WSB quick shifters
The regulations governing transmissions in the World Superbike Championship require engineers to stick relatively closely to the design of the homologated units, on the production machine. Materials and ratios can be changed but the layout of the main components must remain the same, ruling out the possibility of using systems such as a seamless shift. The addition of quick shifters is allowed, however, and their operation and functionality can be a key factor in a bike's overall performance.
As a brief overview, a quick shifter allows for full-throttle gearshifts without having to use the clutch. As the rider shifts gear, the system senses the impending change and cuts the ignition, allowing the shift to take place. The key part of this operation is the sensing of the start point of the shift, the accuracy and reliability of which is essential for a smooth gear change.
In the past, this has been accomplished using a pressure sensor or transducer, both of which rely on the mechanical operation of a switching mechanism by the shifter. There are, however, a number of disadvantages to this type of system that can disrupt shifts and lead to unreliability. The most notable of these is their susceptibility to vibration and shock, isolation from which can reduce the sensitivity of the system. The gear lever also has to move a certain amount in order to initiate the shift. This, combined with the fact that a spring or tensioning system is relied upon to hold the switch open - the strength of which dictates the force needed to initiate a shift - can make it difficult to tailor the system to different riders, linkages and transmissions.
In recent years, there has been a move away from this type of sensor towards the use of strain gauges to detect the onset of a shift. By integrating a strain gauge into a load cell attached to the shift lever, the exact point at which the rider wants to shift can be detected. (A load cell is simply a strain gauge with an amplifier that produces a voltage relative to the force being applied. This is typically 2.5 V for no load and increases to 5 V for a positive load or decreases to 0 V for a negative load.)
There are, however, a number of issues that increase the complexity of using a load cell to actuate the throttle cut-off.
The first is complexity. Unlike a simple switch, the load cell will require additional electronics to process the signal and interface the system with the onboard control electronics. Then there is drift from zero. This means that when the force is released, the load cell does not always return to the no-load voltage.
Next comes temperature drift, a serious problem with all load cells. When the cell changes temperature, the output voltage changes radically relative to force. This means that when the bike is cold, not much force is required to initiate a shift, but when the bike is hot then a lot more force is required. And then there's creep. With age and use, the load cell's zero and load values will change. This means the electronics will require regular recalibration.
However, most manufacturers of quick shifts provide a compact digital signal processor unit that is easily integrated with existing controls and compensates for these shortcomings. Despite these issues, load cells still provide a far superior solution to a pressure switch, with the new generation of shifters now dominating among teams.
Fig. 1 - Load cell or strain gauge shifters are easily added to a bike's existing gear change system
Written by Lawrence Butcher