By Stephen Payne
Principle of Operation:This clutch
uses a fixed ramp comprising of an engagement angle and a
shifting angle.The moveable shieve is positioned by a roller,
linked to the spider. The roller moves up and down the ramp
based on spring pressure, amount of weight attached to the
roller and the RPM of the engine.The engagement area of the
ramp is where the roller contacts initially.The angle of the
engagement area, the amount of weight on the arm and the spring
pressure will determine the actual engagement RPM.The angle
of the engagement area and spring pressure will have the most
affect of engagement RPM.Steeper angles and higher spring
pressure increase engagement RPM, while shallower angles and
softer springs reduce engagement RPM.
The roller is attatched to the spyder
with the arm. Centrifugal force pushes the ramp toward the
Different angle ramps are available
for different engine and snowmobile weight combinations. The
shifting part of the ramp determines the running RPM of the
engine while moving. Shallow ramp angles and/or lighter weights
result in more engine RPM while shifting. Steeper angles and/or
heavier weights reduce engine RPM while shifting.
As engine speed is increased from
idle, the roller will force the ramp to move towards the belt.
The ramp is attached to the moveable shieve, so that they
The roller should reach the end of
the engagement area at the same time as the moveable shieve
touches the belt. This will result in a smooth engagement.
If the roller reaches the end of the engagement ramp and there
is some clearance between the moveable shieve and the belt,
this will cause a harsher engagement. The harshness will be
a function of the belt clearance.
As the roller moves on the shifting
part of the ramp it will cause the moveable shieve to position
closer to the fixed shieve, causing the belt to move up and
change ratios. The up-shift will be a function of throttle
position and load for a given ramp angle and roller weight.
The spring pressure has most effect
on engagement RPM and very small amount on the shifting speed
of the engine. A heavy spring will affect shift RPM a small
The weights are bolted one on each
side of the arm where the roller is located. The weights are
available in different sizes to adjust the running speed of
the engine with a given ramp angle.
There is a large diameter shim between
the moveable shieve and the spider to vary the roller position
at engagement. Two thick shims (.06 ea.) engage at 4500 RPM
with a six degree engagement angle. Engagement will go up
500 RPM for each .03 in shim thickness reduction. The smaller
shim fits over the post to accommodate varying belt widths.
Different sizes of shims are available
to fine tune roller position and belt clearances.
The clutch will come calibrated for
a specific snowmobile. Modification to the engine, driven
clutch or weight of the snowmobile will require fine tuning.
The following are examples:
- Engagement is fine, but shifting RPM is too high. The
corrective action in this case is to increase the amount
of weight on the arm (7 gram to 8 gram), if the RPM is 600
RPM or less. If the RPM is 600 greater than is required,
a ramp with a steeper shifting angle is needed (40 to a
45 degree angle).
- Engagement too low. The corrective action in this case
is to install a stronger spring (200-400 RPM). If more RPM
is required, then a ramp with a steeper engagement angle
is required (15-12 degree).
- Engagement is too harsh. The corrective action in this
case is to remove belt clearance. Changing the thickness
of the small shim to a thinner one will reduce belt clearance
(0.020 inches will have a noticeable effect).