Ski-Doo’s rMotion Rear Suspension
By Kevin Beilke – Editor - SnowTech Magazine - http://www.SnowTechMagazine.com
For 2012 Ski-Doo fitted their MX Z X-packages and XRS-packages with the most capable rear suspension we have ever tested – the rMotion. The rMotion suspension package was only offered to Spring buyers of these sleds and once everyone figured out how well they performed through the bumps, they wished they had ordered one as well. Lucky for us, Ski-Doo is now making the rMotion rear suspension available on many more models – the Spring-only XRS and X packages of MX Zs and Renegade X packages along with in-season TNT and Renegade models.
Some suggested Ski-Doo came out with their rMotion simply as a response to the Polaris RUSH and its progressive rate rear suspension, but Ski-Doo already had their own progressive rate rear suspension on the 2010 Freeride sleds, which is basically a long track version of the MX Z XRS. When Polaris came out with the RUSH and boldly proclaimed the only way to get a progressive rate suspension was to relocate the shock above the track, Ski-Doo engineers set out to prove them wrong and show them how to do it INSIDE of the tunnel and how to make it work better across a wider range of conditions. This is the most noticeable difference between the rMortion and other rear suspensions – it has a super-wide sweet spot. That means it works very well in a wide range of conditions without needing to be tweaked or adjusted.
The real challenge was to offer everything; great performance in big bumps, a smooth ride in small stutter bumps, a wide window of performance without adjustment and great cornering. The rMotion is a coupled suspension for controlled weight transfer, which gives it superior cornering capability with more consistent ski pressure. Pitching is reduced and the chassis remains better balanced through acceleration and braking. Another major factor here is the longer front arm of the rMotion, helping to give the sled incredible balance and consistent ski pressure for confident cornering. The RUSH is a non-coupled design so there is more pitching (rocking) and you don’t get the ski pressure that you might need unless you get your weight forward, demanding a more active rider input.
The rMotion offers the most rising rate (progressive) shock motion ratio in snowmobiling and up to 30% more travel than competitive suspensions. And it does so without raising the sled’s center of gravity (like other progressive rate suspensions) since Ski-Doo was able to keep the shock and springs down inside of the track. This has many benefits – it maintains a lower center of gravity, both for the sled and for gear that is carried on the back of tunnel in comparison to sled with high-mounted racks for cargo. Fitting the rMotion into the traditional REV tunnel also maintains excellent deep snow capability; it allows the use of Ski-Doo’s many cargo bags and fuel caddy; and it allows the use of Ski-Doo’s 1+1 seat. While the Polaris RUSH rear suspension performs well through the bumps, by using the outboard shock and a shorter tunnel it raises the center of gravity, eliminates the cargo capability of the tunnel, reduces deep snow capability, makes it impossible to carry a passenger, and makes spring preload adjustments far more difficult.
What makes this rMotion suspension different from Ski-Doo’s SC-5 suspension?
Four main things;
1. The suspension geometry has been changed to provide the most possible rising-rate motion ratio, all within the confines of the tunnel to keep the center of mass as low as possible.
2. More travel, both front arm and rear arm. The front arm is now 4.5” longer, allowing for a full 9” of vertical travel. Rear arm travel is now a full 10.7” of vertical travel.
3. Separate spring and shock motion ratios – the torsion springs compress at a different rate than the shock does in comparison to a coil-over spring on shock that compress together at the same rate. There are many benefits to being able to have different motion ratios for the springs vs. the shock. This allows adequate ride height (spring) while delivering a compliant ride at lower speeds yet maintaining resistance to bottoming at higher speeds (motion ratio). This is why the rMotion is so comfortable and able to absorb small trail chatter, yet able to absorb the bigger hits.
4. Ease of tuning. Springs, shock valving, coupler blocks, motion ratio, all adjustable and it is easy to do so.
BRP engineers took the “ease of tuning” a step farther by making it easy to adjust in standard form, or even easier with the (optional) Quick Adjust System that gives you the ability to adjust the spring preload and shock valving right up on the tunnel.
On the left side of the tunnel at the rear of the running board is a hydraulic dial that adjusts the torsion spring preload, making adjustment quick and easy along with a greatly increased range of adjustment. This is used to properly set the vehicle ride height, primarily based on rider weight. Heavier riders will find this to be a great addition as the amount of preload available is much greater than before. And instead of getting out the metal tool to crank on the preload blocks you just grab the knob and crank on it, using the advantage of hydraulics to do the work for you. You can quickly adjust the suspension preload in a matter of seconds, and will really appreciate the fine-tuning capability this feature affords.
On the right side of the tunnel at the rear of the running board is a single dial to adjust the rear shock compression damping, both low-speed and high-speed compression simultaneously. This is so much easier than getting out a small screwdriver and making adjustments, or having to reach inside of the suspension to grab a small knob (or knobs) to adjust the shock damping. Again, it really makes suspension calibration quick and easy, as it should be. The Quick Adjust system is an option for the rMotion suspension package on X models, but is not available for in-season models. These models have the more familiar cam blocks for preload and a dial on the shock body for valving adjustments.
By virtue of the torque link, more travel, uncoupled with the blocks set low, and the new motion ratio, the rMotion will be noticeably smoother through the chatter and stutter bumps. The ride quality of the rMotion is nothing short of impressive. Usually when you have a sled that rides this good in smaller stuff and slower speeds it sucks at bigger bumps and higher speeds, and vice-versa. Not this time. As you start to pound it, you are able to still have an increasing capability to go through rough terrain with less vehicle disruption (chassis pitching) or bottoming. This means the performance envelope is now wider, broader, it works acceptably well in a wider set of conditions. You will not have to adjust it as often for changing conditions. This suspension works better across a wider range of conditions than any other we have ever tested here at SnowTech.
And for 2013 the calibration indeed seems to have improved yet again. SnowTech test riders were able to spend a total of 11 days riding the 2013 versions before the snow disappeared and logged almost 2,000 miles (this is after over 2,000 miles on the 2012 rMotion). We can verify there is in fact a noticeable difference between the 2012s and the 2013s, much to our surprise. Trails that haven’t been groomed are far less of an issue, as the rMotion can swallow bigger bumps with less chassis disruption and pitching while being able to maintain ski pressure for cornering. Both the rMotion and the RUSH are progressive rate (shock speed increases as the suspension goes further into its travel), but the rMotion is coupled, the RUSH is not; the rMotion has separate spring and shock ratios, where the coil-over RUSH has the spring and shock motion locked together. Less a matter of right or wrong than it is explaining why each suspension acts and behaves the way it does. Our test riders have logged thousands of miles on both, and each one of them prefers the rMotion across the widest range of conditions for the greater comfort, more consistent ski pressure and ability to absorb larger bumps with less bottoming.
The Ski-Doo models fitted with the rMotion suspension are not only capable bump sleds through the rough, every one of them is compliant and comfortable as well, with few (if any) compromises. About the only valid complaint would be the extra five pounds added with the Quick Adjust system, as the standard rMotion only weighs 52.4 pounds while the Quick Adjust version comes in at 57.4 pounds. Riders over 200 pounds and those who like the ability to tweak the preload and shock settings quickly will benefit the most from the Quick Adjust, or if you have riders of different weights using the vehicle.
Most snowmobile riders place rear suspension performance and ride comfort very high on their list of priorities. You will not find a more capable and better balanced rear suspension across a wider range of conditions. For 2013, when it comes to no-compromise ride quality and comfort on a performance sled the Ski-Doo rMotion is as good as it gets.
The rMotion rear suspension was actually introduced on the 2011 Ski-Doo MX ZX 600 RS race sled dubbed “PCX”, but that was only intended to disguise the rMotion name until the consumer version was introduced for 2012. The version found in the 600 RS is now called rMotion Racing. Both skid frames share the same basic geometry featuring a rising rate rear shock and long front arm, but each are optimized for their intended uses. The rMotion Racing has a reinforced rear arm, improved coupling system, and reinforced powder coated rails for reduced ice build-up.
For more articles by Kevin Beilke, visit http://www.SnowTechMagazine.com