9.7
REAR SUSPENSION
9
REAR SUSPENSION OPERATION
Operation
The primary function of the rear suspension is to provide a
comfortable ride in all types of riding conditions. It separates the
rider from the ground, while allowing for complete vehicle
control. The rear suspension also must provide weight transfer
and maintain track tension.
The rear suspension has many adjustable features for fine tuning
to achieve optimum comfort. The suspension can be adjusted to
suit rider preference and deliver excellent performance for a
given set of conditions. It should be noted, however, that
suspension adjustments involve a compromise or trade off. A
machine set up to perform well in the moguls would not suit the
preference of a groomed trail rider.
Weight Transfer
The shifting of weight from the skis to the track is called weight
transfer. As engine torque is applied to the drive axle the torque
is transferred to the track, pulling it forward. This energy also
tries to pull the suspension forward. The front torque arm reacts
to this force by pushing down on the front of the track, in effect
applying more weight to the front of the track and reducing the
weight on the skis. It is important to note that energy used to lift
the front of the machine is not available to push the vehicle
forward.
Changing the angle of the front torque arm changes the
suspension's reaction to the force. Adjusting the length of the
limiter strap will change the front torque arm angle. Shortening
the strap limits the extension of the front of the suspension;
reducing the angle of the torque arm and increasing ski pressure
during acceleration. Lengthening the strap allows the front of
the suspension to extend further; increasing the angle of the
torque arm and decreasing ski pressure during acceleration.
Limiter strap adjustment has a great affect on weight transfer.
Limiter straps only affect acceleration. It is important to check
track tension whenever limiter strap length is changed.
Front track shock spring preload also affects weight transfer. A
stiffer spring and/or more preload on the spring transfers more
weight to the track. A softer spring and/or less preload keeps
more weight on the skis. Keep your riding application in mind
when choosing springs and setting spring preload. Soft springs/
preload will increase ski pressure, but may bottom out. Stiff
springs/preload will provide more track pressure (reduced ski
pressure), but may result in a less comfortable ride
During acceleration, the rear of the suspension will compress
and the IFS will extend, pivoting the machine about the front
torque arm. Because of this pivoting effect, rear spring and
spring preload also have some effect on weight transfer. Softer
rear springs, or less preload, allow more weight transfer to the
track and reduce ski pressure. Stiffer rear springs, or increased
preload, allow less weight transfer to the track and increase ski
pressure. The main function of the rear torque arm is to support
the weight of the vehicle and rider, as well as to provide enough
travel to absorb bumps and jumps.
Shock valving also has an effect on weight transfer. Refer to
shock tuning information in this chapter. Scissor stops also
affect weight transfer. See scissor stop information also in this
chapter.
Rear Scissor blocks also have a big effect on weight transfer.
See
“Scissor Stop Adjustments” on page 9.8.
Suspension Coupling
On all Polaris snowmobile rear suspensions, there are two
torque arms that control the movement of the rail beam. Prior to
the advent of suspension coupling, these torque arms could
move independently of each other. Rear suspension coupling
links the movement of the front and rear torque arms to each
other.
The front rear scissor stop (FRSS) couples the movement of the
front torque arm with the rear torque arm and limits the amount
of independence between the movement of the front torque arm
and the rear torque arm.
When hitting a bump, the front torque arm starts to compress.
The FRSS links that movement to the rear torque arm, causing
it to compress and raise the rear suspension up as one, allowing
the suspension to hit the bump only once and eliminating
kickback. The factory setting are usually adequate for all riders
in all conditions.
The rear rear scissor stop (RRSS) couples the movement o the
rear torque arm with the front torque arm and limits the amount
of independent movement between the rear torque arm and the
front torque arm.
Adjusting the RRSS either allows more weight to transfer to the
rear for more traction, or allows less weight to transfer to the
rear, resulting in improved cornering performance. And
adjustment dot is located on the RRSS. This dot ID at the higher
end of the scissor stop.
Moving the RRSS to a higher position will reduce weight
transfer, improve chatter bump ride and improve cornering
performance.
Rear Torsion Spring Tension Adjustment
Rear spring tension adjustments are made by rotating the
eccentric spring block. The block provides three spring tension
positions. This adjustment is easier if the long spring leg is lifted
over the roller and replaced after the block is properly
positioned. Always maintain equal adjustment on both sides.
Torsion springs are much like coil springs, although shaped
differently. The rate of the torsion spring is controlled by the
wire diameter of the spring, and the number of coils. Pre-load is
controlled by the free opening angle. See “Torsion Spring
Adjustment” on page 9.9.