BRP ski-doo MACH Z 1000 SDI, Handbook

Page: 32 / 493

03-2
MMC2005-001_03A.FM
SECTION 03 - CHASSIS PREPARATION
SUSPENSION OPERATION/
WEIGHT TRANSFER
The purpose of any suspension system is to iso-
late the rider from the terrain while still allowing
for complete control of the vehicle. A snowmobile
rear suspension has the added requirements of
providing weight transfer and maintaining correct
track tension.
Weight transfer is essentially the shifting of weight
to the track for better traction during acceleration,
and to the skis for positive handling during corner-
ing.
The physics that apply to all rear suspensions are
basically the same. As we apply torque from the
engine to the drive axle, the torque is transferred
to the track and pulls it for forward. That energy
enters the suspension system at the rear axle and
tries to pull it forward (force “C” in following illus-
tration). The rear arm is a pivoting or sliding linkage
that only provides vertical forces at the rear of the
chassis, therefore, none of force “C” enters the
chassis at the rear arm.
1. Drive axel torque
The front arm is mounted with a pivot to both the
runners and the chassis. It is through this arm that
the major reaction to the engine torque is applied.
As the front arm begins to swivel from the load of
force “C”, it pushes down on the front of the track
(force “X” in illustration). This reduces weight on
the skis and applies more weight on the track for
better traction. The rest of the force “C” enters
the chassis through the front arm and accelerates
the vehicle (force “Z”).
If we keep force “C” constant, we can then vary
the size of the vertical and horizontal forces at the
front arm by varying angle “A”. As angle “A” is
made smaller, force “X” decreases, and force “Z”
increases. This reduces the amount of torque re-
action and more weight stays on the skis. As angle
“A” is increased, force “X” increases. The skis
then tend to lift more during acceleration and more
weight is placed on the track.
We can vary angle “A”, within limits, by adjusting
the length of the limiter strap. The limiter strap is
just that, a strap to limit the extension of the front
of the suspension. Shortening the strap decreases
angle “A” and is what we would do to set up a
machine for more ski pressure. For more track
pressure we would want to lengthen the strap to
increase angle “A”. The limiter adjustment has
the largest affect on controlling the amount of
weight transfer.
NOTE: Track tension must be checked whenever
a major change is made to the limiter length.
Front arm spring pressure will also affect weight
transfer. A stiffer spring and/or more preload will
transfer more weight to the track. A softer spring
and/or less preload will keep more weight on the
skis. Springs must also be selected to provide ab-
sorption to the intended size of bumps to be en-
countered. A soft spring will increase ski pressure
but may
bottom out on large bumps, while a stiff
spring will provide more track pressure but may
produce a harsh ride.
NOTE: In this and other Ski-Doo texts, we refer to
the front arm of the rear suspension and it’s spring
and shock absorber, as the center of the vehicle.
The ski suspension is considered the front of the
vehicle and the rear arm of the rear suspension
and it’s spring(s) and shock(s) are indicated as the
rear of the vehicle.
Also, think of the center arm as a pivot point. Dur-
ing acceleration the rear arm will want to com-
press and the front suspension will want to extend
(possibly raising the skis off the ground). Because
of this
pivoting affect, the rear spring and preload
will also affect weight transfer (to a lesser amount
than center arm changes). A softer rear spring
and/or less preload will allow more weight transfer
to the track and less ski pressure, while stiffer rear
springs and/or more preload will allow less weight
transfer to the track and more ski pressure.
1
X
Z
Y
A
C
X
A01F1VA