Chapter 2: The Transmission.
In Chapter 1, we described the complex machinery making up the motorcycle engine. In this chapter, we discuss a mechanism for controlling the engine output. This device is known as the "transmission."
Because the motorcycle engine is a resonant system, the amplitude of its oscillations is proportional to the amount of energy stored in its moving parts. This value is equal to the time-integral of the energy produced in the engine minus that dissipated, i.e., the cumulative storage. If the stored energy should become too great, the magnitude of the crankshaft oscillations will grow without bounds, and the engine will be destroyed. Thus, some means must be provided to dissipate excess kinetic energy and keep the engine oscillations within normal operating limits. This energy-dampening function is provided by the transmission.
The transmission is constructed in two sections, the input section, which is connected to the engine crankshaft, and the output section, which is connected to the rear wheel. The rear wheel connection is a relative innovation, having first appeared on the race circuit during the mid-seventies. It improves the efficiency of the engine by providing road speed and engine load feedback to the power dampener system. The connection is typically made via a chain and sprocket arrangement. This layout has the advantage of being inexpensive, but is vulnerable to dirt, requires constant lubrication, suffers from backlash and tensioning problems, and requires periodic replacement. A superior method is to couple the rear wheel to the transmission via an enclosed drive shaft. Rear-wheel "shaft drive" is primarily found on expensive European touring bikes, but has lost favor in racing applications, where longevity is not a concern, because of its higher weight.
The transmission operates by dissipating the excess energy created by the engine. The rate of energy dissipation varies with the crankshaft oscillation speed, the road speed, and the transmission dissipation factor. The vast majority of transmissions are of the manual selection type, and contain four to six vaned dissipation impellors. A ratio selection lever, called the "gear shift" is used to slide impellors from the input shaft to the output shaft and back.
(The term "gear shift" is really an anachronism, left over from a time when low-cost farm tractor motors were directly coupled to the drive wheels using variable gear sets. Because the tractor loads were so high, and the engines were so weak, power dissipation was unnecessary. However, it was necessary for the farmer to physically exchange gear sets to match the tractor speed to varying terrain or applications. This was accomplished by stopping the tractor and unscrewing one gear set in order to replace it with another, known as "shifting gears." In recent times, only one manufacturer has attempted to build a passenger vehicle with a true gear shift transmission. The Audi 5000 passenger car, with its so-called "automatic" gear shift, contained a frighteningly complex mechanism for shifting gearsets without user intervention. Unfortunately, without a dampening transmission, the Audi power delivery was unpredictable, resulting in unintended accelerations. After several accidents occurred, Audi was forced to retrofit the 5000 with a standard impellor-type dampener.)
As the motorcycle moves, the rear wheel coupling causes the transmission output shaft, and the impellors attached to it, to spin. The impellors are bathed in transmission oil, which fills the inside of the transmission case. The spinning of the impellors causes the fluid to spin as well, so that as the bike speeds up, the fluid spin increases in proportion. At the same time, the engine oscillations cause the transmission input shaft, and its impellors, to spin. As the speed of the input shaft exceeds that of the output shaft, the input impellors experience drag in the dissipation fluid, resulting in the production of heat. The rate of heat production is equal to the rate of engine energy dissipation. So much energy is dissipated that the transmission and engine cases become quite warm. This heat loss is the major source of inefficiency in modern motorcycle powerplants.
The most sophisticated motorcycles have their transmission and engine components in a shared case, with a single oil bath performing the lubrication and power dissipation functions. A portion of the heat developed by the transmission is absorbed by the super-cold fuel-air implosion products, resulting in much higher specific power output. This heat supplements the energy supplied by the Coulomb environmental thermal extraction unit ("cooling system") described elsewhere in this journal. Earlier designs have the transmission in a separate case. Because the thermal conductivity between the cases is so poor, transmission temperatures are much higher in such setups. Thus, these motorcycles require separate, higher viscosity oil in the transmission.
The transmission dissipation factor is controlled by the gear shift lever. In lower "gears", all of the dissipation impellors are slid onto the output shaft. This causes the transmission oil to spin most energetically. With only the drag due to the rotation of the input shaft itself, the crankcshaft revs freely to very high energy levels. If the driver does not shift quickly to a higher "gear", the engine will be damaged. Shifting "up", impellor disks are slid in succession from the output shaft to the input shaft. Thus, motion of the output shaft results in less transmission oil spin. Simultaneously, the greater number of input impellors causes greater oil shear, increasing the drag, and the rate of engine power dissipation. This is why motorcycles accelerate most strongly in lower gears, where transmission dissipation is least.
As the motorcycle comes up to speed, a point is reached where the engine power production very nearly matches that required to overcome aerodynamic, tire, and other external sources of drag. At this point, the transmission input and output shafts move at approximately the same speed. The power dissipation is quite low, because little oil shear takes place between the input and output impellors. The remaining friction is between the moving oil and the transmission cases themselves. Modern design, has reduced this loss to less then a few percent of total engine power production.
Between the crankshaft and the transmission input shaft is a mechanical coupling called the "clutch". It is called this because it consists of a set of expanding fingers which grip the input shaft in much the same way as a bird clutches a branch. The user may decouple the clutch by actuating the clutch lever, causing the fingers to open slightly so that the shafts may spin independently. The clutch serves two purposes. First, it unloads the transmission during shifts so that the disks may be slid without damage. Second, it allows the driver to temporarily disconnect the engine from the transmission. In this condition, the engine revs increase without limit, maximizing available power. This is useful when maximum acceleration is required, or when starting out from a stop. Care must be taken that the clutch must is not held in so long that the crankshaft rev limit is exceeded.
In the next chapter, we will describe the means by which engine power is coupled to the front and rear wheels, and the method for varying power delivery.