Effects Of The Drive Pulley Spring - BOMBARDIER 1996 Ski-doo MX Z440 Handbook

Section 05 TRANSMISSION SYSTEM

EFFECTS OF THE DRIVE PULLEY SPRING

The purpose of the clutch release spring is to return the sliding half of the engine pulley and the
associated moving parts to the disengaged or neutral position at low engine RPM. The spring
tension is calibrated to work with the pressure levers and ramp angles to allow clutch engagement
at the desired RPM. As the engine speed increases, centrifugal forces increase and eventually
overcome the tension of the release spring and allow the pulley halves to contact the drive belt. As
engine speed decreases, centrifugal forces decrease and the clutch spring returns the sliding half
toward the neutral position.
As the clutch shifts out to a higher ratio, the spring balances the shift forces being generated by the
levers and ramps.
The spring tension will affect the entire shifting sequence of the engine pulley. The effect that it has
will depend upon the construction of the spring. Three things must be known about the spring to
be able to predict its effect in the clutch : 1. The spring free length; 2. The spring pressure when
compressed to 74 mm (2.9 in); 3. The spring pressure when compressed to 41 mm (1.6 in) . These
three factors are listed on the accompanying sheet.
The spring free length will give you an idea of the condition of the spring. If the spring has lost
more than 6.35 mm (1/4 in) of its listed free length, the spring is fatigued or has taken too great a
set. The spring should be replaced. The free length of the spring is its overall length when resting
freely on a table top.
In our TRA clutches, the installed length of the clutch release spring is 74 mm (2.9 in) This is the
length of the spring when the pulley is in its neutral position. The pressure that the spring applies
at this length is the factor that controls the engagement speed (all other things kept constant).
When the engine pulley is in its highest ratio position, the spring will be compressed to 41 mm (1.6
in) . The pressure the spring applies at this length will determine the RPM required to reach high
gear; again, with all other tunable factors kept constant.
As you look through the spring chart, you will see that springs are available with equal pressures at
74 mm (2.9 in), but very different pressures at 41 mm (1.6 in). You will also note varying pressures
at 74 mm (2.9 in) and equal pressures at 41 mm (1.6 in). Simply by working with the spring charts,
one can easily see how the shift speed (the speed with which the change from one gear ratio to the
next is made) and the engagement speed can be altered.
As the pressure of the spring when 74 mm (2.9 in) long is increased, the clutch engagement speed
will increase. As the spring rate is increased, the engine will be required to turn more RPM to
achieve a given gear ratio. Again, these facts hold true when all other tunable components are kept
constant.
On chart 1, spring "A" has a pressure of311 N (70 lb) at 74 mm (2.9 in) and a pressure of 1157 N
was installed. The spring has a preload of 712 N (160 lb) at 74 mm (2.9 in) and a pressure of 1201 N
the shift curve from 30 MPH up remained relatively unchanged.
Chart 2 illustrates the effect of keeping the spring preload pressure at 74 mm (2.9 in) constant and
increasing the pressure at the 41 mm (1.6 in) length. In this example, spring "A" has a pressure of
311 N (70 lb) at 74 mm (2.9 in) and a pressure of 756 N (170 lb) at 41 mm (1.6 in). Spring "B" also
has a pressureof311 N (70 lb) at 74 mm (2.9 in) but increases to 1157 N (260 lb) at 41 mm (1.6 in).
The projected effect of this spring change is shown on chart 2. Since the preload pressure at 74 mm
(2.9 in) is equal for springs "A" and "B", the engagement speed is not affected. At 95 MPH,
however, there is a loss of RPM with spring "A" in place.
05-20
the chart indicated, the engagement RPM increased 1000 RPM while