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.
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.
Dunlop releases accurate circumference data for its AMA-spec tires — shown here are the profiles for the Daytona SportBike tires. This data can be used in a traction control or data acquisition system to manipulate the wheel speed signals for more accurate results.
This graph, taken from our Harley-Davidson XR1200 project bike at Infineon Raceway, shows the disparity between actual speed and measured wheel speed. A slip signal compares the two speeds and can be used to determine wheelspin, but the tire’s changing circumference as the bike leans makes this very difficult.
The traction control system described in a Piaggio patent includes a map generated when a loss of traction occurs. The throttle position (θ) and slip (λ) at that time are noted (as θhold and λhold respectively). Proportionally more or less slip is allowed, based on a maximum permissible slip value (λmax) at full throttle and no slip at closed throttle, giving the rider continued control ver the slip at each occurrence.
This chart, from one of the Kawasaki patents, shows how ignition timing, the sub-throttle valve and the rear brake are used to control slip. The various threshold values are based on a multitude of variables and maps.
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.
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.
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.
A set of 50-page patents assigned to Kawasaki (Slip Suppression Control System for Vehicle) describe a comprehensive traction control system that detects slip using wheel speed sensors and controls traction using the rear brake along with the engine’s ignition, injection and secondary butterflies. Using various threshold values for slip, the system modulates power to hold slip at a calculated value, using a multitude of maps to optimize the threshold values and control power.
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.
The three patents outline various alternatives and variables for operation. As described above, in one example the ignition timing advances and retards at a set rate to control slip. In a second example, ignition timing is advanced and retarded in steps as slip goes above or below a number of predetermined thresholds. Or, ignition timing may be changed at a set rate, but a number of “overshoot” thresholds retard timing to corresponding set values. In another example, the slip thresholds change dependent on how much power is being cut over time, as traction is gradually restored. Feedforward operation, with control based on the rate of change of RPM or rear wheel speed, may also be incorporated.
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 Kawasaki system is notable for its comprehensiveness and multitude of maps to account for a number of scenarios, but a patent (System and Method for Controlling Traction in a Two-Wheeled Vehicle) assigned to Piaggio, the parent company of Aprilia, goes one step further. The patent describes a traction control system that, when a traction loss occurs, generates its own maps based on the circumstances and works from those maps. The patent states that a closed-loop system (using feedback from wheel speed sensors) is slow to respond but able to account for variables such as road conditions, tire wear and different tracks. A feedforward or rate-of-change system, on the other hand, is quicker and more instantaneous in its response but can only be used in known circumstances, such as with a specific tire or at a certain racetrack. The Piaggio system combines both types of systems in a unique manner.
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.
The system calculates how much power should be increased or reduced at any given moment based on the closed-loop format as well as the rate-of-change format. These values are mixed, with more of one or the other being used depending on speed, gear, lean angle or the position of a rider selected switch. The mixer gives the system the benefits of both types of systems, allowing quick reaction as well as adaptability to various scenarios. Aprilia’s Performance Ride Control system as found on its RSV4 models no doubt employs much of the system described in the patent, and — as we found in our test of the SE model in our last issue — works extremely well at controlling slip. SR