Steering Tech 2.0

Explaining the black art of handling
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Steering geometry – Arctic Cat has the steering geometry dialed in perfectly on its Sno Pro 500.
Kort Duce
Making a snowmobile steer and handle correctly in many different conditions is still somewhat of a black art. From crispy hard-pack to slushy trails to deep powder, a modern ski has to handle all of these conditions well, without any big surprises for the rider.

A lot of compromises must be made to accomplish this, and that is why skis and runners are in constant development and new designs are popping up from both the manufacturers and aftermarket companies.
Here, we’ll look at not only skis, and runners, but suspensions, geometry and more to help paint a clearer picture of why snowmobiles steer how they do.
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Dually – Placing two runners next to each other in a Dually runner allows you to have 10 inches of carbide in a 5-inch length and makes steering much easier.
wheel
what is “trail?” – On a car wheel the contact area between the tire and the ground is narrow. On a snowmobile ski the contact area is as long as the carbide if on ice, or the full keel if on loose snow. The “trail” distance is the intersection of the steering axis with the ground to the center of the carbide or to the center of the keel. In most cases the carbides are mounted under the center of the pivot bolt. The carbide can be moved to change the “trail” as can the pivot bolt when it is offset back from the steering axis.
3 big developments
Three landmark developments that changed the handling of snowmobiles were independent front suspensions, plastic skis and dually runners. All three changed the way we looked at the steering equation, and all brought big changes in suspension geometry, ski design and carbide runner development.

The early leaf spring suspension had a lot of forgiveness built into it, as the springs would twist and roll the ski under a hard side load. With the new and more solid IFS suspensions the side load in the corner was transferred more aggressively into the chassis. As a result, steering inputs to the handlebars were more aggressive.

Introducing rubber bushings in the swing arms alleviated some of the harshness, but changes in spindle angles also had to be made. The more spindle angle you have, the more the ski tilts when it’s turned, and the more aggressively the carbides dig into the ice.

In addition, managing the steering force on a flat ski is very different from the forces working on a round wheel on say, a car or motorcycle. With a round wheel you have a very short contact patch of flexible rubber, while on a ski you have a long flat runner with carbide inserts biting into hard ice.

On wheeled vehicles the center of the wheel also is a fair distance above the ground, while on a snowmobile the ski mounting bolt may be just a few inches above the ground.

To make the steering self correcting, the kingpin on a car or the fork head on a bike is set on an angle to the pivot axle so the centerline hits the ground in front of the tire contact patch, which is located directly below the wheel center. The distance from the steering axis to the center of the contact patch is referred to as the “trail.” The longer the distance of the trail, the more self correction you have built into the steering.

On a ski, the pivot axis hits the ground very closely to the ski mounting bolt and therefore there is very little built-in trail in a snowmobile ski design. On the other hand, skis have a long flat carbide contacting the ground, and this gives us an advantage over a wheel design, because you can move the location of the carbide in relation to the pivot, actually moving the center of the carbide back from the steering axis to get more trail.

The location and shape of the carbide and the ski is therefore very important to steering input. There has to be more carbide behind the steering axis than in front of it to obtain self-correcting steering. If you locate too much carbide toward the back, steering will be hard. If you locate more carbide in front of the steering axis, the skis will start to hunt or dart and you will be in for a scary ride at higher speeds.
Furthermore, the combination of spindle angle and carbide and keel location determines the actual trail.

Most manufacturers locate the center of the carbide directly under the ski’s pivot bolt, and then play with spindle angle and pivot location. Offsetting the pivot bolt behind the spindle centerline gives more trail and this often is accompanied by less spindle angle to lighten the steering. Yamaha and Polaris both have used this approach.

The longer and straighter the carbides are, the harder they are to steer as the sled is making a circle; and this means the front and rear tips have to break out and shave away some ice as you turn, resulting in hard steering and erratic steering otherwise known as darting.
skis
Normal – Here you see the spindle angle on an easy steering 2011 Arctic Cat Sno Pro 500 (left), and the more aggressive angle on a 2009 Polaris RMK (right) that is used in the mountains.
Two approaches
Two approaches have been invented to correct this.

The first solution was to introduce a “rocker” or a bow to the runner so the carbide digs harder in the center, and as the ski is tilted the runner actually has an arc to it that more closely follows the steering radius.

The second and most recent development is “Dually” runners. By placing two runners next to each other you can have 10 inches of carbide in a 5-inch length. This makes the steering much easier, but you still have a lot of carbide on the ice. Once you get out of hard pack and into looser snow, the shape of the ski keel has to conform to the same rules. There has to be more keel behind the steering axis in order to have self-correcting steering, and the keels also must taper up both in the front and rear.

The depth and shape of the ski keel influences the steering in deep snow and all these variables have to be taken into account to make a ski perform well in all conditions. That’s why designing skis and carbides is still somewhat of a black art, requiring a lot of experience and testing in real life conditions.
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More aggressive – This modified drag sled shows a more aggressive, steeper spindle angle, something most of us would not appreciate on our trail sled.
How OEMs test, implement
Arctic Cat appears the furthest ahead in the non-assisted steering area. Nearly every year when we test the new models, Arctic Cat tells us that it has been working to make the sleds steer even more accurately and aggressively. But recently Cat has managed to do this AND keep the steering input light and keep darting to a minimum.

Al Shimpa, who takes care of Arctic’s racers in the winter as drag race and hillclimb coordinator, usually leads the crew at the new model introductions. He tells us Arctic uses a number of skis with different keel designs for different sleds, and a number of different single and dually carbides.

This year at our annual test and photo week in West Yellowstone the Cat’s were especially impressive, always steering as if on rails, with light steering input and not a hint of darting. Unfortunately, the same could not be said about some of the other sleds we rode. So I cornered Al to find out Cat’s secret. Al has always been honest with me, and the answer was as straightforward as he is. “There is no big secret,” he said, “we just show up early and do a lot of testing for the local conditions, and then pick the best combination for each sled accordingly.”

In West Yellowstone they groomed the trails for us every night, and as a result the trails were flat, hard and grippy in the morning, which influences the choice of carbide runners. In the afternoon the trails would thaw and get softer with deeper ruts, which influences the ski-keel choice. Al and his crew picked ski and runner combinations that would work well in both conditions. They picked the correct runners for the hard pack and the best ski-keel design for the afternoons’ loose conditions.

I was most impressed with their choice for the new long-track F8 EXT. You would expect a long-track to push in the corners, but the test machine steered easily and straight with no darting. This is the result of a lot of testing and years of experience, but the Arctic ski and runner combinations have almost always performed well, even on our long-term test machines without making ski or runner changes over the longer test period. The EXT long-track also handled rough moguls well. The 800 engine pulled as good as the 1100 turbo, but the 800 is lighter and with Cat’s easy steering, the EXT makes for a fun ride.
Each manufacturer has its own approach to steering.

Yamaha moved its pivot bolt location back and offset from the steering axis, which gives more built-in trail, but also somewhat more aggressive and harder steering. This was done on the new Apex, which also received the industry’s first electric power steering, which easily counteracts any harder steering.
The angle of the steering stem also influences the trail. The larger the angle the more built-in trail you will have. Larger angles are usually found only on oval racing sleds where they also lean the carbide more as the steering is tuned for maximum grip. The oval setup is very aggressive and hard on the arms, and therefore not practical on a trail sled.

Picking the ski and runner combination that fits your riding style is somewhat like clutch tuning. Guys with a lot of experience will always be able to pick a good combination, and that is one of the reasons skis and runners are one of the areas where riders can improve handling substantially by testing and experimenting. Fortunately I see more improvements coming in the future, as both manufacturers and aftermarket firms are continually working to develop improved skis and runners, and now even power steering!
olav-14a
Olav Aaen is a long-time contributor to AmSnow.
As a mechanical engineer and president of Aaen Performance, Olav has been heavily involved with snowmobile performance since 1968. Aaen Performance is best known for pioneering performance pipes and introducing the roller clutch to the sled market.
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