During cornering, the ideal Brake Force Distribution (BFD) becomes more rear wheel oriented (cf. chapter 2.1.7). While the rear brake force does not contribute much to the BST effect, the disturbance is mainly generated from the product of front brake force and lever arm86. Concerning an emergency-brake maneuver that requires deceleration at the physical limits, reducing the front brake force in order to mitigate the BST will also compromise the achievable braking distance (cf. chapter 3.6). However, the analysis of typical situations reveals that besides steep brake actuation gradients mostly only partial decelerations far below the ABS activation threshold are applied. E.g. the rider show- cased in Figure 1.2 only had a single finger on the brake lever! Therefore, a brake sys- tem could assist the rider in BST relevant situations by unloading the front wheel from brake effort, both in terms of gradients and absolute level. With the BST chain of effects in mind, the following strategy – from initial brake application to full ABS controlled deceleration – lies at hand:
After activation of either brake, first build up brake pressure in the rear, where also steep gradients are not an issue in terms of stability and BST.
Continue to operate with rear wheel oriented BFD, i.e. a higher exploitation of friction potential at the rear wheel compared to the front wheel µused,rr > µused,ft87, especially for partial decelerations.
85 Spiegel (2010): The Upper Half of the Motorcycle
86 Cf. chapter 3 and Weidele (1994): Bremsverhalten von Motorrädern, Chapter 3.7, p. 61ff, i.e. eq. 69 87 This strategy has already been suggested by Weidele (1994): Bremsverhalten von Motorrädern, Chap-
Let the brake force on the front wheel be built up with a gradient limited to an acceptable level88. Depending on the brake force demand of the rider, also a time delay may be incorporated, as long as the front brake application is early enough to avoid destabilizing slides through ABS activation on the rear wheel.
Finally, for high decelerations, a smooth ABS control is required to diminish BST fluctuations and thus improve course stability and controllability. Since the traction limits set by modern tires89 even allow a brake flip-over at large roll an- gles, a Rear wheel Lift-off Protection (RLP) respectively mitigation function should also be incorporated in the ABS control90, meaning a release of front brake pressure when a lift-off tendency of the rear wheel is detected.
While the ABS functionality is the essential basis for a fully cornering approved brake system (cf. chapter 2.1.8), the complete realization of this strategy ideally requires:
Assessment of the rider’s brake demand.
Active brake force generation, at least at the rear wheel. Rear wheel lift-off mitigation.
Assessment of the cornering state (i.e. the roll angle).
Apart from this, it is worth noting, that already a hydro-mechanical combined brake system (CBS) like the Honda Dual-CBS91 is a benefit with regards to the BST effect. Linking the rear brake also to the front brake lever unloads the front wheel from brake effort and, in case a delay valve is featured, also the rear brake lever activated rise in front brake effort is eased.
However, in the following it shall exemplarily be illustrated, in how far two of the most recent brake systems available on the market address the defined requirements.
88 Roll (2010): Safety benefits of electronic brake-control systems, proceedings pp. 423-513, i.e. p. 463,
Figure 25
89 Weidele / Schmieder (1990): Power transfer between motorcycle tyres and real road surfaces
90 Depending on the brake system manufacturer, this functionality is given different names. The most
frequently encountered are Rear wheel Lift-off Protection (RLP, Continental), and rear wheel lift-up mitigation (Bosch). Also combined forms like rear wheel lift-off mitigation are widespread, while Honda speaks of pitching control for their C-ABS system. From a physical point of view, none of the systems can guarantee to protect the rear wheel from lifting from road contact under any circumstance, and all of them are rather mitigation functions. However, in this study all terminologies are used as suited to the context of the addressed systems, while RLP is generally used as the abbreviated form.
2.3 State of the Art of BST-Countermeasures
Honda C-ABS Brake-by-Wire
Super-sport motorcycles typically feature a short wheelbase and relatively high center of gravity. Under strong braking, they are therefore prone to large pitching motion and flip-over tendency. Introduced in 2009, Honda’s electronically controlled Combined- ABS (C-ABS) in Brake-by-Wire architecture was the first in this vehicle category to address this issue92.
Driving experiments revealed a correlation and time lag between brake inputs and re- sulting pitch effect. Moreover, it was found, that when the slip ratio at the front wheel was increasing during that narrow time window, the undesired pitching did not occur. Creating such a configuration on purpose was not possible with conventional brake systems and therefore the Brake-by-Wire architecture was chosen. The occurrence of pitching is forecasted from the rate of pressure increase, and a reaction is triggered in two steps: Firstly, a very quick increase in front brake pressure reduces the brake force in the tire contact patch through increasing slip. Secondly, this triggers a subsequent pressure reduction through activation of ABS functionality, which is achieved earlier than for contemporary conventional ABS / RLP configurations.
The used hardware setup is as follows and for packaging reasons subdivided into five components: The Electronic Control Unit (ECU) as well as a valve and power unit for each brake circuit. While the valve units contain switching valves, the so-called “stroke- simulators” and pressure sensors, the power units are electrically driven master cylin- ders. Under operation, the hydraulic connection between the rider’s master cylinders and calipers is disconnected by switching valves, while the brake demand is measured by pressure sensors and ordinary lever feel is generated by the stroke-simulators. Final- ly, output pressure is generated by the power units and monitored by further pressure sensors, while ABS functionality is triggered by wheel speed sensors.
The by-wire architecture allows the implementation of arbitrary brake force distribu- tions (BFD) and combined functions upon activation of either brake lever93. The rear brake is always actuated in advance. Front braking leads to a front wheel oriented BFD with a typically rather low and constant contribution from the rear wheel, while strong rear braking also activates the front brake a great deal, yet with reasonable gradients (see also chapter 3.6.6). Moreover, in contrast to conventional CBS, the BFD during brake actuation can be different from that during brake release.
92 Nishikawa et al. (2008): Pitching Control using Brake-by-Wire, proceedings pp. 430-446
93 To a certain degree, the meanwhile discontinued first generation BMW Integral-ABS with integrated
electric brake booster would also have offered this option from a hardware point of view. Among oth- ers, see: Stoffregen (2010): Motorradtechnik, Chapter 11, pp. 384-386.
Compared with the previously defined requirements and strategy for BST optimized corner braking, the C-ABS is only lacking the integration of roll angle information to allow further adaptations to the cornering state. Despite this theoretical limitation, its corner braking performance with easy controllability up to high decelerations was well approved both in racing94 and the test drives in context of this study (cf. chapter 5).
Bosch Motorcycle Stability Control (MSC)
Following the series introduction of a roll angle sensor for traction control systems in 200995, the use of this sensor information for a cornering approved brake system was just a question of time. In 2013, Bosch presented their Motorcycle Stability Control (MSC) together with KTM96. The system is based on the enhanced version of Bosch’s ABS 9 generation of motorcycle brake systems (ABS 9 ME97), that follows the standard
layout with valves and pump known from the Electronic Stability Control (ESC) hydro units of passenger cars, but is just much smaller. In the debut version of MSC, active pressure generation is only implemented for the rear and a sensor cluster is used that measures two turn rates and accelerations in all three dimensions of space. Through an inclined mounting position in the vehicle, rotated by 45° around the pitch axis, one gyroscope is measuring the pitch-rate and the other a combination of roll- and yaw-rate. This arrangement allows to compute information for all six degrees of motion, especial- ly roll and pitch98, which are considered both in the adaptive BFD (called eCBS) and ABS control. Moreover, as an alternative approach to the Honda C-ABS, pitch rate and deceleration signal can be used to improve rear wheel lift-up mitigation.
Even though only very few details about the control strategy of MSC have so far been published99, it fulfills all previously defined requirements for BST optimized corner braking. First tests by motorcycle journalists100,101 revealed a significant reduction in stand-up tendency and in general, that MSC operates smoother than the standard system under cornering in order to avoid everything that might disturb stability close to the physical limits.
94 Tani et al. (2010): Brake-by-Wire System for Race Motorcycle, proceedings pp. 378-395 95 Landerl et al. (2010): Enhanced rider assistance, proceedings pp. 362-377
96 BOSCH (2013): Bosch motorcycle stability control, Bosch Press-Release PI 8314 CC 97 BOSCH (2011): New Bosch ABS for all motorcycle types, Press-Release PI 7438 CC
98 Willig et al. (2012): New Inertial Sensor Unit for Dynamic Stabilizing Systems, proceedings pp. 66-84 99 Matschl et al. (2014): Motorcycle Stability Control, proceedings pp. 128-154
2.3 State of the Art of BST-Countermeasures
Comparing tests between the same vehicle with and without MSC showed that experi- enced riders under controlled conditions and high friction road surface can achieve the same corner braking performance without the assistance of MSC. Moreover, it was found that the control quality of the standard Bosch ABS (or equally sensitive contem- porary systems) already allows safe full lever braking for roll angles up to 35° on roads with “normal” friction coefficient102.
However, this just holds true when the maneuver is done intentionally. Before the back- ground of the BST chain of effects involving startle reactions and confusion of the rider, it is well to be expected, that the improved functionality of MSC will be a great help in a real world BST critical situation.
Furthermore, as the name MSC already suggests, it is much more than just a cornering sensitive brake system. Including the engine management, it also features advanced Motorcycle Traction Control (MTC), allowing add-on functions like launch- or wheelie control. In the sense of a scalable system architecture, it is further already prepared to include additional control systems, such as semi-active suspensions103.
Finally, the inertial measurement of MSC theoretically also offers the possibility to manipulate roll and yaw motion of the vehicle in terms of controlled drifting by strong over-braking of the rear wheel, which could be applicable at the end of the BST chain of effects. However, such a measure brings along all insecurities of rider coupling, motion, and not least acceptance that were already addressed in context of predictive and auton- omous brake systems in chapter 2.3.1.
More details on state of the art of motorcycle brake systems and technology can be found in literature104. Just for completeness, also the dynamic tire characteristics under transient combined slip105 conditions have an influence, but are not further addressed.