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From: henry@ginger.sri.com (Henry "Credible" Pasternack)
Newsgroups: rec.motorcycles
Subject: How a Motorcycle Works, Chapter II.
Message-ID: <6502@unix.SRI.COM>
Date: 5 Dec 89 19:30:50 GMT
Sender: news@unix.SRI.COM
Reply-To: henry@ginger.sri.com (Henry "Credible" Pasternack)
Organization: SRI International
Lines: 129

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.