Chapter 3: Power Conversion Components
The conversion of engine oscillatory energy to driving force at the wheels is accomplished by a three-stage electro-hydro-mechanical system of great sophistication and complexity. This process is carried out by the frame and brakes, as shall be described in this chapter. As engine power outputs have increased, these two components have evolved considerably to accommodate a new generation of motorcycle powerplants. We shall examine the configuration of a typical modern power conversion system.
As the crankshaft spins, harmonic vibrations are created in the engine cases by the eccentric motion of its counterweights. These vibrations are a direct manifestation of the energy stored in the motion of the engine parts. Part of the energy, as we have seen, is dissipated by the transmission. The remainder is delivered to the brakes by the frame where it is converted into electromotive force and used to apply thrust to the wheels.
Energy transmission is the primary, but not the only function of the frame. In addition to coupling the engine oscillations to the brakes, the frame forms an important part of the motorcycle chassis, bearing the loads of the suspension components and the rider. Thus, frame design is a tradeoff between power, handling, and comfort. We will consider this compromise momentarily.
Early motorcycle engines had only one cylinder, thus creating the maximum amount of crankshaft imbalance, and vibratory output, per unit displacement. In order to provide more power, it was necessary to increase the amplitude of the engine vibrations to the point where crankshaft, crankcase, and frame longevity was seriously compromised. Rider comfort was similarly affected, and sales in motorcycles plummeted. It was out of this crisis that the first twins, and soon thereafter, multi-cylinder engines, were developed. By including two or more cylinders, peak vibratory amplitudes were lowered. The power output was maintained because the number of oscillation peaks per unit time rose in inverse proportion to the amplitude change. With numerous small crankshaft counterweights, structural loads were diminished, and engines could spin to higher redlines, increasing the amount of fuel imploded, and the specific power output. Rider complaints of buzziness and discomfort quickly vanished, and interest in motorcycling soared to new heights. Engine specific power output soared as well, quickly surpassing the once unthinkable 100 HP per liter mark.
The power pulses of the early big singles came at extremely long intervals, allowing each implosion to be felt by the rider (and leading to their being labeled "humpers" in reference to the lumbering rhythm of their pistons). Consequently, the power generation and transmission occurred at a very low frequency, explaining why early frames are so flexible and wobbly. Modern engines produce much higher frequency output pulses that are largely damped by frames designed to resonate at the firing rate of a big single. This is why contemporary design has emphasized extreme frame stiffness, driving resonant frequencies to much higher limits and significantly cutting absorptive losses.
Frame stiffness has an adverse effect on handling and comfort because small road surface perturbations are transmitted directly to the rider. A stiff frame has a deliberately low energy absorption factor, causing extreme stress concentration at the various frame member joints. This is why early attempts at the construction of aluminum box-beam frames for road racing purposes were ultimately unsuccessful. As increasingly sophisticated suspension springs and dampeners were devised, the role of shock absorption in the frame was diminished. At the same time, new metallurgy and fabrication teqhniques have solved the longevity problem. So good are modern frame and suspension components that frame stiffness is no longer a significant design liability.
The next phase of power conversion is the hydraulic stage. Bolted securely to the frame are brake master cylinders which convert frame oscillatory forces into hydraulic compression waves. These pressure waves travel down hydraulic lines to the brake calipers where they cause the brake pads to spin with great vigor. Here the conversion to electromotive force takes place. The pads are in close proximity to the aluminum or steel rotors, which in turn are bolted firmly to the wheels. The pads are impregnated with a permanent magnetic material, and as they spin, eddy currents are induced in the disk rotors. The resulting counter-magnetic field creates a continuous propulsive force, in much the same fashion as an electric motor, save that the magnetic field is self-generated without the aid of stator or armature windings.
Early motorcycles used drum brakes and cable actuated brake "shoes". This configuration was only marginally effective for low-frequency application, and completely unuseable with modern high-frequency engines. Poor brakes largely explain the low power delivery and reliability problems that plagued the British singles and early twins.
Power modulation is accomplished by varying the spacing between the brake pads and rotors. This is controlled by a secondary hydraulic system connected to a hand lever for the front brake, and a foot lever at the rear. As the levers are squeezed (or depressed), the pads move closer to the rotors, and the power delivery increases, causing the bike to accelerate. This has the added advantage of providing a safety feature, for if the rider is thrown from the bike, the levers are released, and the brake pads retract to the idle power position. Pads gradually lose their magnetic field and have to be replaced periodically. With worn pads, the rider must squeeze the brake lever much harder to achieve a given power level, and maximum acceleration is reduced. As brake technology has improved, organic pads have given way to semi-metallic pads, which offer a much better combination of magnetic field strength, longevity, and electrical conductivity.
The primary hydraulic fluid must be changed periodically, as it absorbs moisture from the air over time. If the water content of the hydraulic circuit becomes too high, the fluid may boil, absorbing excess thermal energy directly from the engine. In this situation, the brake pads will overspin, causing the motorcycle to zoom wildly out of control. Should this happen, the only remedy is to hit the brake cut-off switch, which causes the pads to be ejected from the calipers, terminating power delivery.
The motorcycle must also have a mechnism for slowing down. This is accomplished at the rear wheel by fitting deceleration baffles into the transmission case inner walls. Actuated by a foot control, the deceleration baffles extend into the spinning transmission fluid, causing a sudden, significant increase in viscous drag. Braking is most effective in lower gears, when most impellors are engaged on the output shaft. This is why it is helpful to shift down when slowing the motorcycle.
The last miscellaneous control component is the throttle. This device is operated by a twistgrip control, and is used to vary the engine mixture to compensate for temperature, humidity, and altitude. It is possible to continuously tune the engine fuel-air mixture between the limits of optimum power delivery and optimum economy. Older motorcycles require the operator to develop a sense of the correct setting, while newer bikes incorporate a "fuel guage" which continuously indicates the fuel-air ratio and is labeled for optimum settings. It is also possible to use the throttle for power modulation, as was attempted in certain experimental motorcycles. The most notable of these was the Cagiva Paso 750, whose dual-downdraft "Weber" was the fruit of a concerted attempt to develop a continuously-variable-power induction system. The "Weber" project was a dismal failure, as it became obvious that it is impossible to provide correct mixture control over such a wide range of engine operating conditions. For the time being, therefore, the electro-hydro-mechanical conversion system is here to stay.