Racing into the future

The ‘84 Ski-Doo Formula MX / Pro Stock was advanced!
RELATED TOPICS: VINTAGE | SKI-DOO
1984 Ski-Doo Formula MX Pro Stock
Ski-Doo’s 1980 lineup of high performance Blizzard leaf spring sleds looked sharp and were certainly fast enough with liquid cooled, rotary valved, twin carbed Rotax powerplants. A trio of Blizzards covered the displacement spectrum of 340 and 440 liquid and 500cc fan cooled sizes, called the 7500 Plus, 9500 Plus, and 5500 respectively.

The fan-cooled 5500 was infamously updated in 1981 with Bombardier’s first IFS consumer model, the Blizzard 5500 MX. The correctly named trailing-arm front end provided about four inches of travel paired to the industry-leading 10 inches of rear movement.

Good trail sleds they were, but great race sleds they were not. Even the cross-country racing legend Gerald Karpik was known to prefer to race the older late-70s vintage “RV” based sleds for battling it out in the Midwest ditches. The leaf spring and the smooth riding, but heavy handling MX variants were just not competitive against the Indy trailing-arm and SRV telescopic strut suspensions of the day. In response, over the summer of ‘81 Karpik and his team built a pair of racers based on stock Blizzard motors, drivetrains and 5500 MX tunnels, but with an improved trailing-arm front suspension and front chassis. These were often called the “Karpik Specials” and were very successful, winning major “snowcross” races at Alexandria, the SOO I-500 and others during the ‘81-82 season.

■ Racing sled for home

As the consumer Blizzards were improved with the 440cc 9500 being replaced with the 521cc 9700 for 1983-84, more was cooking up in Ski-Doo’s race department. Snowmobilers, racers and press folks were all anticipating a “next generation” machine from the team in Valcourt. A preview was on the way in the fall of 1983 when Bombardier released a new 56 hp class racer, the Ski-Doo Formula M/X Pro Stock.
1984 Ski-Doo Formula MX Pro Stock
The opening line to the owner’s manual for the Formula M/X Pro Stock stated: “This vehicle has been designed for and should be used on an oval lake enduro or snow-cross racing only.” Bob Anderson, a championship oval racer from 1982-92 bought a Pro Stock new in the fall of ‘83 for competing in oval and snowcross races in winter of ‘83-84. Anderson stated the sled “felt low and the front suspension felt fantastic.” On the track the sled handled great - very precise but took some time to properly dial in, according to Anderson. This was a huge leap forward in chassis technology, getting Ski-Doo in the ring with the other IFS stock class racers of the era.

Bombardier assembled approximately 100 Pro Stocks for 1984, and another 50 for ‘85. Aside from a major leap forward from the leaf spring Blizzards and definitely taking a decidedly more high performance turn over the trail cruiser Blizzard MX, the Pro Stock also gave consumers and racers alike a sneak peek of the soon-to-be released PRS (Progressive Reaction System) IFS chassis-based Formula lineup for the ‘85 model year. Consumers got the first taste of the Formula’s look with the conventional leaf spring front end “SS-25” for the 1984 model year. However, the Pro Stock previewed the sharp handling and progressive-rate IFS front and rear suspension in the Formula Plus and MX for 1985.
1984 Ski-Doo Formula MX Pro Stock
■ Geometry gains
Aside from being a limited production, handmade, power-limited racer, the 1984 Pro Stock differed from the production 1985 Formula MX in many small ways. One of the most obvious was the front suspension. The racer had “Pro Stock” on the side of the trailing-arms, the spindles were front and bottom steer, compared to top and rear steer for the production sled. The Pro Stock had the front coilover shocks and progressive-rate linkage sitting flat in the belly pan, facing rearward, nearly tucked under the sides. Interestingly, each of the two years of the sled had a slightly different configuration.

The production consumer PRS chassis had the shocks and linkage in a completely different location at the front of the sled, at a forward angle. This indirect action setup enabled progressive linkage, reduced un-sprung weight and kept the shocks away from snow spray and trail debris. It also added a lot of joints and levers to wear and add play, as well as adding to overall chassis mass. Additionally, while the under-hood packaging protected the shocks from some environmental conditions, it exposed them to others such as exhaust gases, heat, oil, belt dust and trapped dirt. Check out the bottom of your pan to see this trade-off for yourself.

The production PRS rear suspension was rated for 6.5 inches of progressive-rate travel, a reasonable amount for the mid-80s. However, this was a significant reduction from the approximate 10 inches on the Blizzard MX. This travel reduction was probably to benefit handling, cornering and lowering the center of gravity, but it was years before Ski-Doo returned to travel numbers like the that.
1984 Ski-Doo Formula MX Pro Stock
The PRS rear skid ended up with three shocks, two in the rear and one in the front, all coilover and progressive-rate. This was state-of-the-art for the time and received quite a bit of press when the Formula lineup was released to the consumers.

One interesting note was the track length on the Pro Stock and eventually the 1985 PRS sleds was pretty short. Ski-Doo selected 114-inch length for these sleds, but it was an inch and a half wider than the norm with a 16.5-inch width. Even the leaf spring El Tigres, Liquifires and Interceptors of the early ‘80s ran 116-inch tracks. The Invaders, Indys, ’83-and-up Vmaxs, SRVs and ‘85 AFS Cats ran 121-inch tracks, which ended up an industry standard for trail sleds. Ski-Doo stuck with the short 114-inch length even with a stout 521cc Rotax pulling it around in the Blizzard 9700 and then the Formula Plus. I understand the idea of reduced rotational mass and a shorter machine and wheelbase for crisp handling, but the offset was smaller contact patch and floatation, especially as suspension travel was increasing.

■ Steps in the right direction
As intended for 56 hp stock class oval and the early days of “snowcross” racing, the 1984 Ski-Doo Pro Stock racer was a big improvement over leaf spring sleds of the era, but even with the more precise front ends, and progressively damped rear skids, the Indys of the day maintained a slight competitive edge on the race tracks. At the consumer level, the Formula MX and Plus were considered winners with sharp handling, solid good looks and a definite step in the right direction for Ski-Doo.

If any of this tale seems familiar, perhaps it reminds you of the 16 early release 2002 REV Mod 800cc SnoCross race sleds introduced before the regular production 2003 model year. But that’s another story.
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