as well as more or less slip required at certain lean angles. The system even takes into account how the rear tire deforms with acceleration, with a compensation map for each gear that looks much like a thrust chart. To improve drivability, the second slip threshold, which the system works to maintain over time, changes with throttle position or a rider-selected switch. This allows the rider, through the use of the throttle (or TC level) to control the amount of spin in a given situation. While some parts of the system described in these patents — such as the rear brake control — are obviously not found in the ZX-10R’s elaborate traction control system, certainly other aspects are included.

The system monitors front and rear wheel speeds along with lean angle, RPM, gear, speed and throttle position, and takes into account changing tire circumference “in an inventive manner”. When slip goes above a minimum value (as determined by a map based on throttle position, speed and lean angle) or if the throttle is opened rapidly, the system activates and makes a note of the throttle position and amount of slip. From maps based on the recorded parameters, a reference slip is noted that represents the maximum amount of slip allowed at full throttle in the referenced conditions. A map is generated that allows proportionally more or less slip as the throttle is opened or closed from the values noted at activation. This map is intended to give the rider control over the slip based on throttle position.