AmSnow.com is now SnoWest.com
¦ 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.