Fine Tuning The FAST M-10 Suspension
While time spent tuning and adjusting any suspension will increase the enjoyment of your new scooter, 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.
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 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 AND 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 PRELOAD
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. I have found 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.
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.)
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 GO?
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!
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!
Basic M-10 Set-Up
The basic adjustments for M-10 set-up are the rear shock coil-spring pre-load, the Full Range Adjuster/FRA (indexed sliding blocks on both sides of the suspension) and the cross-over (X-over) tube length (threaded setting under the rear shock coil spring).
Determine your weight (with gear) and find the appropriate preload, FRA and X-over settings in the calibration chart supplied with your M-10 or on the decal up under the hood. Set the coil-spring preload to the specified distance for your weight and riding style. Next, set both FRA blocks to the specified setting for your weight and riding style (use the “soft” setting for comfort, “firm” for aggressive). Remember to re-tighten the FRA retaining bolts. Check rear bumper sag; it should be between 6” (soft) and 4” (firm). If not, increase or decrease FRA settings to fall within the range.
Measure the X-over tube length, and check this setting with the adjustment chart. Adjust as needed to fall within the chart specifications.
These are only preliminary adjustments, experimentation should follow initial set-up to obtain optimum results. Most adjustments are easier to figure out when measured in millimeters (mm). If the M-10 is new, it will take anywhere from 25 to 200 miles to properly break-in the springs and shocks and the suspension usually will become softer which may require the FRA setting to be increased 1/8 to 1/4 of a setting higher.
By Mike Lehman - SnowTech Technical Consultant