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January 17, 2011
Challenge: Make RMK Lighter
Utah seniors design, build, test carbon tunnel
About the only thing that probably comes close to getting paid to design pieces and parts of snowmobiles and then getting to test and ride those parts on a sled would be getting college credit for doing the same thing.
That’s what a group of seniors majoring in mechanical engineering at the University of Utah did last year.
This group—project leader Sean Wilson along with Dan Hirst, Julie Painter, Trevor Thomas, Marshall Maughan and Erik Brockhoff—had the idea of designing and fabricating a carbon fiber tunnel for a Polaris RMK. The purpose was to—you guessed it—lighten up the sled.
In the team’s project summary at the end of the spring semester, they stated, “The purpose of the project was to reduce the weight of a snowmobile intended for use in steep terrain with deep snow without sacrificing strength or stiffness.”
More specifically, the executive summary stated, “The primary goal of the Carbon Fiber Snowmobile Senior Design Project was to redesign, fabricate and integrate portions of a snowmobile chassis made from carbon fiber composite materials to replace the existing components made from steel and aluminum in order to reduce weight.”
Polaris donated an 800 RMK 155 to the engineering students for their senior project and much of the work was done in a local shop. As stated on the project summary, “The portions of the chassis chosen to be replaced were the aluminum tunnel and steel tubular structure. The replacement parts had to be made just as strong or stronger than the originals, while also being lighter in order to exceed the stock design’s capabilities.”
Here are some highlights—in the team’s own words—from its project summary as well as one of several presentations it had to make during the two semesters of working on the carbon fiber snowmobile.
In order to select a material configuration that would satisfy the necessary requirements, samples were manufactured, tested and the results recorded.
The material chosen for the tunnel was a sandwich composite comprised of carbon/epoxy face sheets and a Poly Vinyl Chloride (PVC) foam core. The material produced a strength to weight ratio nearly five times better than that of 6061 T6 aluminum alloy.
The 1-inch OD, .050-inch wall, mild steel structure was replaced with 6061 T6 aluminum wrapped in carbon/epoxy composite. Due to the higher yield strength of 6061 T6, stiffness was focused on rather than strength. About 25 percent of the stiffness difference created by the lower modules of elasticity of the aluminum was reduced by the material configuration selected. The weight of the structure replaced was reduced by about 43 percent.
The carbon/epoxy composite material does not conduct and dissipate heat in the same manner as aluminum alloys. This created a problem for keeping the engine cool because the stock tunnel also doubled as a heat sink/cooling fin with coolant being plumbed through its structure. A dual fan aluminum radiator taken from a 1300cc sport bike combined with an electronic thermostat control was used to solve this cooling problem.
The composite components manufactured were reassembled with the stock components to produce a fully functioning, working, running and rideable sled. The sled was ridden on two occasions by the design team to verify structural integrity and cooling system function. The newly reassembled sled performed very well. No damage to the composite components could be seen and the temperature readout integrated into the stock cluster never displayed a temperature greater than 215 degrees F (on a very warm and sunny day ~45 degrees F).
Factory Composite Replacement
Steel tube structure factory weight: 14 lbs. Hybrid aluminum/carbon/epoxy weight: 8 lbs.
Aluminum tunnel: 30 lbs. Carbon fiber tunnel w/radiator: 19 lbs.
Total weight reduction: 17 lbs.
There is another possibility of weight reduction due to less coolant being required for the engine cooling system. The stock cooling system contains roughly 13 lbs. of coolant with a certain amount of that being held in the tunnel. The amount of coolant contained in the radiator would appear to be less; however, a quantifiable amount was not measured and would not be factored into a dry weight.
The project has been deemed a success. Based on initial test riding, the goal of reducing weight while maintaining strength through the utilization of composite materials has been met. It would also appear that the use of this composite technology could be expanded into other areas of recreational vehicle production. The ability to quickly create relatively durable tooling and prototype parts for complex geometries would lend itself greatly to developing and testing new designs without the need of expensive tooling for initial prototypes.
Wilson, the project leader, kept SnoWest apprised of the progress from the initial tear down of the new RMK to the test ride last spring. One of the original plans called for replacing the stock seat with a lighter seat, as mentioned in an e-mail last March. Wilson wrote, “We have come up with a creative way of cooling the engine while incorporating a lighter weight seat that should also increase the strength and stiffness of the tunnel. We will be using a Hayabusa radiator with carbon fiber ducting to encapsulate the cooling assembly as well as support the seat. It should be lightweight and really cool.”
As you read, the radiator was necessary for cooling due to the removal of the aluminum tunnel and that was installed but the team wasn’t able to do the seat exchange
Then this e-mail, later in March, where Wilson said, “The first test ride went really well. The cooling system worked as intended and the carbon structure matched up to the stock aluminum. The steel hoop for the seat will be eliminated and the seat structure will be integrated into a carbon cowling to cover the radiator we had to install on the tunnel.”
After the second test ride, Wilson wrote, “On Friday, April 2, we took the sled out for a more aggressive ride and video shoot. We made a couple of changes and additions since the first test ride. I managed to bend an A-arm and rolled the sled three times while riding but see no signs of damage to the carbon. Everything, including the cooling system, seems to be working well.”
We attended the Senior Design Day at the University of Utah, where senior engineering students’ projects were on display. Gauging by the number of people viewing the displays, the Carbon Fiber Snowmobile Team’s project was one of the most popular that day.
And according to the Carbon Fiber Snowmobile Senior Design Team’s faculty advisor, distinguished professor K.L. DeVries, it was a “unique” project indeed for University of Utah students.
DeVries said, “For undergraduate students, it was rather unique. Not only did they need to design carbon fiber parts to exactly fit and replace the original metal parts but they had to design the techniques, etc., to manufacture parts and attach them to the snowmobile.”
Utah’s DeVries said he was pleased with how the project turned out. “I was very satisfied,” he said. “The students demonstrated the use of computer techniques in design, they manufactured the parts of equal or better strength than the parts they replaced while maintaining appropriate rigidity at a 50 percent (from 34 to 17 lbs.) savings in weight. They installed the replacement parts on a functioning snowmobile and operated the machine, clearly demonstrating that the replacement parts were completely functional.”
We asked Wilson, the project leader, looking back over the course of several months of designing, building and testing what he would do differently on the snowmobile project. “Given the opportunity to go back and do it again, knowing how much work it was going to be, I would have started organizing team meetings and planning for the project during the summer rather than waiting for the school year to begin,” he said. “We had a lot of high-stress crunch time right up to the presentation day to get everything finished up because of some unforeseen delays in the project.”
In talking with Wilson and other snowmobile project team members at Senior Design Day, they were stoked with how things turned out with the sled, despite the delays and minor setbacks. Wilson would later tell us of one aspect he was particularly pleased about on the project. “The replacement seat support and rear cowling were taken from 3D computer model to finished product in about 10 days and it actually fit. At the beginning of the project, I never would have thought that our team could rapid prototype something out of carbon that fast and have it look exactly how we wanted. I was very proud of our team after those components were installed the weekend before the due date.”
However, not all was perfect, as Wilson pointed out. One “setback” he noted was, “When we first did the finish work on the plug, the male piece that the mold is made from, the epoxy paint hadn't cured completely and began to peel up. Keep in mind that we had spent at least 80 hours designing, machining, sanding and painting this plug, only to have the paint peel up, rendering it useless. This was a frustrating setback, but the team had no choice but to spend an entire week reworking it to get it right.”
But it all worked out in the end and the team got to see the result of all their hard work when they put the sled on snow last spring. It really comes as no surprise that the two test rides were the highlight of the project’s tenure. Wilson said, “The most fun I had during the project has to be, hands down, the first test ride with the team. We had just spent eight months tearing down, redesigning and reassembling an entire sled and not only did it go back together, but it worked. I have never been so happy, relieved and excited in my whole life.”
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