SECTION 06 - DRIVE SYSTEM
MMC2005-001_06A.FM
06-3
BASIC FUNCTIONS OF THE
SYSTEM
The TRA Clutch
We call it a clutch but that set of pulleys is a lot
more than simply a clutch. Once the system reach-
es its low ratio speed, the clutch function ends and
the pulleys become a completely automatic trans-
mission searching for the highest gear ratio that
can be pulled at the engine’s given output. In the
case of our TRA clutch, the pulleys will begin shift-
ing from a 3.8:1 ratio in low gear to a .8:1 overdrive
ratio in high gear. That is a lot of ratio change. A
typical six-speed motorcycle gearbox, for instance,
will change from a 2.38:1 ratio in low gear to a .96:1
overdrive ratio in high gear.
The ratio changing is done by opening and closing
a drive and driven pulley and forcing a fixed length
drive belt to turn around different diameters on
each pulley. The force used to
close
the engine or
drive pulley is centrifugal force. It is the job of the
ramps, rollers and lever arms to convert and con-
trol the centrifugal force.
Each engine will produce its maximum horsepower
at a particular RPM. Power will decrease at engine
speeds on either side of the peak power RPM. The
usable width of the power band will dictate where
the clutch must be calibrated to keep the engine
performing at its peak. In the power curve the
mildly-tuned engine has its peak horsepower of 64
at 5800 RPM and has a usable power band width
of 1500 RPM. The race tuned engine produces its
peak of 92 horsepower at 9300 RPM, but only has
a usable power band width of 400 RPM. The race
engine will have to have a much more accurately
calibrated clutch to be able to keep the engine run-
ning within a 400 RPM range compared to the
1500 RPM wide range of the mildly-tuned engine.
The goal of clutch calibration is to keep the engine,
at full throttle at its peak horsepower RPM and, at
the same time, to select the highest possible gear
ratio as dictated by the load on the drive axle. The
speed diagram illustrates what the goal of good
clutch calibration is.
POWER CURVES MILDLY TUNED VS. RACE
TUNED
In the speed diagram, the inclined line labelled low
ratio indicates the vehicle speed at each RPM
when locked into the 3.8:1, low gear ratio. At 8000
RPM, the vehicle speed would be just under 20
MPH if held in this ratio. The high ratio line com-
pares vehicle speed with engine RPM when the
transmission is locked into the .8 :1 high gear. At
this ratio, the vehicle speed would be just under
80 MPH when the engine is turning 8000 RPM. In
calibrating the clutches, the objective will be to
maintain as horizontal a line as possible between
the low ratio and high ratio lines. This transition
line or shift speed must be as close as possible to
the engine peak horsepower RPM.
Engagement speed of the clutch is always set as
low
as possible to avoid track slippage and to pro-
long drive belt life. The clutch must be engaged at
an RPM that is high enough, however, that the en-
gine will be producing enough horsepower to
overcome drag and allow acceleration without
bogging. In the speed diagram, the acceleration
period between 0 and about 20 MPH illustrates
the actual clutching period of the transmission.
During this time the rollers in the clutch are on the
initial angles of the clutch ramps and the drive belt
is actually slipping in the engine pulley as engine
and vehicle speeds increase to about 9000 RPM
at 25 MPH. The transmission then begins upshift-
ing to the high ratio at a constant engine RPM.
Engine speed should not increase above the cali-
bration RPM until the high ratio is achieved. If the
engine RPM exceeds the calibration RPM once
the high gear position is achieved, it is an indica-
tion that the chaincase gearing is too low. If clutch
calibration is accurate, engine speed should never
vary more than 50 RPM from the peak power
RPM. This is the optimum shift curve.
The following section will discuss each of the tun-
able components of both the drive and driven pul-
leys and provides some insight and data neces-
sary for tuning the system.
Содержание ski-doo MACH Z 1000 SDI
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