To get the inside scoop about the new raised Pro-RMK chassis, we sat down with three engineers from Polaris: Marty Sampson (snowmobile development and testing manager), Cody Telford (RMK development engineer) and Luc Wilson (senior design engineer). They talked about the intense design and testing process – that took them all the way to South America – to make the sled agile, durable and fun to ride. For the complete story about new technology on 2016 snowmobiles, pick up the October 2015 issue of Snow Goer magazine.
1. You first hit the snow with the raised RMK chassis in June 2011. What were your first impressions? What was great? What needed to be fixed?
Sampson: “When we built that first sled we knew the simple part of the raised chassis, which was getting the chassis out of the snow for clearance would be a good thing. We didn’t know a lot of the rest of what we were going to go find out about it. So we built it because it was an idea, a theory, a concept and we wanted to go learn what would we like and not like about it.
“There were a lot of theories flying around about how it would feel and what it would do. So we built that first one simply because we knew it would give you more clearance to go through the snow better. We took it out in either May or June of ’11, so late in the season. And one of the things we were wondering about was when you raise the sled that much, would it become more unstable or more tippy in off-cambers? We fairly early on realized that although it might be a little more tippy in off-camber situations, it wasn’t a dramatic difference, almost not noticeable that it was.
“We also realized that there was more to it than just the snow clearance because we didn’t have deep snow, but we nearly immediately recognized that it enhanced your ability to sidehill on very steep sidehills [because of clearance to the chassis]. The clearance that it built in, with everything else being the same, that became kind of the big piece of … the conversation was, ‘Wow, look what you can do with this that you couldn’t do with our other sled, because of the clearance.’
“We were sidehilling down a very steep slope and you were able to maintain control of the sled because the plastic wouldn’t pick the track up off the snow. When we tried to do it on the Pro-Ride we had with that day, the side panel and running board would pick the track up off the snow and you’d slide down out of control. So there was a major difference in both steep sidehilling and downhill sidehilling.” (This testing was in Wyoming)
[About what didn’t go right] “At that point because of the snow condition, where we were and what we needed to learn we didn’t have anything that we necessarily thought we needed to go fix on it. What needed to be fixed was the testing conditions. When you’re talking about mountain sleds, you have to be very careful about making changes and not knowing what it does in a wide variety of conditions.
[About the path they took next] “We recognized that it was late in the spring. We recognized some things that it did very well that we really thought was a competitive advantage for our customers. So we said for us to be able to make this big of an architecture change and the timing that we needed to make it. From that point in June, we decided we needed to get it in real snow to find out it if it worked everywhere and did everything that we needed it to do. That’s when we started planning to go to South America, that summer, in better snow conditions to figure out if there were any other things that it did very well and some other things that it didn’t do as well.
“We were down there about a couple weeks and we got into some really good snow at that time and we learned more about it because the snow conditions and the conditions were more winter like, so more real. And we learned some things about getting the chassis height right and getting the driveshaft right to get it to pop up on the snow quickly. From there we started to optimize the raised chassis portion of the conversation. So we were testing ability to pop up on the snow, ability to carry speed over the snow, ability to climb hills, ability to sidehill up and down hills and looking at performance of the whole package in deep snow. And there were some things that we liked about our Pro-Ride that it did well that the raised chassis didn’t seem like it was optimized for, and so we focused on those things to get that equal to or better than what we had.
“To go into specifics on what we adjusted everything, it’s kind of tough because it’s optimizing locations of suspension components and optimizing locations of [the] driveshaft and optimizing [the] front suspension to match to get it to do the two things we talk about a lot, which is to get it to pop up on snow from a very low speed or a dead stop to pop up on snow really quick and then carry its speed over the snow. From that initial concept, everything was changed some to optimize those items.”
2. Was ability to get on snow one thing Pro-Ride did better?
Sampson: “They were different. The mesh between popping up on the snow and carrying speed over the snow, there are compromises in that both ways, right? You can get something to pop up on the snow very quickly and not carry the speed. And you can get something to carry a lot of speed over the snow but it might trench for a long time while it’s getting up there. So the mesh between those two is the balance of what makes a mountain sled work right. And the thing that the raised chassis didn’t have was the right balance between lift and ability to carry speed over the snow. And the Pro-Ride had that balance better because it had years of being optimized.”
3. You kept the same rider position and skidframe approach angle as the original Pro-RMK. Is it safe to assume you experimented with different positions and angles but were unable to find ones that worked better?
Sampson: “A lot of times when you build a concept you’re forced to change some of those aspects just to get pieces and parts to fit, and from that at times you realize that this change or that change, works in your favor or not. The answer to your question is yes, but somewhat by default. When you build a concept and you start changing driveshaft locations and chaincase center distances [and] rail mount positions, a lot of times you end up changing the rider position by default. In the case of this sled it did change the rider position somewhat and we worked our way back to an ergonomic package that’s more similar to the Pro-Ride than what we started with in the concept unit. It’s not exactly the same as Pro-Ride.
“Our post position is somewhat forward, and our seat position and our tank position to where your feet are is somewhat forward of where it was on Pro-Ride. And that’s based on getting the whole balance of the overall sled where we think it’s optimized. The answer to your question is yes, we did play with different positions to get the balance of the sled right, and it is very similar to Pro-Ride, but there are some changes in the system to give you the perfect balance.
“That’s a pretty big change in the world of perfect sled balance and optimization, but people don’t play that back to us because it still feels very familiar to them. And they don’t really realize that because we’ve moved where your knees are a little bit to match that, so what you feel when you’re on the sled feels like it’s the same as Pro-Ride, but to get the balance of the sled working the way we wanted it to and a lot of that had to do with the raised chassis and how it reacts to the snow, we wanted to move your control point for your hands forward, and that was a conscious decision to make the sled feel right.
“Everything from where the plastic interfaces with your knees, because when you put your hands forward the plastic needs to move some with it and the tank needs to move with it. And we wanted the seat a little bit forward from where we had it in the past for clearance as your step across the back and to put you in the right spot on the seat with the new post position. A lot of work went into that even though when you get on the sled and ride it, it feels the same. You ride it and you’d be like, ‘Hey, they didn’t change anything in the ergonomic package.’”
4. Talk about how engine placement, chassis design and mass of parts help to offset the C-O-G effect of raising the chassis.
Sampson: “There’s not massive offsets in the C-G, with the center of gravity coming down massively, but it came down slightly. The engine placement went a little forward and a little down in relationship to the rest of the chassis. So there’s some offset, both forward and down build a little stability into it [in comparison to Pro-Ride].
“In comparison to our competition, the fact that the raised chassis gives you the benefit of having the leverage over the sled to be able to control it better both on an off-camber and when it’s tipped into the hill. So you have more leverage over the sled, [so] it’s like having a longer axe handle. So that piece offsets [the raised C-O-G], and the second piece that helps us [is] the fact that we aren’t going to narrow front ends and flexible tracks to get them to turn into the hill easier, also lets you potentially have that higher C-G and still have a stable vehicle because our sleds are still rigid and inherently stable when they’re on the flat ground.
“If we had done that and put a narrow front end and put tMotion in the rail and let the track bend at the edges, it would be extremely unstable because it’s made to be rigid and have high leverage for the rider. It would probably be a handful if it was set-up like that. That being said, our Pro-Ride had a high level of inherent stability, it was still fun to ride on the trail and it held itself upright when it was in an off-camber. There’s small compromises in the off-camber with the higher C-G of the raise chassis, but they’re more than overcome with the leverage that the rider gains over the sled.”
5. Talk about why the center of gravity requirements and the desires of trail riders are so much different from those of a powder rider, and how that affects your design parameters when basically “sharing” a lot of elements on the same chassis platform.
Sampson: “A trail rider typically want’s straight-out, inherent stability. When you’re going around corners you want the sled to stay flat, keeping both skis on the ground and not tip up. So, low C-G, inherent stability is key for trail riding. Mountain riders want some level of inherent stability so the sled is stable and predictable when you’re on the flat and off-camber and when you want the sled to sit on the ground, but you want a high level of agility. So, agility is what [Chris] Burandt calls flickability, the ability to change direction quickly and easily and roll from one direction to the other. And agility can be had in several ways, giving the rider a high amount of leverage over the vehicle can give it a high level of agility. So, we like the low C-G that’s built into the chassis that the trail riders like.
“We actually want the same thing the trail rider wants, we just use it differently. We build in a longer axe handle so you can move the sled around with leverage, they both weigh exactly the same, they both have a low C-G, but we’re just giving you a longer handle [with the raised chassis]. You have a high level of agility built in with a low C-G built into the base chassis.”
6. When deciding to raise the chassis, how many different versions were tried? How was the current position selected? What happened when you went a little too far, or not far enough?
Sampson: “Lots of playing around with different positions of different items. It’s not like we had seven sleds that we built in succession. [We] made little changes on all of the samples that they had.”
7. How do you decide which is the right setup?
Sampson: “Testing. Riding in the deep. Riding in the conditions that our customers ride in. Sometimes it really is [a randomly drilled hole] and you go compare it to what you have for a comparison sled, and you move stuff – sometimes a lot, sometimes a little. You slide it around. And it’s tricky when you’re moving everything together. It’s tricky to move drive shafts around and suspension points around.
“We change stuff, and then we’ll maybe change it up on the hill, we’ll maybe change it in the shop. Maybe it’s a drill, maybe it’s a torch, maybe it’s a hacksaw and we’ll run up to the hill and do performance comparisons. If that proves good, then we’ll go ride for a while. It’s very rare where we’ll do one quick comparison and give it a thumbs up because in the mountain sled world there’s a lot that goes into what changes on the sled.
“So you might have a really big gain in performance, say, in hill climbing and then you go ride in the trees and there’s something that you do or don’t like about it. And then you go ride down hill or you side hill and you’re like ‘Hey, this feels different.’ We spend a lot of time sitting on the side of a hill talking about what felt right and what didn’t feel right.”
8. Related to that, a Pro-RMK is designed to ride in the backcountry but we all know that most riders have to trail ride to get to the best powder areas, and there are even some flatlanders who buy RMKs and use them on trails but also so they have a great machine when they go out west once or twice a year. So, when tampering with center of gravity or raising the Axys chassis, is this part of the equation – the fact that it’s still got to be trailable?
Sampson: “We’re pretty embedded in what our customers are doing with their mountain sleds and you’ll get quite a few people that tell you ‘I never ride a trail with my mountain sled, so I don’t care what it does on the trail.’ But a lot of those parameters that let it go down the trail fairly well also make it a good overall sled. So even if they aren’t riding on a groomed trail, if they’re riding single track in and out and they’re riding across off-cambers in the mountains, you still want some of those same parameters that we call inherent stability.
“If you can make a vehicle inherently stable and still super agile, that’s kind of the Holy Grail. Being able to have it really nice and stable and fun on the trail or on the flat, or hold that off-camber well without rolling away from you, and then react to your input by being rigid, agile and predictable so it takes very little effort to get it to do what you want it to do, that’s exactly how you want it to work. That is not any different than any other highly dynamic motorsport. If you look at road race bikes for example, they build in as much stability as they can to make them stable and predictable and then they build in as much agility as they can to get them to corner and turn from side to side.
9. How do you know when you might need to take away a little straight-line stability in order to get a little more agility?
Sampson: “It’s changed over the years. If we had gone directly from [a 2003 Vertical Escape] to the current sled that we have and we hadn’t brought our customer base with us and let them learn how the new sleds react to your input and how to ride them with higher levels of agility that we’re offering now, they probably would have rebelled. You have to stay in touch with your customer base and be sure they’re coming along with you.”
10. We have seen some snowmobile manufacturers in the past, when trying to use a like chassis platform for both the flatlands and mountains, go backward in one of those two areas. How do you as engineers make sure that doesn’t happen – that you’re not making too big of a compromise in an effort to save manufacturing costs, and in turn hurting your dedicated mountain sled effort?
Sampson: “All of the teams are involved with the architecture up front. And we could have come with either the trail sled or the mountain sled first and it wouldn’t have mattered because the architecture on both chassis was worked through before we committed to that being our next platform. We’re doing that work as a team on both trail and mountain to make sure that the architecture work is going to complement both of those vehicles equally well.
Telford: “During that early stage you determine which parts you’re not able to compromise on.”
Sampson: “While we were working on raised chassis, there were members of our team that were also working on [Axys-based] Rush and Switchback, and we knew which parts we wanted to try to keep common and as we worked through those we figured out which parts couldn’t be common to have the goodness that both of those platforms offer.”
11. In manufacturing and, no doubt for Polaris, cost savings is obviously important, so it’s somewhat surprising that Polaris would choose to incur the extra cost of designing, manufacturing and stocking similar-but-different body panels. Was that a difficult feature to get approved the bean counters?
Sampson: “It was very, very easy. We’re dedicated to make the sleds work for that customer. It’s kind of interesting that you think that it might have been difficult in those cases for us because 10 or 12 years ago we started making the mountain sleds substantially different than the trail sleds. Back in Vertical Escape timing [2003], they had different IFS on them, that was kind of the first departure where there wasn’t as much common.
“Since then, as Polaris has looked at snowmobiles and wanting to own the markets in both trail and mountain, those conversations on the bigger pieces of what is not going to be common to make sleds specific for how the customer needs them to work have been very easy and typically we can identify most of it before the program starts. It’s just a dedication by the greater Polaris to make the right sled for the right customer that has made those conversations very easy for us.
“Since those times accounting wise, we’ve been able to plan for and understand that a whole bunch of the vehicle can be the same, can be very common and use a lot of the same parts. We’ve been able to be very efficient at engineering them so they’re specific for the customer needs and it becomes fairly easy to do if you know most of it up front. And that allows you to plan for your commonalities. You can really commit to the part that you’re going to have common and your volumes go way up on your parts that you have common and you don’t have to worry that you’re not going to have those parts common because you’ve already got that in the plan. If there’s something that pops up along the way, these days, it’s usually something pretty small that you maybe plan on being common or not and might change your path a little bit, but the big pieces of it, they’re nearly a non conversation at this point.”
12. Is there anywhere else that a raised platform might be beneficial? Such as snocross racing?
Sampson: [Laughs] “That’s fairly interesting. There is another place, and it’s going to surprise a lot of people. The raised chassis on the hill climb course [is] going to be crazy. They are so much faster on the new Assault, it’s ridiculous because it’s not bouncing off the sides of the ruts and lifting the chassis out of the snow and it keeps the track on the snow a lot better because the running boards aren’t hanging up as much.”
13. Were additional weight-loss efforts made to the raised Axys chassis that went beyond the original Axys?
Wilson: “Every gram has been accounted for and grams add up to pounds. The rear axle, overall, saved close to 3 pounds on the rear suspension. In the front was about 1.6 pounds, mostly from control arms. As you refine your vehicle and look for additional ways to reduce weight, it gets more and more difficult without sacrificing durability. In achieving this we were able to use some more advanced tools to really understand the structures and what the parts needed to do, so, utilizing some structural optimization we were able to take weight out of areas where it keeps the part rigid and doing what it’s intended to do.”
14. Explain how the skidframe’s rail design affects lift.
Wilson: “It’s a combination of things. Architecture, [the] relationship of the skidframe relative to the rest of the chassis is a big contributor to improving lift. Track design is another huge contributor. Suspension geometry, changes in motion ratio that also help control weight transfer and help get the sled up and out of the snow and not waste unneeded motion in just moving the sled but actually getting it to pop up and go on top of the snow. It took a lot of development work to get that correct balance, try and maintain stiffness, make sure that inputs and rider energy go directly into the sled to influence it.”
15. Mountain sled brakes don’t seem to be as vitally important as they are for trail sleds, but Polaris focuses a lot of effort on RMK brakes. Why?
Wilson: “Power and fade resistance isn’t as important due to the speed of the vehicle. [But] a good mountain rider will use his brake quite a bit in order to control the vehicle while he’s maneuvering, doing sidehills and keeping that sled under control. The primary criteria that we were looking for in this new brake system was weight reduction. We were able to save a little more than half a pound with this lighter weight caliper.
“It’s a different style of caliper. Our previous version was a floating piston design whereas the new one is a dual opposing piston and by doing that we were able to rigidly mount the caliper to our chaincase or backer plate so the benefits are there are less moving parts, there’s no sliding pins so there’s a little bit quicker braking reaction to the rider’s input.”
Sampson: “The criticality of brakes on a mountain sled are completely different than a trail sled and we actually test and develop different because of that. But I would say it’s highly critical. We test and develop the brakes on RMKs completely differently than we do on the trail sleds, and that’s why you see things like the brake duct that’s on the trail sled is specifically not on the mountain sleds because our braking events are low-speed events so we need that air flow to actually come out of the hood instead of go into the hood.”
Wilson: “Controlling track speed just via the throttle doesn’t get you quite that amount of confined control that you can by using both the throttle and the brake. So if you watch a good mountain rider you’ll actually see his brake light come on quite often. When you actually start to learn how to use that effectively it’s really beneficial in keeping that control.”
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