Geometry and why carbides help hinder steering

Steering Tech 1.0

New Tech – Yamaha added electric power steering to its Apex for this season, but carbide and ski design, along with front-end geometry and shock technology evolved first to aid in handling.
When Polaris came out with the RXL race sled with its IFS front suspension, it made a giant leap past the competition. Polaris totally dominated oval racing for several years starting late in the 1977-78 season. When the competition scrambled to catch up, they ran into a host of problems.

While Polaris sleds cornered like they were on rails, the catch-up crowd from the other manufacturers was all over the place with front-ends that darted violently and flipped riders off like cowboys on bucking bulls.

Overcoming IFS problems
The IFS debacle went so far that some drivers from OEM’s other than Polaris refused to race the new IFS designs because they were dangerous. Many of the other manufacturers went back to the proven leaf spring suspensions because they could not understand how Polaris was able to get the IFS design to handle properly. For the other OEMs, the traditional leaf spring ski suspension was more forgiving, as the springs twisted under side load. This twisting and flex allowed the drivers to use very aggressive carbides because it masked potential problems.

In short, leaf springs were a primitive design that worked fairly well, but it wasn’t the best design.
With the new IFS design there was no such flex, and many racers found that the long aggressive carbide runners they used on leaf spring skis would just rip the handlebars right out of their hands if the carbide runners were placed just a little wrong on the skis. As a matter of fact, Polaris almost gave up on the IFS design, and at the first race of 1977 at International Fall, Minn., its race team ran both leaf spring and IFS sleds. If I remember right, the leaf spring sled won!

By the time the first Pro race of that year came in Ironwood, Mich., Bob Eastman and his team at Polaris had the IFS figured out and started a dominating run by sweeping the Pro classes. For the rest of that season we all watched in amazement as Polaris’ Midnight Blue Express ran away, while the green, yellow, and red/white competitors flipped and squirmed down the track in a desperate attempt to catch up.
What did the Polaris factory racers know that the competition could not figure out?

With IFS designs, steering geometry becomes all important. Long runners made the skis vibrate and the sled “hunt” down the racetrack’s straightaway, and aggressive caster angles that normally compensated for spring twist with the leaf springs now dug in aggressively in the turns and flipped drivers off their sleds.

Polaris had made several modifications to their runners. First they were not quite as long. Second, and most important, their runners and skis had “rockers” built into them. This meant that the runners were not straight, but looked more like the rails of a rocking chair. On some racetracks the runners could be bowed as much as 1⁄4-inch in the center. This meant that the load was centered under the steering post centerline, and as the ski was turned and the carbide leaned over it had the effect of steering more like a wheel in corners. It also took load off the carbide’s front and rear edges and eliminated the violent shaking racers experienced with straight runners.
In the beginning – Polaris led the way with its RXL, first in race form and then trail versions. This newer 1991 RXL also used independent front suspension and utlized electronic fuel injection.
Pre-historic – Yamaha’s Bravo is a value sled that used the leaf spring design. OEMs continued to use leaf spring designs on several models even through the 1990s. (this model was sold in 1992)
The rocker configuration was the result of careful experimenting, and remained a carefully guarded factory secret for years. Polaris also had a second feature built into its IFS suspension that made it easier and more forgiving in turns. The bottom suspension arm tying the ski spindle to the chassis was much shorter than the top arm. This is opposite of what you will find in a racecar design, where a short top arm is normally used.

On racecars the shorter top arm pulls the top of the wheel in under cornering to compensate for negative camber and maintain grip in their rubber tires. If you do this on a snowmobile with solid carbides, the carbide actually digs in harder as the chassis rolls, and flips the driver into a snow bank.
By keeping the bottom link shorter, the Polaris IFS would actually fold the carbide under the other way and take some grip away. When the Polaris drivers entered the turns aggressively and the chassis started to roll, the front-end would go into a slight understeer, instead of grabbing harder and flipping the sled and driver.

Competitors would try to follow the Polaris boys into corners and come out wide-eyed or upside down while the blue sled disappeared down the straight. With the rocker and the shorter bottom arm Polaris had a technical advantage that was hard to discover and it took many seasons for competitors to catch up. Even 10 years later, when I did a story on the Polaris IFS and mentioned the rockered carbide, I got a call from Polaris’ race director, who was not happy about me revealing “factory secrets,” although by then it was well known and everyone was running some amount of rocker in their skis.

New awareness
What the IFS revolution brought forth was a new awareness of the importance of steering geometry and carbide design.

For a vehicle to be stable in a straight line, you need some kind of self-correcting geometry. In wheeled vehicles this is usually referred to as “trail.” If you draw a straight line through the steering axis of a motorcycle fork, or the kingpin of a car, you’ll find that the tilt (caster) angle makes the steering axis hit the ground in front of the tire contact patch’s center, which lies directly under the wheel’s center axle. The distance between where the steering axis intersects with the ground, and where the center of the tire patch is located, is “trail.” The more trail you have, the more self correcting you have. But with more trail also comes harder steering effort.
Today – These Yamaha images show the geometry of its latest IFS design, its caster and the pivot points of the ski. The pivot point was moved back 15mm to increase trail.
A ski represents a little different picture, because the pivot point is only a few inches off the ground. Also, the ski is flat and not round like a tire, so it has a long contact patch. The distribution of the keel and carbide location in front of, and behind, the steering axis therefore determines the ski’s self correcting action.

More carbide and keel has to be located behind the steering axis to get the desired self-correcting action at speed. Long and deep keels would work well in deep snow where carbides have no effect, and more carbide would give better control on ice and hard pack. Front to rear distribution could be as much as 30-70 or as little as 45-55. The 30-70 ratio would provide the highest self correcting, but be harder to steer, perhaps to the point where power steering would be useful.

The 45-55 ratio would be used for tight cross country trails and snocross racing.

Hunting and darting have been a common complaint lately as sleds get heavier and offer more advanced front suspensions. It reminds us of the problems from the early IFS days.

Longer carbides will grab with the front and fall into grooves on a hard packed trail, which provides unwanted steering input. One popular solution is adding shorter dual carbides to the skis. With two shorter carbides next to each other less steering input is needed, and they do not have the same tendency to fall into a groove as the other carbide would hold it up. If there are a lot of grooves, both may fall in, but the shorter overall length still needs less steering input and the dual carbides transfer out of grooves more easily.

There is a lot of development going on in the steering department. Yamaha has introduced electric power steering, while Ski-Doo seems to have a good jump on the rest when it comes to easy “dart-free” steering without the use of extra power. In addition, the aftermarket ski and carbide manufacturers are busy advancing steering technology.

Stay tuned for Steering Tech 2.0 in a future issue when we take a closer look at some of these new developments.
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