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From: henry@ginger.sri.com (Henry "Credible" Pasternack)
Newsgroups: rec.motorcycles
Subject: How a Motorcycle Works, Chapter III.
Message-ID: <6512@unix.SRI.COM>
Date: 5 Dec 89 22:16:32 GMT
Sender: news@unix.SRI.COM
Reply-To: henry@ginger.sri.com (Henry "Credible" Pasternack)
Organization: SRI International
Lines: 140

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.