The IFS Design Challenge

From leaf springs to ‘controlled motion’ suspensions
Unser family Flying Wing snowmobile design
Racers are always looking for a winning edge design.

In the early-mid 1970s, Indy car racer Bobby Unser got involved with snowmobile racing in his off-season from his busy Indy car schedule. Chaparral hired him as a manager for their race team. Right away he introduced new designs spawned from his Indy-racecar experience that gave handling advantages.

All the Unsers are avid snowmobilers, and both Bobby’s younger brother Al and his nephew Al Jr. have been involved with sled manufacturers and aftermarket companies since the early 70s.

■ From race cars to race sleds
Bobby has had the most influence as he right away decided there had to be something better than leaf springs to control ski suspension action and steering. Others were following a similar line of thinking, but few had a practical solution. Racecars used two A-arms to control the front wheels, but the Chaparral’s design borrowed ideas from both Indy cars and Dirt-Sprint cars.

The now-familiar IFS suspension first raced on the Chaparral, had the ski spindles mounted on a long swing-arm that pivoted at the chassis center. It was controlled by two radius rods mounted in the center of the front bulkhead. Motion of the suspension was controlled by coil-over springs on hydraulic shock absorbers. It was a clean design that spread the load across the chassis, instead of concentrating it on the front bulkhead as double A-arms would do. As a result the sleds could still be built with a light chassis. When the new Chaparral showed up at the races in 1972, they got attention, but also were met with some skepticism.

I remember standing in the Rhinelander pits studying the new Chaparral suspension when Polaris racer Bob Eastman walked up. I asked Bob what he thought, and after a while of looking he said: “Very interesting, but very complicated, and also way too light. I think they may break and we should then be able to beat them with our stronger leaf spring setup.” Bob was quite correct for the time being. There were early development problems, and leaf springs were the dominant choice for our racers for the next five seasons.

While the IFS ski-spindles swung in arches, controlled by the length of their swing arm and radius rods, leaf spring spindles were fixed in the front cross member, and all suspension motion went through the leaf springs. This meant that there was no bump steer or scrub, as the steering rods and spindle did not move with the suspension action. The leaf springs would also bend under side load, so the carbide runner would not hook so aggressively during hard cornering. Leaf springs also went through development with multiple leafs, longer travel and better shocks, to end up with very dependable and predictable performance.

With the recession following the Arab oil embargo in the 1970s and several bad snow-years, Chaparral went out of business, and the Unser IFS designs faded from racing circuits. There were two exceptions; Gordy Rudolph and Gilles Villeneuve. Gordy worked at Chaparral, and when they folded, he took the IFS concept with him back to Illinois and built race sleds out of his shop for himself and other privateers. Gilles built a twin track racer with front A-arm suspension copied from his formula Atlantic race car. The A-arms were short and represented many compromises in bump steer, in scrub, in camber and caster variations. But the width of the twin track sled made it more stable and not so easily upset by the front suspension motion.
Gilles then went back to a regular single track leaf spring sled for the ’74 and ’75 season, and promptly won the 1974 World Championship at Eagle River. Perhaps many thought this was the end of the IFS experiment, until Gilles showed up at the 1976 Sno-Pro races at Kilkenny in New Hampshire, and convincingly won the 250, 340 and 440 Pro classes with his liquid Kohler powered Skiroule IFS equipped sleds.
IFS REV chassis
■ IFS begins its takeover
The Gilles IFS sled was an improved version of the Unser-inspired Chaparral design and immediately proved the worth of Unser’s concept. This time Bob Eastman saw the changes coming and bought a sled and suspension pieces from Gordy Rudolph as a development base for the 1977 RXL race-sleds. The original Rudolph sled was short and very light.

As the Polaris racers aggressively tested the new prototype they ran into problems that others had been unable to solve, and as a result had given up earlier efforts. While leaf springs have some give, the IFS moving spindles hooked up hard. This made the sleds unpredictable and dangerous to ride, with some calling the direct steering feedback “unrideable.” With the IFS you had problems not present with leaf springs. First, unless the steering rod joints are lined up with the pivot axis between the swing axle and the radius rods you will get bump steer. As the spindle and steering arms move when hitting a bump, the steering arms will move independently and cause unpredictable weaving action. Depending on the mounting of the radius rods controlling the camber action of the spindle, you can get increasing positive camber which hooks the carbides hard during chassis roll. It can get so aggressive it flips the machine and driver in a corner.

Scrub occurs when spindles move up and down in arches, and changes the distance between the carbides causing “hunting” of the front end. Between the bump steer, the “scrub” and aggressive camber changes in roll, some early test drivers were downright scared of the IFS sleds.

This was quite a challenge for Eastman and his race crew. In amazingly short time they managed to get a handle on the problems using testing, redesign, intuition and new ideas. First, they made the sleds more stable by moving the IFS spindles five inches forward and installed a longer track. By mounting the steering rods out front in the center, they lined up with the inside mounting points of the radius rods, reducing bump steer.

To reduce camber change in roll they made the bottom radius rod shorter, tucking the spindles slightly in during roll. Instead of hooking hard, the carbide would hook less under the critical roll-load and let the front end under-steer slightly, instead of flipping the driver into the haybales.

Another serious problem was chatter of the skis in corners. This occurred with straight aggressive carbides as both the front and back of the carbide would naturally try to go straight, while the ski tried to run in a radius around the corner. Either the front or the rear of the carbides had to let go violently, and this set up the strong chatter action. The solution was a well kept secret to Polaris’ domination. The Polaris racers built a quarter-inch or more curve into the skis and carbides, making them look like the rails on a rocking chair. This was called rocker in the ski and carbide, which concentrated the load more in the center of the carbide. All this development work was done furiously in a few months, and when the Polaris team showed up at Ironwood, Michigan the sleds ran straight, steered predictably and allowed drivers to race hard without fear of sudden surprises.
Ski-Doo REV chassis
The result was total Polaris dominance. When the competition tried to catch up, they entered race-sleds with IFS designs that flipped drivers on corners and were so unpredictable many racers abandoned the new technology and went back to their leaf spring race-sleds. It took the competition several years to catch up to what Eastman and his crew had sorted out in a couple of months.

■ Move to stock trail sleds
The Ski-Doo IFS sleds improved by the early 80s, but Arctic Cat’s Z suspension was aggressive and unpredictable, until they started on A-arm designs when they rebuilt the company in the mid-80s. As the decade rolled on, the early Chaparral layout (raced by Polaris) became perfected on trail sleds. With longer suspension travel controlled by coil springs and oil shocks, ride quality greatly improved.

As time went by, patents expired, and the major manufactures adopted the simple Chaparral layout. Although Arctic had stayed with A-arm suspension since the mid-80s, the width of the bulkhead in the front prevented long enough radius arms to get good control of all the design parameters as suspension travel grew.

By 2004 the Ski-Doo REV- revolution had changed designs dramatically. Not only with the driver-forward positioning, but also with narrower bulkheads allowing longer A-arms. Longer A-arms means less scrub during travel, but bump steer has to be controlled with new “rack steering” designs, which lines up the steering rod joints with the A-arm pivot axis.

New Cat designs upped the ante by moving the A-arms further apart. Since the spindle was longer, the camber changes were smaller as the suspension moved through its longer travel. The latest development in front geometry may not look correct at first glance. The A-arms in these designs are not parallel; the top arm is mounted higher on the chassis and tilts down to the spindle. This does the same thing as when the Polaris crew mounted a shorter radius rod on the bottom of the RXL. As the ski spindle moves up during travel, the top is pushed out and the camber change is negative, preventing the ski from hooking too hard during rolls, and smoothing out the ride in corners.

Sled suspensions have evolved at a steady pace since Chaparral introduced the Unser IFS in 1972. All the problems are now well-understood and movement in three dimensions can be predicted on computer programs. Travel both on the skis and the rear suspension has grown to a level where big jumps can be taken with confidence, whereas in the earlier years broken ankles and busted chassis would be a more predictable result.

Where do we go next? Shocks are the new battleground. This is happening already, but is far from finished. There will be bigger bodies, new multi-rate springs and dampening designs, coatings, and more on the stock sleds.
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