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Sprocket
Ratio Theory
General
In the early days of the snowmobile, the only
changes made to the drive system to increase
performance were changes to the sprocket ratio.
From that stage, manufacturers have moved
ahead in understanding the complete drive
system. The ways to change and improve the
operation of the drive and driven clutches are
much better understood now, and the perform-
ance gains we see today are mainly a result of
modifications in these two areas.
Sprocket Changes Affect Clutching
The sprocket ratio on the machines today is
chosen by the manufacturer after all the data for
that machine is known. The horsepower curve,
operating rpm, clutch ratio and sled weight are all
used to calculate the sprocket ratio that will
produce the best performance for that machine.
Field testing in different snow conditions is then
conducted to confirm the best sprocket ratio to
use. After the sprocket ratio has been selected,
the drive and driven clutches can then be
matched to the snowmobile.
_
NOTE: Any change in sprocket ratio will
-
have an effect on the operating rpm of the
clutches.
Many people have the misconception that by
increasing the sprocket ratio (example: from
17:39 to 19:39) the rpm of the engine will go down.
The opposite is true on the Arctic drive system.
From the discussion of the driven clutch, we said
that when driven load (load on the track) is
increased above torque delivered, the driven
clutch becomes dominant and overrides the drive
clutch, resulting in a back-shift while full rpm is
maintained. This explains why the rpm will
increase when the sprocket ratio is increased. By
going to a large sprocket ratio, the driven clutch
senses a heavier load from the track. If the driven
clutch is dominant to the drive clutch, it will
override the drive clutch and shift into a lower
ratio. This will then increase the rpm of the
engine.
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Before improvements were made in the speed
sensing ability of the driven clutch, a change to
larger gear ratio did, in many cases, decrease the
rpm of the engine. The reason for this was that the
torque sensing driven clutch was not dominant to
the drive clutch.
In the clutch systems of today, the brain of the
drive system is the torque sensing driven clutch.
The driven clutch knows what the load on the
track is and reacts to that load by shifting into the
clutch ratio the engine has power to handle
without losing rpm.
Drive Belt
Theory
The drive belt plays an important part in drive
system performance. The belt dimensions and
construction are two variables that influence the
performance of the drive system. Drive belts have
been vastly improved in the last few years. With
the tremendous increase in horsepower, drive
belts have been developed that far exceed
original estimates of possible power ratings for a
single belt drive system.
Drive Belt Dimensions
The drive belt dimensions are very closely
calibrated when the drive system is matched to
the machines at the factory. The two dimensions
that are important to the performance of the
machine are the
a.c.
(outside circumference) of
the belt and width. Both of these dimensions will
influence the shifting characteristics of tile
clutches.
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