They have spent years grasping at straws trying to gain frontend control!
Compare this article with what is being sold in 2009 and you will see they are still singing the same old song with the same old results.
Supertrax, December 1996
The 1990’s have witnessed a revolution in snowmobile technology. During this tumultuous decade so far, we’ve been there to see the advent of gas shocks with remote reservoirs and floating pistons, we’ve watched as the concept of coupling was applied to a new generation track suspensions and we’ve been witness to an era when suspension travel extended from just 5 and 6 inches to 10 and 11 inches and more.
As we journey through the second half of the 90s, multiple rate springs, controlled roll center ski suspensions, long travel tunnels, torque-canceling links and remote adjustments for ski pressure, spring rate, shock damping, preload and more have begun to appear, keeping the technology momentum going.
How far have we come in terms of ride and handling? Here’s a look at the details that make 1997 a vintage year in the progress of the pilgrims searching for the holy grail of snowmobile suspension excellence…
HANG TIGHT, I THINK WE’RE ONNA ROLL
Relegated to the background just a little bit by all the hype surrounding development of the new extended travel technology for the track suspension, ski suspension design has recently shifted to the front burner as engineers in both the aftermarket and at the factories, work to be steering and handling performance in line with the new high standards set for snowmobile ride quality in recent seasons.
The biggest buzz surrounds the development of controlled roll center technology and it is here that we see significant developments in 1997 as designers work to create new ski suspensions and components to deal with longer travel, increasing demands for precise and low effort steering plus minimum body roll and ski lift.
The object of roll center control is to raise the position of the theoretical point the chassis rotates around in a body roll situation. While the outward manifestation is often seen as spindle angle change as a ski moves through its travel, the underlying physics call for creating a situation in the chassis where that key roll point is high enough and close enough to the cg to make the snowmobile corner flat.
Polaris introduced its unequal length radius rod ski suspension on the XCR440 in 1995 to solve two problems with the IFS in high-performance situations. One problem has to do with the scrub created as the distance between the spindles changes in the equal length system and the other has to do with the fact equal length systems typically display very low projected roll centers, some theoretically on or below the snow!
While we began to hear the phrase controlled roll center, or CRC, being used to describe unequal length ski suspensions at that time, the fact is the Arctic Cat AWS ski suspension is unequal length geometry and has been since before the advent of the Prowler and the AWS technology in 1990.
The AWS design that first appeared on the Prowler used a ball joint set-up that has since been abandoned in favor of a bolt-through wishbone concept that appears in high-performance models in 1997 as AWS-IV or V. The first bolt-through was the AWS III and this featured unequal length wishbones set at about an 18 degree angle to the snow front to rear. When AWS IV appeared, this angle was dropped to about 2 degrees and about this time the front bulkhead was narrowed to increase the length of the wishbones for more travel.
The set-up on the ZR440 is an all-alloy, long travel version dubbed AWS-V and Cat says the roll center theory that determines the very effective geometry on this unequal length front end and the other AWS units is essentially the same compromise between camber change, scrub and roll center control that was worked out years earlier by the Sno-Pro racing team.
Polaris started out in the CRC game with an unequal length set-up for the 1995 XCR440 that went to a significant positive camber at the top of travel and was subsequently refined to set-up with very little camber change through effective travel in last year’s XCR440.
This story is being written before we will have a chance to measure the 1997 XCR440, but we suspect the latest in CRC from Polaris will stay with this trend and features only slight camber changes. But the story at Polaris has another focus. The IFS pioneer has moved in another direction for 1997, one that also relates to flat cornering and more precise steering.
A control roll center IFS is in many ways like the wishbone set-up only stretched-out and angled by the trailing arm. The 1997 CRC Polaris sleds, which include besides the XCR440SP, the XCF, the XC440 and 600, the XCR600, the Ultra SP and SPX and the 700S, display a shorter trailing arm for 1997. This change is less about controlling roll center than controlling how turning forces are fed into the chassis. For several complex reasons only geometry geeks understand, the reduced trailing arm length brings the cg and roll center effectively closer and the pivot for the arm moved farther forward reduces lift at the front of the track contact area.
CAST YOUR FLEX TO THE WIND
Yamaha says they went with the longer arm to minimize spindle angle change as the suspension moved through its 9 inches of travel and this is a factor that can’t be ignored in the quest for accurate handling with IFS. Because the angle of the ski spindle goes ever more shallow as an IFS suspension compresses, there is an inherent change in the way forces are directed into the ski that many believe relates to push in corners. Obviously, the more distance between the spindle and the pivot point there is, the less angle change there is, and in this area, Yamaha is the clear leader.
Yamaha chose to ignore the roll center issue in this first IFS and worked instead to take some of the flex that has characterized this type of suspension in other brands out of the new babies (all named “Max). The cast aluminum bulkhead that has received lots of attention from us and everyone else is aimed primarily at this factor and there is no doubt the inboard mounts for equal length radius rods are extremely rigid.
The fact is, Yamaha engineers were looking for rigidity when they engineered the steering pivot, shock tower mounts and radius rods into this one-piece casting… and they got it. From the saddle, this lack of flex in the ski suspension is noticeable right away and, while we obviously haven’t had much chance to fiddle with set-up on this all-new front end, we expect the ultra-rigid Yamaha to be one of the most predictable and consistent IFS units ever.
HOW MUCH DOES YOUR INDY TRAIL?
Regardless of the approach taken by each of the manufacturers, one goal in front suspension development has continued to be to optimize steering effectiveness and minimize effort. Some recent detail refinements visible on the new sleds reflect this interest, most notably the trend to trailing spindle mounts and adjustable spindle pivots now seen on Arctic Cat, Polaris and Ski Doo.
One major variable factor in this location is the value of the spindle angle. Steeper angles at the ski spindle generates greater turning forces around the ski axis and it is for this reason the bolt that attaches the skis to the spindle at the place usually called the saddle was moved to a position trailing the center point of the spindle first by Arctic Cat and now by Polaris.
Cat’s AWS and earlier AFS ski suspensions have generally displayed steeper spindle angles than other sleds and the trailing saddle bolt has been used on Arctic sleds to give the skis some centering force to counteract a tendency for the skis to “take charge” in high force turns. As long as the spindle angle is at around 25 degrees or more, this is not much of an issue, but when the spindle angle gets up towards perpendicular – around 18 degrees preferred by Cat engineers – then adding trail reduces a tendency to “seek” that comes with steeper angles.
As steering effectiveness has sharpened up on the Polaris IFS with the advent of more CRC front end and rocker ski, the need for more aggressive centering effect made the trailing saddle bolt design part of the equation.
The new MX-Zx displays an essentially straight angle saddle orientation without significant trail for 1997, but Ski Doo engineers have added a twist to this new rocker ski – equipped sled that relates to ski orientation not totally unlike the trailing saddle bolt. On this new Ski-Doo cross-country sled, a unique slot appears where the saddle through-bolt connects the ski to the spindle. This design permits the rider to adjust the exact location on the ski where the spindle pivot point is located, effectively moving the center pivot point fore and aft on the new composite rocker ski. This movement changes both the exact position of the carbide runner as well as the position of the center of the “bow” in the rocker ski itself. It is a detail that we expect to see expanded to include other Ski-Doos and in time, other brands as well.
The 1997 model year is not, for some, a pivotal year in snowmobile technology. While the tendency is to see this particular vintage as a year of incremental change and development, there are some “devilish details”, particularly in the suspension area, that suggest a little gizmo here and an adjustment there and yet another dimension change over there adds up to a year when the art and science of snowmobile suspension snaps into its sharpest focus ever.
The best summarization, of the point I am trying to make, came from a snowmobiler "Millions of dollars of engineering still cannot replace good old common sense."
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