BRP ski-doo MACH Z 1000 SDI, Handbook

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SECTION 03 - CHASSIS PREPARATION
MMC2005-001_03A.FM
03-15
CORNERING DYNAMICS
The ideal situation, while going through a turn, is
to keep the snowmobile as flat as possible without
the skis or track losing contact with the driving sur-
face.
As you enter a corner and turn the skis, the rest of
the vehicle will want to continue straight ahead. If
the skis do not bite the surface, they will start slip-
ping and the vehicle will not turn as tight as the
skis are turned. This is called
understeering or
pushing. If the skis bite very well and the track
starts sliding out, then the vehicle is
oversteering
or is said to be loose. If the ski and track traction
is balanced, then the vehicle will maintain a good
line though the corner. Because the center of grav-
ity of the vehicle wants to continue straight ahead
and because the center of gravity is above ground
level, weight will be transferred to the outside of
the vehicle. This causes the machine to roll to the
outside. As the radius of the corner gets tighter
and/or speeds increase, the machine rolls more,
and more weight is transferred to the outside of
the vehicle until the front or back loses traction or
the vehicle tips over.
Roll can be reduced by installing stiff springs on
the front suspension and/or a lot of preload, but
this will cause a harsher ride than necessary. Low-
ering the center of gravity will also reduce roll but
there are practical limits as to how low the center
of gravity can go. Most vehicles are equipped with
an antiroll bar or
stabilizer bar. Common terminol-
ogy will refer to it as a sway bar. (It is inaffect an
anti-sway bar) The bar is mounted to and pivots
on the chassis. The ends of the bar have lever
arms from 3” to 7” in length. The ends of the le-
vers are connected to the front suspension. As the
outside suspension is compressed during a cor-
ner, the bar is twisted and forces the inside spring
to compress also. The bar is “borrowing” spring
pressure from the inside spring and adding it to
the outside spring. The suspension can now resist
more chassis roll (see following illustration).
By having a sway bar in the suspension, softer
springs can be used to achieve a good ride be-
cause the bar will help control roll in a corner. The
bar has no affect on ride when traveling straight
ahead over bumps that are even from side to side.
However, if only one ski encounters a bump, then
the bar will transfer energy between the springs.
This leads to another design decision. The diame-
ter of the sway bar determines how much spring
pressure will be
borrowed from the opposite
spring. A smaller bar will twist more and not trans-
fer as much energy. A larger diameter bar will
transfer more energy which will reduce chassis
roll, but will produce a harsher ride on uneven,
bumpy terrain. A smaller diameter bar will give a
more compliant ride on the nasty bumps but it will
allow the chassis to roll more in corners. A cross
country sled will use small to medium diameter
bars while oval and lemans racers will use large
diameter bars.
The length of the lever arm also affects the stiff-
ness of the sway bar. A shorter lever will stiffen
the bar and a longer lever will soften the bar. Many
lever arms will have 2 holes to mount the connec-
tor linkage. The hole closest to the bar will act stiffer.
When changing the sway bar diameter you must
also change the ball joint blocks and bushings.
For snowcross racing some racers prefer to dis-
connect the sway bar. This will let the front sus-
pension act more independently, as the suspen-
sion is no longer coupled.
Another little known fact that has a large affect on
roll is the limiter strap length. As mentioned earli-
er, if the limiter is lengthened, the front suspen-
sion will extend during acceleration, which reduc-
es ski pressure. If this vehicle was in a corner
when power was applied, it would have quite a bit
of chassis roll and the inside ski will start to lift off
of the ground. Shortening the limiter will have a
very large affect on controlling roll. A general
guideline for initially setting limiter length for good
ski pressure and reducing roll is to have the front
and back of the track touch the ground at the same
time when you set the back of the vehicle down.
If the front of the track touches much sooner than
the rear, there will be quite a lot of weight transfer
and chassis roll during hard cornering.