 |
With Polaris now offering the FAST M-10 rear suspension
on many of their models, there will be a large number of
snowmobilers discovering what the M-10 has to offer.
Those new to the M-10 will find that their previous rear
suspension tuning experience doesn’t directly apply to
the M-10, and there will be a learning curve of how to
get the most out of their new suspension, whether it be
in a new Polaris or in any other brand of machine.
While time spent tuning and adjusting any suspension
will increase the enjoyment of your new sled, it will be
especially rewarding on sleds with a FAST M-10. A
properly dialed in M-10 will most likely provide the
best ride comfort and control you have ever experienced.
In this article we will try to go over the basics of
rear suspension tuning and how to apply this to the
M-10.
Many Polaris dealers have little, if any, experience
with the M-10 suspension, thus they are not familiar
with the various adjustments or how to cure specific
rider concerns. At a minimum, they should be setting up
the M-10 equipped machines for the weight of the rider;
the M-10 is a weight-specific suspension. Most
calibrations are “ideal” for about a 25-pound range of
rider weight. If you weigh 250 pounds and the suspension
is set-up exactly the way it left the factory, you will
not be happy with the performance.
The M-10 will follow the same basic rules of physics for
overall machine set up as any other skid frame, but
there are some very unique differences and adjustments
that you need to be aware of.
Background:
Most other OEM rear suspensions utilize a “rising-rate”
design where the shock speed increases as the suspension
compresses. This arrangement provides for a mechanical
advantage during the compression stroke. While this
design works well to resist bottoming, it also leaves
the shock at a disadvantage to control the release of
energy stored in the wound-up torsion springs. It is the
duty of the springs to store the energy of the impact;
the duty of the shock is to control (the storage and
release of) this energy. Another way of looking at it is
that this; springs carry the load, shocks control the
load.
The M-10 differs in that it uses a triple-rate coil
spring arrangement (basically a position sensitive
spring rate) combined with “falling-rate” shock speed
geometry. By decelerating the shock and spring speed
through the compression travel, the shocks become far
more sensitive to vehicle speed. This enables the
suspension to use more of its travel in nearly all
conditions, fast or slow. This is perhaps one of the
biggest differences a rider may notice when he/she first
rides an M-10; the suspension is using so much of its
travel at lower speeds that the rider is convinced the
M-10 is set up too soft. “If I’m using almost all of the
travel at this lower speed, it’ll bottom at higher
speeds!” (comparing the sensation to previous
suspensions). Using “old” logic, they crank up the
springs or FRA, only to make the suspension too firm in
all conditions. This is an important difference; the
M-10 will use more of its travel more often; this is
normal!
By allowing a faster shock speed on the rebound stroke,
the mechanical advantage is reserved for the release of
the energy stored in the springs; the rising rate is
applied on the return stroke of the suspension. The
result is excellent control of suspension “kickback”.
FAST believes it is the kickback (seat slapping you in
the rear) that is most upsetting while traveling over
rough terrain. While you may be able to get an M-10 to
bottom, it will rarely (if ever) kickback.
The M-10 was also the first suspension to utilize
“coupling”. This function couples the front and rear
suspension arms together to travel simultaneously as a
parallelogram, limiting the “angle of incidence” and the
resulting kickback effect. All other coupled suspensions
are a variation of this original design; they all
attempt to simulate the parallelogram in an effort to
reduce vehicle pitching and attitude. After one arm or
the other (it doesn’t matter which, the function is
two-way) compresses so far, the other arm will also
travel, keeping the parallelogram intact. The vehicle
remains fairly level through the bumps (pitch control)
without a “rocking” action. During acceleration, weight
is transferred to the track in a very controlled manner
and more ski pressure is retained for greater control.
For presentation purposes, we will discuss the M-10 as a
single suspension, but there are in reality several
different variations that exist. The original FAST
versions are perhaps most easily distinguished by the
Allen head FRA adjuster bolts, while the later FAST
versions and the Polaris version can be identified by
the hex head FRA adjuster bolts. There are several
detail changes between the versions, and FAST now uses
many components from the Polaris 121” version. This is
due to reduced production costs associated with the
larger-scale production afforded by Polaris.
Components & Adjustments:
Let’s take a look at the primary M-10 components,
summarize their function, and go into the tuning of
each. The front arm has a preload adjustment on the
shock spring, and the limiter strap length can be
adjusted. These will have the major effect of how much
weight is transferred to the rear suspension under power
and how much ski pressure there is during deceleration.
It will also affect steady-state tracking of the sled
and performance in fresh snow. The rear arm has a spring
preload adjustment on the rear shock, and there is a
crossover tube adjustment that determines when the
overload spring engages. The Full Range Adjuster (FRA)
sets the angle of the rear shock, which affects BOTH the
rate of the shock and spring. These will be the items
you will want to adjust to achieve your desired ride
quality traits. While both front and rear arm
adjustments will have an effect on both ride and
handling, the front arm will have the largest effect on
handling and the rear arm will have the largest effect
on ride.
Front Spring Reload:
Increasing the front spring preload will increase the
amount of weight transfer to the rear suspension, which
will increase track traction. Under acceleration, the
limiter strap length will have a much larger affect than
spring preload, but the spring will have an effect. The
spring will have a larger affect on steady state and
deceleration. When the throttle is chopped, weight will
transfer to the front of the sled. A stiffer front arm
spring setting will keep more weight on the track. A
lighter setting will allow more weight to be transferred
to the skis, making them bite harder. Under steady state
cruising, a stiffer setting will take weight off the
skis thus reducing the effort to turn the handlebars.
Generally a softer front arm setting will allow the arm
to compress more in deep snow, thus reducing the
approach angle which helps the sled get up on top of the
snow.
Limiter Strap Length:
Increasing the limiter strap length will allow the front
arm to extend more under acceleration. This will also
transfer more weight to the track and lighten the bite
on the skis. A long limiter also allows the front arm to
extend into holes and low spots in the trail, which
increases ride quality. Contrary to popular belief,
inside ski lift in a corner is not solely a function of
the front suspension design and adjustment. The rear
suspension can play a huge part in how the machine will
react to power and steering inputs. A long limiter strap
length will allow the rear skid to transfer weight to
the rear of the sled and then as you execute a turn, the
inside ski will lift. Many times a simple shortening of
the limiter will make a world of difference in how flat
a sled will turn. However, a tight limiter will also
start to adversely affect ride quality.
Personal preference will dictate which set up is right
for you. A tighter limiter will also generally be used
in deep snow to reduce the approach angle and allow for
better “planing”. However, in some snow conditions
(something with a base or crust below fresh, loose
stuff) it may be beneficial to have a longer limiter to
allow the skid to dig down to the more solid snow and
get traction. This to be more applicable with shorter
tracks with shorter lugs. With a 2” lug there is plenty
of traction and the better approach angle seems to be
much more important.
A side note; too much front arm spring pressure and too
long of a limiter setting can put excessive pressure on
the front of the slide rails and two things may happen.
One, accelerated hyfax wear can occur in marginal snow
conditions and two, the sled can have its pressure
points changed to the skis and front of the rails, thus
effectively shortening the “wheelbase” of the sled which
can produce darting and ill handling.
Rear Arm:
The overall spring and shock rate of a rear arm must be
considered when determining how adjustments here will
affect the sled. I will talk about the complete rear arm
assembly to cover the basic physics and get to
particulars a bit later. From a handling standpoint, a
“stiffer” rear arm will not allow the skid to compress
or “squat” as much under acceleration, thus providing
less weight to the track for traction and keeping the
skis planted for more bite and keeping the sled flatter
in the corners. A “softer” rear arm will do the
opposite; more squat, more traction, more ski lift, less
ski bit.
As we mentioned earlier, the rear arm adjustments will
have the most effect on the ride quality of the sled.
And the basic premise of tuning here is you will match
the “stiffness” of the rear arm to the rider weight and
riding style. A heavier rider/cargo will require more
spring rate in the rear arm as will a more aggressive
rider going faster and/or riding bigger bumps. On a
non-M-10 suspension, this would be adjusted with the
rear spring preload, coupler block settings, or control
rod position adjustments. You will generally set the
“stiffness” of the rear as soft as possible to achieve
the best ride quality and yet prevent “bottoming” when
hitting bigger bumps.
What Does This All Mean?
Let us now apply this knowledge to setting up an M-10.
The outcome of this procedure will be to achieve a well
set up trail sled. Now certainly that means a wide
variety of ideas depending on who you are and how you
ride, but with the above and below information, you
should be able to tune for your application. The first
thing I would recommend is to thoroughly read through
your owner’s manual to familiarize yourself with the
nomenclature and features of the M-10. There are very
good initial set up guidelines in the manual. If your
new Polaris is fitted with an M-10, there is also a
handy reference decal on the underside of the hood that
you can refer to.
Proceed as follows for your initial set up:
Front Spring Preload:
Start with the factory setting. This will work for 99%
of applications and is a fine-tuning parameter as
described above. Note; the newer M-10s used by Polaris
are fitted with a straight rate (160 pound) spring. A
softer 140 lb. spring is available for very light
riders, really smooth trails or deep snow operation.
Many of the FAST M-10 versions use a dual-rate spring
that tend to provide a more comfortable ride in the
chatter-type bumps.
Limiter Strap Length:
Generally, you’ll find the limiter strap set using the
fourth hole from the top for lighter ski pressure, and
in the third hole from the top for more ski pressure.
Here’s how to set it: with your ski suspension preload
set where you like it, lift the rear of the sled off the
ground. Slowly lower the machine and just as the front
of the track (at the front of the rail) touches the
ground, there should be about a one-inch (1”) gap
between the rear of the track (under the rear axle) and
the ground. This will produce excellent ride comfort,
good track traction and depending on the sled weight and
riding style, good handling. If you are a more
aggressive rider and really like a flat cornering sled,
especially on smoother trails, then you should shorten
the limiter so that when you perform this procedure, as
you set the sled down, the front and rear of the rails
touch the ground together. As mentioned before, this
setting will sacrifice some ride quality, but for my
personal taste the flatter cornering is more preferable
and this starting point has worked on every M-10 I’ve
used in a variety of makes and models of sleds. If in
doubt, start with the longer strap setting and tighten
from there.
Rear Spring Pre-Load:
Where rear spring preload is the primary adjustment on
most other rear suspension designs, it is not so on the
M-10. For the most part, you will set the rear spring
preload for your weight and use the FRA for day-to-day
adjustments.
The rear spring preload on an M-10 is generally set at
the factory to 11mm or 7/16” on FAST versions, and 8mm
or 5/16” on Polaris versions. Major adjustments are
generally not necessary except for very light riders
(FAST versions) or riders over 275 pounds (Polaris
versions), then the preload will be adjusted. Check the
set-up chart that came with your sled or suspension.
Unless you are a very light or heavy rider, more of your
effort should be concentrated on the FRA position and
crossover tube length. Remember this.
Crossover Tube Length:
This length determines when the overload spring will be
engaged. This adjustment has no effect on spring preload
or general ride characteristics, only bottoming
resistance.
At full extension of the suspension and for
approximately the first 2/3rds of the travel, the weight
of the sled is supported by the main dual-rate coil
spring. As you approach full compression, the crossover
tube will contact the inner overload spring. This design
allows for softer initial travel for small bumps and
then brings in the additional spring rate for full
travel on big bumps. For an initial setting, I would
adjust this length corresponding to the rider weight as
shown in the manual. If you’ve studied the chart, you’ll
notice that the crossover tube length gets SHORTER as
rider weight increases. Why? Read on.
(Before proceeding, make sure your track tension is set
to the correct specifications.)
F.R.A. Position:
By far the main tuning component on the M-10 is the Full
Range Adjuster (F.R.A.) position. This is the adjustable
lower rear shock mount. The shaft that the rear shock is
mounted to can be adjusted higher (forward) and lower
(rearward). Increasing the FRA (towards #4) moves the
bottom shock mount forward, which increases both the
shock speed AND spring rate. Reducing the FRA setting
lays the shock down for a softer ride and less spring
preload.
Very small changes here will have large effects on your
ride quality. As the rider weight increases, you need a
higher FRA setting. Because this has such an effect on
the rate of the rear suspension, large increases in the
FRA setting will usually require a DECREASE in the
crossover tube length to reduce the amount of energy
from the overload spring. (Minor day-to-day tweaks to
the FRA won’t require crossover tube adjustments.)
Again, for a starting point, refer to your manual and
set the FRA according to your rider weight and driving
style. With the newer Polaris-style M-10s, this
adjustment is easily made with a single 9/16” wrench.
Loosen the hex bolts that secure the blocks to the
rails, and then loosen the lock nuts on the adjuster
bolts (much like track tension adjusters). Set the FRA
blocks to the desired setting, tighten the lock nuts,
and tighten the hex bolts (35 ft. lbs. is the torque
spec here).
At this point you want to shoot for four to six inches
(4”-6”) of sag with the rider(s) on board; 4” for a
firmer ride or more aggressive style, 6” for a softer
ride or smoother trails. Sag is determined by lifting
the rear of the sled just to the point of full extension
of the skid. Take a measurement from the ground to the
rear bumper, with no load on the suspension. Next, sit
on the sled and bounce it a bit. Have an assistant take
a second measurement at the bumper (with the rider on
the sled). The difference between these two measurements
is your sag. Increase the FRA to get less sag and
decrease the FRA to get more sag. Note: the additional
weight of tools in your seat storage or saddlebags full
of gear needs to be added to your rider weight when
making this adjustment. If you do lower speed riding on
smaller bumps, then lean towards the “soft” settings on
the set up chart.
You are now ready for a test drive. Make sure the
suspension shafts are well greased. One critical point
to remember is that a brand-new M-10 requires a break in
period of between 50-200 miles. During this period, the
ride will get softer and the springs may take their
“set” all at once and you could find yourself completely
collapsing the rear end. Don’t panic; just increase the
FRA setting to return to four to six inches of sag.
How'd It Do?
90% of your work should now be done! After break-in, you
can start fine-tuning to achieve the best ride possible.
Generally, set the FRA as soft as you can to only bottom
lightly on the largest bumps you encounter. This will
produce the best ride quality and will definitely
require a change depending on rider weight and
speed/size of bumps. A heavier, more aggressive rider
charging through one foot whoops will need a much higher
FRA setting than a lighter rider trail cruising on four
inch stutter bumps. Generally if you set up for your
80th percentile bump, you will have good comfort all
through the range and still have big bump capability. If
you are hammering ungroomed trails at high speeds, you
will tend to have higher settings and will sacrifice a
small amount of ride quality on the small stuff, but
this compromise will be much less than what you would
experience on other suspensions. The M-10 works well
across a very wide range of conditions. A poorly set up
M-10 usually performs better than just about anything
else!
Fine Tuning:
A method I have found that works quite well for
fine-tuning the M-10 is this: Shorten the length of the
crossover tube and find a section of trail with small to
medium bumps (two to five inches). Ride back and forth
over this section of trail at speeds that you would
normally ride and play with the FRA until you have your
best ride comfort. Learn what FRA settings produce the
best ride for a variety of speeds. Then find yourself a
section of bigger bumps. Take it easy at first, as your
crossover tube is short right now and the unit will
bottom easier. Start increasing the crossover tube
length until you get good control and comfort in the
bigger bumps. This should now be very close to the best
combination for you and your application. If you do
nothing else with you M-10, at least experiment with the
FRA settings to find the “sweet spot” for your unit.
Now go out and enjoy winter!
 Click Here
Click Here
Back To Tips &
Safety
Support Us By Supporting
Our Sponsors.
Please Let Our Sponsors Know You Found Them On
AbsoluteSnowmobile |
|
|
Support Us By Supporting Our
Sponsors.
Please Let Our Sponsors Know You Found Them On
AbsoluteSnowmobile
|
|
