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PVT System
Final Gearing
The final drive gear ratio plays an important role in how much
vehicle load is transmitted back to the helix. A tall gear ratio
(lower numerical number) typically results in lower initial
vehicle acceleration, but a higher top-end vehicle speed. A
lower gear ratio (higher numerical number) typically results in
a higher initial vehicle acceleration, but a lower top-end vehicle
speed.
Choosing the proper gear ratio is important to overall drive
system performance. When deciding on which gear ratio to use,
the operator must factor in the decision where the snowmobile
will be ridden, what type of riding will be encountered, and the
level of performance the operator hopes to achieve.
Gearing a snowmobile too low for extended high-speed runs
may cause damage to the drive belt and drive system, while
gearing a snowmobile too high for deep-snow, mountain use
may cause premature belt and clutch wear.
Typically, it is recommended to gear the snowmobile with a
slightly higher ratio than the actual top speed the snowmobile
will ever achieve.
1:1 Shift Ratio
A 1:1 shift ratio occurs when the drive clutch and the driven
clutch are rotating at the same RPM.
The mathematical vehicle speed for a given gear ratio at a 1:1
7
shift ratio is represented in the chaincase gearing charts located
in the Final Drive Chapter.
Low / High Ratio
Low ratio is the mechanical position when the drive belt is all the
way down into the drive clutch, and all the way out on the driven
clutch. High ratio represents when the drive belt is all the way
out on the drive clutch, and all the way in on the driven clutch.
Driven Helix / Ramp
The helix cam is the primary torque feedback component within
the driven clutch, regardless of driven clutch type. The
beginning angle of the helix must transmit enough torque
feedback to the moveable sheave in order to pinch the drive belt
while minimizing belt slip. The flatter or lower the helix angle,
the more side force will be exerted on the moveable sheave,
while the steeper, or higher the helix angle, the less side force
will be exerted on the moveable sheave.
7.3
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