The snowmobiles that we ride today bear little resemblance to the machines that were sold and raced in the mid 70’s. However, if you...

The snowmobiles that we ride today bear little resemblance to the machines that were sold and raced in the mid 70’s. However, if you open the hood of your sled at home and peer into the depths where the tunnel and front bulkhead meet, you will find that this layout was born on the oval race tracks over thirty years ago.
The limitations of this chassis design are just now being discovered on the new high flying Sno-X sleds. Centralized mass (moving the engine and rider closer together) is being touted as the next revolution in snowmobile design. There is no denying the huge influence that racing has on the snowmobiles we ride on the trails. Until recently, oval and cross-country racing were the dominant test beds of snowmobile design. To beat the competition, you had to figure out a way to go faster around a corner or survive a 500 mile snowmobile race between Winnipeg and St. Paul. Much like today, the manufactures built “race-only” machines that were sold to compete in both disciplines. Ski-doo built the “Blizzard”, Polaris the “TX” and Arctic Cat the “EXT”. These machines would offer new styling and machine upgrades that would for the most part heavily influence the design and look of the following year production snowmobiles.
In the 70’s there were at times 50-plus snowmobile makers, far more than just four (five-six) like today. Every little town between Maine and California staged a snowmobile race in the early 70’s. All of the manufactures were competing for the same slice of the sales pie. To make an influence on the customer’s decision, you had to be winning races. “What wins on Sunday, sells on Monday” was never more true back then.
Sno-Jet was one of many sled builders back in the 70’s that needed to develop a race program on a modest budget with the goal of winning races and increasing sales. What came out of that program would revolutionize snowmobile design.
The Sno-Jet “special project team” directed by Duane Aho was given a clean sheet of paper to build their first purpose-built oval race sled. Management had no plans for this sled to go into production, but rather the goal was to promote the brand by winning races. Sno-Jet’s engine supplier was Yamaha, and because Yamaha was also racing on the oval tracks, the Sno-Jet group was supplied with the previous year factory race engines. Clearly, the Thunder-Jet program was not going to win races with superior horsepower. With that line drawn in the “snow”, the team then went about building a chassis that would give superior power to weight ratio. The lack of horspower also meant they were not going to win any races down the back straightaway. However, if the center of gravity could be lowered the sled could be designed to corner faster.  These two engineering concepts, (high power to weight and low CG) continue to keep engineers busy. The Thunder-Jet was the first snowmobile that presented solutions to both problems.

Power to Weight Ratio
Remember Grade 9 physics? Newton’s second law of motion states force = mass x acceleration. For the Thunder-Jet team that meant to improve the throttle response (acceleration ) of the sled they had to reduce the weight of the snowmobile. How light do you ask?  The 650 cc Thunderjet tipped the scales at 280 pounds dry. No, that is not a misprint! The engine developed 90 hp, for a power to weight ratio of 3 lbs/hp. For comparison, the 2007 Ski-Doo Mach Z @ 570 lbs with 170 hp equates to 3.4 lbs/hp.
The T-Jet tunnel and bulkhead was constructed from 2024-T6 and 7075-T6 aluminum. The use of high tensile strength aluminum alloys allowed for structural components to be formed from extremely thin gauge sheet and then heat treated to double the tensile strength of weldable aluminum alloys used by the competition. The chassis was assembled using construction methods for aircraft, as the entire tunnel and front bulkhead was riveted together. Riveted construction also allowed for ease of replacement of sacrificial components damaged during the race season. The ’73 race sled used aluminum skis and a stamped aluminum slide rail carriage. The entire chassis weighed a scant 40 pounds.

How Low Do You
Want to Go?
The use of high tensile strength aluminum alloys allowed the design team the flexibility to manufacture chassis parts that allowed the engine to be mounted so low that the top of the sparkplug measured 23” from the ground.  When development of the chassis was taking place in the spring of 71, the design team built a “ test mule” that allowed the group to change the ski stance and the ski spindle to track distance to determine which produced the most stable and driver friendly combination. Bob Mackin (Senior Mechanical Designer) of the project was of the opinion that, “If the sled is hard to handle, the designer has failed and so will all but the best riders.”
The final chassis design resulted in a sled with a 32” ski stance. Instead of a simple box-shape, the tunnel was narrower at the top and wider at the bottom. This allowed the rider to move around easier on the sled when cornering. However, the location of the engine is what turned out to be the T-Jets most copied feature.  To increase the stability of the sled and make it “fly more level” when running on a bumpy track, the engine was placed further ahead of the tunnel by mounting the engine to the high tensile aluminum stampings that connected the tunnel to the front spindle cross member. This layout combined with off-setting the engine over the left ski, put the engine weight more biased over the skis. Snowmobile designs at that time still had some engines mounted on top of the tunnel or mounted just forward of the tunnel. This extreme forward mounting of the engine still forms the basic layout of today’s snowmobiles.
Two chassis lengths were built. The 650 and 440 were built with a 116” x 15 1/2” track and the smaller 300cc single was built with a 102” x 15 1/2” track. To minimize the rolling resistance of the track which eats up power quickly, the slide suspension had minimal travel and large diameter idler wheels were used. The seat provided most of the energy absorption for the rider.

Laughter, then respect
The Thunder-Jet styling team then gave the sled the famous “shark mouth” look. The belly pan was fabricated from fiberglass (not very common at that time) due to cost restraints. Sno-Jet was looking at keeping the machine costs to a minimum, and the tooling costs to form an aluminum / steel pan could not be justified as the chassis was to never be used for production. The side panels of the hood were hinged to allow access to the clutch without having to remove the hood. In fact, when the sleds first appeared at the first races across the snowbelt in 1972, the reaction was the same. Peter Morris (Design Draftsman on the T-Jet program) puts it best:

“When the Sno-Jet boys unloaded their T-Jet in the pits, everyone hovered around it in amazement. Here was this freaky looking sled, from a relatively unknown manufacturer. Who could take this seriously? If this was a decent design, wouldn’t the big boys like Bombardier, Polaris, and Arctic Cat be on top of it first? Hell, the engine is almost on the ground… and offset from center. The cowl doesn’t look like anything we’ve seen before. All this spawned a ton of ribbing in the pits, from the nay-sayers and other teams: “Hey, it looks like someone backed their pickup over your sled?”…. “The danged thing is so low, it might be easier to go over you than around you!”… “I know a machinist that could probably do a better job of centering up that engine!”
“All this good-natured joking and laughing at the strange new kid on the block, ended VERY abruptly, I’d say somewhere between the first lap and the second lap of the first event the T-Jet starred in! The T-Jets put on a clinic that dropped jaws on all the ‘Big Boys’ teams. They came in first and second in every race they entered that day, as I remember it. I believe Yvon Duhamel (Ski-Doo) and a Polaris racer named Bob Eastman were in attendance, too. Now, some of the same people who were in the pits earlier, throwing barbs at the funny new sled, were back after the first race goggling in amazement… asking serious questions, and trying to peek into and under every square inch of this weird new iron. Some of the playful joking was still there, but it now took on a very different note: “Why, this thing’s nothing but an engine between your legs!… A Crotch Rocket!”

The decision by management to not implement the design innovations of the race sled in consumer models, was in hindsight, a mistake. Had Sno-Jet senior management used some of the forward thinking of the group that designed the T-Jet, perhaps the company would have survived the shakedown of the manufactures in the mid 70’s. Sno-Jet was eventually purchased by Kawasaki as a means for their entry into the snow market, and for a short while the Sno-Jet machines were sold as “Kawasaki Sno-Jet” before the all-Kawasaki sleds (Invader & Intruder) hit the snow.
Today, with the introduction of new snowmobile designs coming fast and furious to the marketplace, the influence of the Thunder-Jet is still shaping the snowmobiles of the future.

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