BRP ski-doo MACH Z 1000 SDI, Руководство пользователя

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06-32
MMC2005-001_06A.FM
SECTION 06 - DRIVE SYSTEM
EFFECTS OF THE DRIVEN PULLEY
CAM
The purpose of the driven pulley cam is to sense
the torque requirements of the drive axle and feed
a portion of the engine torque, which has been
applied to the driven pulley, back to the sliding half
of the pulley. It is this side force that signals the
downshift and provides side thrust to give traction
to the drive belt.
The cam is acting like a screw pushing against the
sliding half of the pulley. A large cam angle will act
like a coarse thread while a small cam angle will
act similar to a fine thread. The smaller the cam
angle, the greater the side force on the sliding half
of the pulley and the slower the upshift will be.
This will result in higher engine RPM.
A larger cam angle will allow the pulley to upshift
at a lower engine speed. Less side force will be
exerted on the sliding half of the pulley and the
pulley will upshift more rapidly.
On downshift, a smaller cam angle will backshift
more easily and, again, tend to keep the engine
RPM higher. A larger cam angle will be harder to
downshift and will load the engine and reduce the
RPM.
If all other variables in the pulleys are kept con-
stant, a cam change with a smaller angle will result
in a slower upshift and a faster downshift. Engine
RPM will remain higher. A change to a cam with a
larger angle will result in a faster upshift and the
downshift will be slower. Engine RPM will be lower.
Remember the drive pulley signals or controls the
upshift of the transmission while the driven pulley
signals the downshift largely because of the effect
of the cam.
The standard factory cam will probably work well
for most
woods type cross-countries, while a
smaller angled cam may prove to be better for
high speed lake cross-countries.
Top speed and low ET’s are drag racers’ and radar
runners’ most important concerns. Because back-
shifting is not at all important in these races, most
racers experiment with larger cam angles for the
fastest possible upshift.
Multi-angle cams are sometimes used by racers
needing a good holeshot. They generally work
best on vehicles where no track spin is encoun-
tered. As a vehicle idles on the starting line, the
exhaust temperature cools thus slightly lowering
the optimum HP RPM of the engine. Because of
this, a steeper (larger) angle cam can be used to
upshift more quickly, and lower the RPM to work
with the cooler exhaust. As the exhaust heats up,
the optimum HP RPM increases. A multi-angle
cam reduces to a shallower (smaller) angle as the
clutch shift out and the RPM is increased to match
the
hot HP curve of the engine. This phenomena
is more pronounced on engines with narrower
powerbands.
Oval and snowcross racers need the best of both
worlds. A good holeshot is critical but backshifting
must be quick in order to have good response out
of the corners. They may have to change cam an-
gles depending on what type of track layout is en-
countered.
Driven pulley cams are helices. A helix is mea-
sured in lead. Lead is the distance a point moves
along the axis of rotation in one revolution of the
helix. (Screw threads are a helix).
The helix angle is computed from the lead and the
circumference of the helix.
1. Circumference (C)
2. Helix angle A
3. Lead (L)
A01D1WA
3
1
2