Snowmobile suspensions are a lot like Dick Clark. Something on them has changed over the decades, but you're not exactly sure what.
It's easy to spot the differences between old and new front suspensions, but the rear suspensions look pretty much the same. Sure, they've gotten longer (especially in the last eight years), but we're still dealing with the same basic principles: slide rails, torque arms, scissor arms, torsion springs and shocks.
Slide rail technology has adapted to a lighter, stronger material. The use of computer-aided design has allowed engineers to manufacture lightweight rails that are stronger than anything from previous model years. Take a look at a rail from the 2009 Arctic Cat M-series sleds. The webbed rail has material added to high-stress areas without adding weight to the rail. In fact, the '09 rail is lighter than the '08 rail.
Another aspect of rail design that has had a tremendous impact on mountain sleds and engineers alike over the last 20 years is approach angle.
Approach angle is the angle between the level ground and the track as is rotates around the drive axle and returns to the slide rails. There are several ways of affecting approach angle and as demands rose for better deep-snow mobility from mountain consumers, the manufacturers' engineers have toyed with all of them. Slide rail length, lead angle, angle radius and mounting locations can impact whether the track can climb on top of the snow or trench through it. Additionally, approach angle is affected by front and rear torque arm tunnel mounts and limiter strap tension. Live angle (what happens to approach angle when the sled is in motion) is dependent on shock valving, torsion spring rate, scissor stop blocks and tunnel mounts.
Each component has been more closely scrutinized as technology developed. Limiter straps are a prime example. Limiter straps control the extension of the front torque arm from the front of the slide rails. Shorter limiter straps mean that the slide leading end of the front suspension (slide rails) stays closer to the torque arm. In the early 90s, limiter straps were designed with fixed lengths. It portrayed the thought that suspensions were designed with showroom floors in mind. If the approach angle looked good on paper, it was good enough for the snow. But you probably remember how mountain prep went on a 1994 Polaris XLT SKS: drill out a lower hole in the tunnel for the front torque arm mount, move the lower arm of the rear scissor back an inch, bend the ends of the torsion springs down to give the skid more preload and punch new holes in the limiter straps so they could be lengthened out.
Right now you're probably doing the mental math on how longer limiter straps improve approach angle. Well, they don't, which leads us to another aspect of rear suspension function and design: weight transfer. Weight transfer is the fore and aft transition of weight, as forced by inertia. Under acceleration, the front end of the sled unloads and the weight transfers back to the track. When the sled is at speed and the inertia has equaled out with the momentum, the sled's weight is neutral (balanced between front and rear suspensions). Under braking or deceleration, the weight transfers back to the front of the sled. The front torque arm of the rear suspension acts as the fulcrum of the fore and aft weight transition. The longer the limiter straps are, the more drastic the weight transfer action is. Back to the XLT example, the harder the weight transferred back to the skid, the better the sled would accelerate because of the added traction. Of course, that was before aggressive deep lug tracks were standard. Now we're back to shortening limiter straps to keep the front end down. But for these reasons, adjustable limiter straps are standard equipment on modern mountain sleds.
But as we just mentioned, other component designs changed again what had become the norm. The 2003.5 Ski-Doo Summit Rev changed a lot of the rules of suspension operation and tuning. Once deep-lug tracks were standard, the need for aggressive weight transfer changed. If a sled with a two-inch track transferred too hard, ski lift would be too high, causing a loss of control and the track would trench. Limiter straps became more of a tuning tool.
Engineers have put a lot of research and development into scissor stops and transfer rods to further enhance and control weight transfer, but most of that technology is geared towards trail and race sleds. More components are required and one of the main ideas in mountain sled suspension design is light weight. Trail sled skids are not light by comparison. Yamaha went away from transfer rods altogether with the 2006 ProMountain rear suspension, opting for the lighter and more conventional torsion-spring design.
Suspension arm travel is another area where great strides have been made, especially in the last 13 years. More of a ride characteristic than a deep snow aspect, longer suspension travel has smoothed out the long trails to the mountains. However, the longer travel and improved shock technology was a necessity in order to keep up with the advances in engine displacement and horsepower. Fast is smooth and smooth is fast. You can't have a fast sled if you can't keep it smooth over the terrain.
Shock technology has changed from steel bodies filled with low-viscosity oil to high-pressure rebuildable gas shocks in aluminum bodies with remote reservoirs and external dampening adjusters. Some models even feature shocks that use either compressed air (Fox Floats) or nitrogen (Walker Evans). Old shocks overheated and faded easily, but stock shocks on even the most basic models far surpass the old stuff. And the new technology . just check out the '09 Polaris Assault RMK.
Front suspension technology and design has led the rear suspension in every way. For many models and makes, it was normal to see more attention paid to the front suspension than the rear.
Here's the trend snowmobiling has seen in front suspensions: Everything started with leaf springs and ended with A-arms. Trailing arms were popular on three of the four OEM's lineups for several years (only Arctic Cat held out). Trailing arms were considered the best way to control suspension arm movement given the geometry of the time period from the late 80s to the early 2000s. But A-arm suspensions have won the war, offering longer travel, lighter weight and less drag.
Front suspensions have to perform several functions. First, they have to absorb bumps and dampen the movement (imagine how well the old leaf springs managed that). Second, they have to maintain steering control under the full range of motion. Third, the design has to keep the designed geometry in tact throughout the motion range.
When the front suspension moves to absorb bumps, there are a few things that happen. The ski stance has a tendency to widen out as the shocks compress. Ski alignment changes. The spindle usually moves forward as it travels and will also rotate inward through the motion. These are all side effects of geometry. A couple of decades ago, the wayward movement in each of these categories was pretty noticeable. As the front suspension compressed, the ride quality deteriorated. But modern designs have eliminated most of the unwanted movement (thanks again to computer design software) and what wasn't eliminated was designed to work towards an advantage (like lengthening the snowmobile's wheelbase as the suspension compresses).
When you talk front suspensions, you are talking about two independent mechanical groups that are contact points with the ground. The rear suspension moves in unison, acting as one component. But each side of the front suspension can contact different terrain simultaneously. It has to be designed to absorb on one side without negatively affecting the other side. Stabilizer bars and linkages were designed to connect the two sides. That idea lasted for several years, though many modern mountain sleds have dropped the bar in favor of a truly independent front suspension. It's a controversial subject that still hasn't been settled.
Even going back to the early 90s when mountain sleds were not purpose-built machines, suspension design and function is completely different (although many components still look the same). We mountain riders now have the best suspension designs and components available to us than any other segment in the sport, except limited-build racers.
And if you consider how much has changed in 10 years, it makes you wonder what technology will provide in another 10.