SnowTech Interviews the Polaris Factory Experts!
2019-2021 Polaris INDY XC, and MATRYX models in the 129″ and 137″ lengths feature the latest evolution of the fully-coupled Polaris rear suspension – called the PRO-CC rear suspension. Offered in both 129” and 137” versions, the new Indy PRO-CC rear suspension is less sensitive to rider weight and phenomenal in the chatter bumps with the separation of the torsion springs and shock action (compared to the coil spring mounted on the shock on the RUSH & Switchback models). They feature top-line Walker Evans needle shocks with piggyback reservoirs front and rear, and a non-needle piggyback at center.
To get a more in-depth analysis on the PRO-CC equipped AXYS models, SnowTech Canada’s Hal Armstrong spoke with Marty Sampson, Product Manager, Polaris Snowmobiles and Matt Prusak, Staff Engineer, Design, Snowmobile Engineering. Marty addressed the suspension from a customer viewpoint what Polaris is trying to achieve, and Matt explained from an engineering/development perspective how Polaris achieved it.
Marty: As we looked at the Rush we really liked a lot of things about how the sled works. We liked its core capability, we liked its chatter ride if the bumps were smaller and how it worked in the big bumps. It had some areas where it was constrained and what we wanted was to have a much wider envelope of feasibility from smooth trails, through the chatter into the bigger chatter then the big bumps. We wanted all that and have a nice plush ride that didn’t bottom in a very wide range. In essence we wanted a suspension that was not as focused as Rush that would appeal to a wide range of riders. So that’s what we asked the engineering team for. How do we take what we learned with Rush and Switchback and even the Cross-Country Team with the XCR and Sno-X sled and widen out the bandwidth on it and make it work in more conditions for more people? The Team in Roseau started work on a rear suspension and solving the focused areas of the RUSH with the PRO-CC rear suspension that came in the (INDY XC) 129.
Matt: Really when you have a good sled that you experienced with the Rush or Switchback with the external suspension, probably the biggest challenge up front is not screwing up what you already have. So, we made a conscious decision as an engineering team nearly ten years ago to move towards a family of rear suspension architecture that shares similar geometry. The front half of the PRO-CC skid is identical in geometry to the Rush and Switchback and Switchback Assault and its roots trace back even further than that. Our target was to keep a focus on rider balance and the playful” throttle on- throttle off” are customers love with Rush and Switchback but still being able to navigate a high-speed corner well. So that was probably the first and most important part of PRO- CC was carrying the front torque arm, front track shock geometry across and that’s been successful for us in several projects.
SnowTech: So, when you laid out the front torque arm geometry, what is the difference compared to previous rear suspension designs? Is the location where it mounts on the rail beneath the rider the big change?
Marty: That relationship is unchanged from MY 2015 and later Rush and Switchback product. It’s been the same for all AXYS trail performance vehicles
SnowTech: So, the longer torque arm is just giving more leverage so it’s more responsive to rider position?
Matt: Yes, but I want to be careful using the word, just talking about length. Just as important or more important is the position on the vehicle relative to the rider. Its length is important but longer is not necessarily better. There are tradeoffs there. We have some competitors that have extremely long front torque arms and those same competitors have challenges on how the vehicle behaves. It might corner well but the rider is not as influential on the vehicles behavior. So longer isn’t always better.
Marty: Torque arm length isn’t the only lever built into that geometry. So, there is a lot to do and you’re alluding to how it’s mounted to the chassis and rail.
Matt: Big picture, we feel good about our vehicles behavior on all trail performance models going back to MY15. Call it the secret triangle if you will or secret mounting that we have carried thru on all trail performance and x-over model.
SnowTech: So, if I looked at an earlier Polaris sleds (‘09 Dragon, PRO-X) is there significant difference in that front torque arm geometry?
Matt: It would be different not only in that geometry but on those sleds the rider sits farther rearward as well, so now with AXYS we made the rider further forward. They sit kind of on top of the front torque arm now and that is important for the vehicle’s behavior with respect to the rider. You want a vehicle that corners well but still is lively when you get onto the throttle. Lighten the throttle going over the bumps or you power precise line through a corner.
SnowTech: Do you remember the AD Boivin Expert rear suspension with the really long (single) front torque arm? Is that where rear suspension is heading? More emphasis on the front torque arm as the main suspension component as it is becoming more influential?
Matt: All rear suspension is a game of tradeoff. If you go too radically in one direction, you tend to sacrifice other aspects. So, if you went to an extreme long front torque arm that mounted to where the rear scissor mounts you might make certain aspects of the suspension good and other areas bad. So, a change that makes a sled corner could work the opposite and it drives through corners and feels like 800 pounds. We just feel that with the AXYS trail performance platform we have the tradeoffs minimized. That’s why we talk about Rider Balance. The rider has a huge influence on whether the sled bites hard in a corner and remains positive or the front end and run through bumps and behaves much more freely.
Another important part of PRO-CC that is new relative to AXYS is getting the front torque and rear torque arm to work together. You naturally get that with a coupled suspension and that is nothing new, but that is one area as a team we were focused on improving with PRO-CC. There are a couple of benefits to rear coupling the vehicle such as transfer control and such. We very interested in front coupling the vehicle from the benefits realized there such as bottoming resistance of the front torque arm and the rear suspension not hitting the same bump twice with the front arm kind of picking up the rear arm. Understanding this is nothing new but in this architecture it had real tangible benefits that the front torque arm performance we feel is a big improvement there compared to its predecessor. Again, with any architecture change you must understand the tradeoffs. We had the opportunity to build an entirely inside the skid uncoupled suspension (like IGX144 on the Assault) but as the team evaluated it we understood we wanted to make a big improvement with the front torque arm performance and coupling was a big part of that for this application.
SnowTech: Moving to the back of the suspension, pull rods (timing rods) are being used on the rear torque arm. Anything different on the geometry on the pull rods compared to the past?
Matt: Couple of big differences. The pull rods themselves are really influential to the speed of the rear track shock, most commonly referred to as the motion ratio of the rear track shock. If you recall, with a lot of the marketing info on the AXYS (with the hinged PRO-XC suspension) we talked about it being a progressive suspension. I think there is a bit of a misconception in the industry in terms of the rear track shock motion ratio. Inside the track suspensions are by and large also progressive. The pull rods and geometry how it relates to rear torque arm has a big influence on the motion ratio of the rear suspension. I would still call the PRO-CC a progressive rate suspension. As Marty said, we were targeting a much broader sweet spot with this suspension. The geometry of that pull rod has a big influence on how wide that sweet spot is and how closely you dial it in for a given condition. We made that slightly different for PRO-CC.
SnowTech: If the pull rods were removed what would I notice on seat of the pants riding the sled?
Matt: Without pull rods, an inside the track suspension would does not appeal to a wide range of conditions. It would be harsh initially and bottom out easily. Suspensions with pull rods do a better job at controlling shock speed and resist bottoming while still being plush.
Marty: How we control motion ratios is a big part of this suspension design – the amount of shock stroke at different points of the suspension travel. Suspensions without pull rods, typically they did not have much travel and they were very stiff and needed to be because the shocks were very regressive. As they got down in the stroke they got more spring rate but you did not get much out of the shock for damping it just wasn’t stroking very far.
SnowTech: Some designs use a single pull rod beneath the shock. What’s different on the PRO-CC geometry compared to the XTRA-10 that we all remember?
Matt: Similar concept, just different in terms of execution. This rear suspension has significant more travel, rider in a different position relative to the rear suspension and CG of the vehicle. Similar parts, different execution, different architecture of the vehicle.
Marty: We ride the sleds in a different position today. Over the years from the XTRA-10 to today the rider has moved from the rider positioned over the rear arm to the front arm. We changed the work the two are doing today. It’s vastly different now. On the Edge/ IQ chassis all of the rider load was over the rear arm when a rider was coming down on the seat. Everything was so different back then.
SnowTech: Good point. So what would be the weight distribution be on the front arm today?
Matt: I would say double the amount of load on the front torque arm compared to the XTRA-10.
Marty: The old sleds, when you sat on it, the front end would extend. Today they (both front and rear suspensions) squat down when you sit on them.
SnowTech: Rear spring mounting location – is this replicating Ski-Doo’s rMotion where the spring is under activation on small stutters before it gets into shock motion on the rear arm?
Matt: We’re not necessarily targeting replicating rMotion. We’re trying to build a better version of the AXYS snowmobile with an in the track suspension. When we talk about a progressive motion ratio, it is that. It’s a motion ratio where the shock is travelling slow when the skid is fully extended and speeds up through travel. We know rMotion has that, PRO-CC has this feature also. They’re accomplished different ways and I wouldn’t say we did it to replicate rMotion, we made conscious decisions to be different there to make the sweet spot as wide as possible and maintain the Rider Balanced aspect of the vehicle.
SnowTech: As the suspension moves through its travel, how much is absorbed by the rear springs initially vs. shock travel?
Matt: Near the bottom of its stroke the shock is moving over twice as fast as it starts, it’s ramping up quite hard through its travel. The force at the bottom of the hyfax is dominated by the springs early on and the shock begins to dominate deeper into the travel. The springs are position sensitive and the shock is largely a speed sensitive thing. So, when you come off a big jump there is quite a bit of shock force early in the stroke because the velocity is relatively high, the biggest thing is that the speed of the shock is changing through travel and speeding up significantly.
Marty: Just so you understand from our perspective. There is a lot of time and effort and engineering going into a rear suspension. We spend years into these things. These things just don’t just happen overnight.
SnowTech: The torque link is on the PRO-CC and is not on the IGX 144. What is the advantage of the torque link vs. non-torque link suspension?
Matt: You will find torque links on suspensions that are relatively short. We had them on the 121” Indy from MY13 and further back. We don’t have them on the 144 IGX, because as the vehicle gets longer the vehicle does not pitch as easily on the throttle. The torque link helps counteract that. Torque links work best on shorter wheelbase vehicles with track lengths of 137”, 129” and 121” – our trail performance vehicles – especially when studded and riding on hard packed trails. As the vehicle gets longer, the vehicle and footprint on the ground is getting long enough to just bite. The vehicle just has more control over pitch with longer tracks so you don’t need them.
SnowTech: Do torque links influence track tension as the suspension is compressing or extending?
Matt: It absolutely does influence track tension. That wheel, instead of rigidly mounted to the chassis, is now swinging on an arc as the suspension collapses. Yes, it affects track tension.
SnowTech: What are the differences with shock valving on the front and rear arm on Rush PRO-XC vs. the 129 PRO-CC?
Matt: Shock valving (on the INDY XC) was calibrated to a high level to be very similar to the PRO-S Rush with PRO- XC suspension. Overall the PRO-CC has a much wider sweet spot compared to PRO-XC. For MY 20 the Indy 128 XCR however has a big difference in shock construction, valving and overall vehicle calibration and springing are meant to match much more to XCR. More focus on aggressive riders, larger bumps while still maintaining good cornering performance.
SnowTech: Why go with the 129” in MY 19 instead of the 137”? Everyone seems to be moving away from the 129”. What were the reasons for first introducing a 129” track instead of the 137”?
Marty: There is still a viabl segment in the trail market in the 121” and 129”. Most had moved away from 120/121 on high performance sleds. There is still a lot of volume in that segment. We felt that looking where our customers were landing, the Switchback was strong but Rush wasn’t as strong, so we were able to get the INDY XC 129” to market in the first year and follow that in 2020 with the 137” and also the XCR’s. The 129” length is where we had the most to gain for a first-year introduction.
SnowTech: Is there any point with the PRO-CC suspension where the suspension just turns into falling rate?
Marty: A lot of what we learned and generate data happened as we developed the PRO- XC suspension. That information carried into the development of the Pro-CC. It’s not just a black and white problem. What did we learn with those motion ratios and the way we stroke the shock, the way we operate the rear arm, how we couple it to the front arm and make it all work together to give the best ride we can have and best bottoming resistance. Even though they don’t look the same, there are a lot of similarities to how the two suspensions function and part of that is because we have the same team and same learning carrying all the way through the suspension development.
SnowTech: How much is similar on the PRO-CC to the RR race sled suspension? That was a 136” suspension. Is the new 129” Pro-CC share parts and geometry with the RR from 2019?
Matt: There are some similarities. The front torque arm geometry used in PRO-CC is shared with Race as well. If you look at the MY 20 Indy XCR front torque arm, it’s the same exact part number for MY19 and MY20. It does directly transfer over.
SnowTech: The rails themselves are real pieces of art. What can you tell us about them? Step us though the rail designs
Matt: We optimized using structural tools to remove mass and keep strength. You will see cues from the RMK world. It uses the same basis extrusions from the Mountain sleds. The high-level goal was to deliver the lightest weight and strongest rail beam we could deliver. Computer modeling played a huge role.
SnowTech: The PRO-CC was in development for what 3-4 years?
Matt: It’s hard to say that. The lessons learned from PRO-XC have been in development for over 10 years. We did not abandon the PRO- XC learnings, but incorporated those lessons into PRO-CC.
SnowTech: The new tunnel. The running boards look like the RMK and Assaults.
Matt: The decision to use the extruded running boards makes a huge difference in chassis stiffness. We call them POWDERTRAC TR. There like the POWDERTRAC XT used on RMK, with the big difference being how they are kicked up at the rear and tapered different from RMK. Big improvement in stiffness, cleanouts, ice buildup, traction under your feet. They are also much stronger and if you look close it eliminates the need for a fishbone support, that goes away to clean up the tunnel side and makes the integration much cleaner.
SnowTech wants to thank Marty and Matt for giving us some additional insight into the development of the PRO-CC suspension and why it was developed. Perhaps the biggest take-away is that the new PRO-CC suspensions are the continued evolution of the lessons learned from previous Polaris rear suspensions, including the hinged PRO-XC progressive rate skids from the Rush and Switchback sleds. The PRO-CC skids deliver a broader “sweet spot” and will perform better in more conditions for more people.