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In the late 1990s I spent some quality time with Gerard Karpik of FAST during a program where we evaluated new sled suspensions. As it is today, suspension development was a hot topic at that time and much that we learned is still being used in current suspension designs.
Testing pays offThe Karpik brothers, Gerard, David and Randy, gained respect in the 1990s as leading suspension engineers. Much of their success was due to the performance of their M-10 long-travel suspension. Trail riders loved the M-10 as it was easier on their backs and better in rough terrain.
What impressed me during our testing was the systematic method the brothers and TeamFAST.com developed to evaluate suspension performance. It was not a "lets go for a ride and see what we think" type procedure common among magazines and to a large extent used at the factory level.
The FAST system consisted of a grid, where performance was evaluated at slow, medium and high speed, over stutter bumps, medium and large moguls. Each group got a result assigned to it between one and six, one being the worst and six being the best. After establishing the test track and evaluating the sleds through each of the nine combinations, a clear pattern developed for each machine. Sport machines did well in faster big bump conditions, but not so good on smaller bumps. Family/touring sleds had a better ride on stutter and medium moguls, but weren't so good on larger moguls at higher speeds. Not surprisingly, no sled did well in all areas but some compromised better than others. We managed to quantify the performance, and name the better sleds, which was the exercise's purpose.
This evaluation process was further refined by FAST to be even more accurate and quantifiable, making it an important tool in the firm's suspension development, like a dyno would be for an engine builder.
The three Karpik brothers have extensive backgrounds in race sled development stretching over decades on factory race teams. Randy Karpik has been crew chief and chassis development engineer for Yamaha for several stints and is involved in Yamaha's present racing program. David Karpik was Brad Huling's crew chief during the Scorpion glory years, as well as the lead designer of the dominating 1982-'86 Ski-Doo Twin Trackers, and Gerard was a cross-country champion and Ski-Doo factory racer from 1978 to 1986.
As a successful cross-country racer, Gerard was involved with both chassis and suspension development. When he started TeamFAST in 1986 he kept trying new suspension ideas but one goal was never attainable; building a good working long-travel suspension. Many had tried, including the Big 4, but in most cases they ended up with a wild ride, not an improvement.
Year after year, Randy and Gerard tried new ideas and improved their evaluation process. After a while, a trend developed. It's a widely held misconception that you need a rising rate geometry on a rear suspension. In theory, this would give you a soft ride on small bumps and a strong spring to handle the big moguls. The Karpiks had been working on suspensions that had a rising rate of about 40%. In testing they decreased the rate to 30% and then 20% and the suspension would "kick back" less and less wildly as the progression decreased on the big stutters and larger moguls. Finally, the linkage was arranged to give a straight rate and this showed immediate and positive results. Settling in on a straight rate they then focused on dialing in their patented "coupling" system.
In a conventional snowmobile rear suspension, the rear link "floats," allowing the slide rail to pivot over moguls. As the travel got longer this had a number of disadvantages, the biggest being too much ski lift and too much rear "kick back." By introducing "stoppers" on the rear linkage, forcing it to go solid and work as a parallelogram at a specified part of the travel, these problems were addressed. At this stage, the design had developed into a complicated maze of linkages and a major re-design was needed.
What the Karpiks learned was that one of their main design criteria "a rising rate geometry" delivered an unanticipated and largely negative result. Gerard credits his brother Randy with suggesting the simplest and cleanest solution to the problem, to drop the linkages in their entirety and position the shock as vertically as possible in the suspension and to deal with the challenges a Straightrate design delivers. This simplified design not only cleaned and lightened the system, the overall function improved in nearly every area with the exception being high gravitational-low velocity bumps, or G-bumps.
It was now time to prove the system on the racetrack, and the opportunity came when FAST was asked to build a couple of snocross race sleds for Swedish racer Krister Johansson for the European Championship circuit in the early 1990s. Gerard felt this was the optimum test for what was by then being called the M-10 suspension. He credits Krister as a very astute and knowledgeable driver who gave excellent development feedback on everything from construction improvements to set-up and shock calibration.
The results speak for themselves as Krister won both the Swedish and European snocross championships in both 1992 and '93. Ski-Doo then came up with a budget to bring Krister and Finnish driver Toni Haikonen over to compete on the U.S. snocross circuit. Toni became the first "super star" on the fledgeling U.S. snocross circuit, dominating for several seasons.
FAST now had good development and solid testing with proven race results to back up their products. Having been able to combine both a soft ride in the stutters with superior mogul performance, the M-10 suspension quickly gained a large following among trail riders. As the racetracks got rougher and the jumps increased to today's giant crowd-pleasing proportions, Straightrate spring rates have been exchanged for rising rates just to absorb the big G-outs on landings. Although these G-outs are normal for snocross pro's, it's not something the average trail rider will ever experience, and the original M-10 Straightrate spring rate philosophy remains intact on most consumer models. That said, suspension development has not become stagnant at FAST.