Torque or Horsepower?

This subject is definitely confusing more than just the regular tuners. Some experts have lately pronounced that "Torque Doesn't Matter," you need only be concerned with maximum power.

To end the confusion right now, let's set the record straight: for maximum performance you clutch for maximum power, but it does matter a great deal what your torque curve looks like if you want an automatic belt transmission to work well enough to keep you on that power peak. It's easy to forget that your limiting factor is how well your transmission works, and that the modern snowmobile uses a torque-sensing helix cam to help keep the correct pressure on the belt. Understanding how the torque-sensing helix works is the key to understanding clutching.

The torque-sensing helix was introduced into the world of snowmobile transmissions in the late ‘60s, and made an immediate difference on belt life and efficiency. Belt life instantly doubled and efficiency gained an easy 10%. You not only had 10% more power to the snow, but whereas you previously could only transmit 35 hp, you could now easily transmit 70 hp without belt problems. Later improvements in belt construction and the introduction of Kevlar cords almost doubled that figure again, and that's pretty much where we are today. The "torque-sensing" helix adjusts the side pressure on the belt according to the torque the engine produces. Depending on the angle of the helix, a certain part of the torque can be converted into side pressure. The smaller the angle, the more belt side force it generates. This is why "Torque Backup" is important. Torque backup is the difference between the lowest rpm with maximum torque and the rpm at which maximum power occurs.

In order to make a snowmobile accelerate and maintain top speed, there are a number of forces always trying to slow it down. You are fighting inertia, friction and drag, all of which try to pull you off the power peak. This is why it is important to have a powerband where maximum torque occurs at a lower rpm than the maximum horsepower peak. With good torque backup, the secondary helix feels an increase in torque as drag tries to pull the rpm down. With increased torque feedback the secondary drive will start to push the belt into a lower ratio, actually changing to a "lower gear,” which will maintain the engine speed at the horsepower peak.

This is where a lot of confusion starts, mainly because it is not easy to understand what the difference is between torque and horsepower, and there is a BIG difference. Torque is the result of the gas forces above the piston working on the crank arm and creating a momentum. This momentum force can be measured on the dyno brake and is usually given in foot-pounds. Work is something different. It is a question of how often in a given time period you can lift a certain weight. The more times you can lift a weight in a minute, the more work you produce. The same with an engine. Torque tells you how big a load you can lift, and the rpm tells you how often you can do it in a minute.

The horsepower curve is a result of multiplying torque load with engine speed to find the optimum combination of load and speed that will produce the most work. Therefore, the horsepower peak is a purely theoretical figure. It only tells us at what part of the rpm band we have the best chance of producing the most work.

The components in the drive train cannot "feel" horsepower, but they can "feel" speed through the flyweight mechanism and react to torque through the secondary helix cam. It is possible to make an engine with such a narrow powerband that the torque peak falls almost together with the horsepower peak. In these cases the torque must fall faster as the engine speed increases, and by definition this gives a very narrow powerband. Not only does the power fall fast as the engine revs beyond the horsepower peak, but this has almost the same effect as throwing on the brakes when the engine overrevs. As the speed then decreases, so does the torque, and with less torque feedback there is no backshift into a lower ratio to compensate. This results in an engine and a clutch that live in disharmony, constantly chasing each other around, trying to agree on a place to coexist.

If, on the other hand, the torque curve is flatter with a peak at least 500 rpm below the horsepower peak, this usually also means that the power does not fall off as fast when the engine revs beyond the power peak, and instead of the power dropping off, it still pulls hard. When the rpm drops, the torque then increases to downshift you into a lower ratio while maintaining the engine at a power peak. With the engine pulling hard beyond the power peak and downshifting with increased load as the rpm goes down and the torque increases, you have a very happy coexistence between the engine and its transmission. The wider the torque backup, the easier the engine is to clutch. Having maximum torque down lower on the powerband also gives great acceleration out of corners and off the line. Hard acceleration is used in 90% of your trail riding, while there are fewer places where just sheer top-end is needed. This has given us a place for the "Trail Torquer" pipe that fulfills just those conditions. With strong midrange, a wide torque band and hard acceleration, they make trail riding even more exciting.

The engineers who develop your snowmobiles have to take a number of factors into account when determining what kind of power plant to use for your machine. This depends on the weight of the machine and the kind of driving it is intended for. The heavier the machine is, the wider a torque band you need to make it work right. A larger, long-stroke twin engine may be a better choice if you run a lot on tight trails, because it combines good torque with light weight. A good example of this is the popularity of the Arctic ZR 700 (and other big-bore sleds with wide powerbands). Good acceleration out of corners and good clutching is the reward for this combination. A triple engine will weigh more and produce more power at a higher rpm, and this can be used to advantage on lakes and in oval racing, drags, and speed runs where the demands for abrupt on-again/off-again acceleration is less. How much of a compromise between maximum power and a strong torque backup you need for your type of riding or racing often determines if you have a successful trail machine or a winning racer. All things being taken into account, you will likely have the most fun and be a more successful racer if you lean toward having a wider powerband and more torque backup.

In our 25 years of snowmobile racing, we have consistently been more successful when we sacrificed a few top-end ponies and a narrow powerband for a wider powerband with more torque backup. "Torque Not Important?" This statement could only be made if you spend most of your time on the dyno instead of on the trail.

What is "coupling" of the rear suspension and why is it important? What is the difference between "two way coupling" and "one way coupling"?

Suspensions as a whole are nothing but a big bunch of compromises. We'd like the ride quality to be super plush over the small bumps and never bottom out over the big bumps. We'd love to hit the throttle and get total traction with the track not spinning a bit and be able to keep the skis an 1/8" off the ground. We'd like the sprockets to never jump a cog in deep snow, but wear the hyfax at a nice slow rate. It seems as if anything you do to try to accomplish any one of these will compromise the performance in another area. It is all a matter of which combination you feel provides the best value for your applications - what works best for YOU. But first, a little bit of history.

The FAST M-10 was the first true parallelogram suspension. The M-10 introduced coupling to the snowmobile industry and has been the benchmark ever since.

Early bogie wheel suspensions were nothing more than track tension devices with very small amounts of travel, and in some cases no travel at all. The first slide rail suspensions pioneered by Arctic Cat started with about 3" of travel and the ride was improved. When they went to 5" things again got better, but when the travel went above 5-6" the ride didn't necessarily get any better. As engineers kept adding more travel, all the way up to 10" and 11", the ride quality didn't seem to really improve. No matter what the engineers tried, the vehicles didn't seem to work as good as they wanted them to.

Angle Of Incidence:
After developing quite a few suspensions that didn't hit the mark it was finally decided that limiting the "angle of incidence" of the slide rail as it hits a bump was the key factor to further improve the ride quality and make the longer travel suspensions perform. (The "angle of incidence" describes the angle of the slide rails to the ground as it hits a bump. As the front section of the rails hits a bump, the front of the rails rise but the rear of the rails are still at the bottom of the bump, causing the rail to be presented at a specific angle - the angle of incidence. The greater the size of the bump, the higher the front arm will raise and the greater the angle of incidence.) The greater that angle is, the more secondary kick there is to the rider when the rear hits the bump and the greater the loss of control there is to the rider, and greater the loss of speed of the vehicle. Herein is what was limiting the long travel concepts prior to "coupling". It is called many different names, but keeping the slide rails as parallel to the chassis as possible was the answer.

The ideal solution was to be able to carry a specific amount of weight (vehicle plus rider plus energy of the impact) on the rear of the suspension yet be able to limit the angle of incidence. How do you do this? One of the easiest ways to limit this angle was to put a very soft spring in the rear of the suspension. The problem with this solution was that when you land off a jump on the tail (of the sled) the rear of the vehicle bottoms extremely hard. The next round of experimentation was to try to connect the two suspension arms (front arm and rear scissors) together by putting a sway bar between the two arms, and there seemed to be an advantage. The next step was to try to mechanically couple the arms together directly. The arms were connected together via some transfer rods and/or links and some experiments even used cables. There was a huge advantage noticed when the arms were coupled in this manner as the coupling was limiting the angle of incidence.

This is where the parallelogram concept came into being, which all coupled vehicles are a variation of. (Parallelogram meaning; that if you have equal length arms and equal length bars, the link has to stay parallel at all times.) By limiting the movement of the rear arm, both arms would move "together" as a parallelogram, even though only one of the arms actually encountered the bump. The end result is that the suspension rails remain fairly parallel to the chassis.

Front to Rear Coupling:
This type of "communication" from the front arm to the rear is best described as "front to rear coupling". As the front of the suspension hits a bump, the front tells the rear "get out of the way, there's a bump coming!". As the front of the rails start to rise, they are only allowed to rise so far before the rear of the rails begin to move. On a traditional suspension design, this is accomplished by allowing the lower section of the rear arm to only move so far forward - and then stop. This causes the rear arm to move with the front arm (think parallelogram) and pull the rear of the rails up, reducing the angle of incidence and keeping the rails parallel to the chassis. Limiting the forward travel of the lower rear arm pivot is accomplished in different ways on different suspensions, but the end result is the same. On the M-10 it is controlled by the top of the slot, on the Yamaha Pro-Action Plus by the upper gap on the control rods, on a Polaris XTRA-10 & 12 with the F.R.S.S. (front rear scissors stop) adjustable scissor stop blocks, and now Arctic Cat uses a reinforcing cross bar positioned in front of their lower link.

Rear to Front Coupling:
The second side of the coupling is "rear to front" coupling. If you land on the back of the vehicle with a traditional suspension the front arm is independent of the rear arm and only the rear arm is going to be collapsed. To limit bottoming in this situation you have to have enough "spring" to carry the weight of the entire vehicle, the rider, and the forces involved with the impact of the landing. By coupling the rear arm to the front you afford an overall better riding package. After the rear arm has displaced so far (movement to the rear) you stop the travel causing the front arm to start to displace (think parallelogram), but again control the amount of displacement of the rear arm. In effect you are borrowing all of the resistance to bottoming of the front arm shock and spring in addition to the rear arm. This "borrowing" or "combining" of spring rate and shock valving is very similar to what occurs when the front ski suspensions are linked together through a sway bar. Individually, each arm would require a stiffer shock and spring to handle the impact energy all by itself. When linked (coupled) together they can "borrow" spring rate from each other to resist bottoming, allowing the use of softer springs individually. This way you get a softer ride initially, yet a cumulative rate that is more resistant to bottoming when coupled together. This rate combining occurs in both directions of coupling, front to rear and rear to front.

Another consideration of rear to front coupling is weight transfer. By limiting the rearward movement of the arm you are reducing transfer (and traction) but at the same time maintaining ski pressure. For applications where transfer is desirable (drag racing, hill climbing) early coupling of this type is not. This leads to the great compromise of having to choose between ride quality, weight transfer and ski pressure, which can really be a matter of personal preference for your application.

When to Couple and When Not To?:
The timing of when you couple and not couple seems to be part of the magic of what makes one system work better than another. You should not be coupled at all times or the ride quality will be firmer than need be. In this case you are going right past the softer "un-coupled" portion of your travel and are loosing the best ride quality. This is why you can sometimes, on a coupled suspension, find that by increasing the preload of the rear springs that you will end up with a softer initial ride quality. What is happening is that by increasing the preload you are moving the suspension away from a coupled condition to an uncoupled one, requiring some travel to get to the combined rate section of the travel. If you don't have rear to front coupling you have to increase the resistance to bottoming of the rear arm for the tail landing situation. Consequently, when you're not in a tail landing situation the rear arm spring and shock mechanism is stiffer than it needs to be (harsh ride quality). You likely can detect these qualities on those suspensions that do not employ this type of logic. By the same token, these suspensions lift the skis and really hook up well - if that's what you want. What this tells us is that the suspensions that provide the best coupled ride quality do not provide the best weight transfer. When you increase weight transfer on a coupled suspension, you will end up with greater amount of travel that will be softer - maybe too soft which could reduce the overall ride quality.

Current OEM Suspensions

Polaris XTRA-10:
The Polaris XTRA-10 suspension is a rising rate two-way coupled suspension. The black scissor stop blocks on both the front and rear sides of the rear arm provide the limiting mechanism that provides the coupling. It delivers real nice performance in the big bump environment, but compromises much of the lower speed ride quality. Our impressions are that the calibration is intentionally set more for the high speed big bump environment. The adjustability of the coupling blocks on the lower section of the rear arm could be considered a strength but knowing what to do with these blocks when can be a task. One weaknesses of this suspension arrangement is that there's little in terms of friction reduction on the shafts and bushings. In a stutter bump environment the ride quality is just not there in comparison to other coupled designs.

Polaris XTRA-12:
The Polaris XTRA-12 rear suspension is a falling rate two-way coupled system. The obvious strength of this system is the ride quality which seems to be calibrated exactly the opposite of the XTRA-10. Polaris does not seem to be able to get the best of both worlds in one package so they have decided to develop two systems, one for each end of the spectrum. While the XTRA-12 appears to be a parallelogram, the most obvious difference with this arrangement is that the XTRA-12 doesn't have a shock on the front arm. When it comes to comfort, diminishing the effect of the spring and shock on the front arm can afford you much ride quality. By going with this design (eliminating the spring and shock on the front arm) the comfort level has definitely been improved. Initially this design decision may have been more of an attempt at cost reduction as the original XTRA-14 used only one shock in the entire skid frame. What Polaris likely found out with this pilot build of XTRA-14s was that they couldn't get enough work done with only one shock where it was located. In an effort to give the suspension greater bump absorption, a second shock was added to the rear arm to control the energy. What has been gained in comfort by not having a front shock seems to have been lost is resistance to bottoming of the front arm. It may be great for running 40 mph through the stutters, but if you want to go faster through the great big whoops it just can't handle it. Excellent for trail speeds and ride quality, but high speed control and resistance to bottoming is not a strength. Recent tweaks to this design has reduced the ride height of the machines so equipped, reducing the center of gravity and body roll, resulting in noticeable handling improvements. In certain conditions this can be the smoothest riding suspension available; as the speeds and/or the bump size increases, the attractiveness of this suspension diminishes and you will quickly find the limits of the suspension.

Yamaha Pro-Action Plus:
The Yamaha Pro-Action Plus is a true parallelogram suspension with two way coupling. It seems to use a fairly linear rate of shock speed to rail travel. These units are very smooth and since they use a shock on both arms the suspension is able to retain its composure with a bit wider of an operating range into the rough than the Polaris XTRA-12. Still, it appears that in going for the comfort that Yamaha has traded off some of the high speed control of the front arm. This trade off varies from model to model, but it seems to hold true through the line. In our mogul testing this was apparent as the front arm would bottom when we pushed the units hard. There are also a fair share of links in this suspension, and while Yamaha has to be given credit for "sticktion" reduction techniques such as needle bearings and quality bushings, we wonder as to the logic of all the links. We twisted a few of these on our '96 XT and were happy to see the beefy improvements to the '97s, and now the '98s have added a front bumper stop to the rails to provide greater control to bottoming as well as improvements to the center shock to provide better bottoming control. Some of the '97s that were ridden in extreme conditions would damage the front shock as there was no mechanical limiting device other than the rubber stop on the shock itself. The "sliding gap" on both sides of the control rods performs the coupling function on this suspension design. As the front of the suspension hits a bump, the gap at the bottom of the control rods is closed, causing the rear of the rails to move up, keeping the rails fairly parallel to the chassis. In tail landing situations and during acceleration, the gap at the top of the control rods is closed. Once this happens, the front arm will travel up into the tunnel with the rear arm, effectively transferring part of the energy to the center shock and keeping the suspension rails parallel to the chassis.

Adjustments to the control rods take time, requiring replacement or restacking of the plastic shims on the control rods. These are "shop only adjustments", creating a window of opportunity for aftermarket vendors to come up with quicker methods of adjustment. The primary use of these quick adjust control rods is to get more weight transfer, however as the weight transfer is increased there seems to be a fairly proportional decrease in ride quality (the suspension will bottom easier). More weight transfer is obtained by providing a larger upper gap, allowing more weight transfer to take place before the arms are locked together. The Mountain Max versions of this suspension use only 2.5mm spacers in the upper gap for more weight transfer, while the XTC versions of this suspension use 10.0mm of spacers in the upper gap for less weight transfer.

Skidoo SC-10:

Skidoo SC-10 Cross Country Suspension with AD Boivin ETS coupler kit.
Skidoo has their SC-10 suspensions which come in both falling rate and rising rate designs. The falling rate design clearly provides smoother ride comfort yet lacks resistance to bottoming, while the rising rate designs provide good resistance to bottoming but lacks the low speed calibration for ride comfort. Another case of inability to make one unit to do it all well. Push the rising rate versions hard and they feel fine, but they do tend to ride a bit firmer than a fully coupled design.

With the Acceleration and Control Modulator (ACM) on the rising rate versions of the SC-10, Skidoo entered the coupled arena but only with rear to front one way coupling. The adjustable ACM allows the rider to control the amount of weight that is transferred to the rear arm (vary the moment of coupling through the amount of rear travel of the lower rear arm) before rate is borrowed from the front arm. The tighter the ACM is, the sooner the "coupling" occurs keeping the weight transfer at a minimum and ski pressure at a maximum. As the ACM is loosened the amount of weight transfer increases, as does the ski-lift.

The Skidoo A.C.M. is a rear to front coupler. Tightening the adjuster nut limits the rearward movement of the rear arm link, limiting weight transfer to the track. This arrangement works great for tail landings and dialing in traction or ski pressure, but does nothing for reducing the angle of incidence when the front arm hits a bump before the rear arm. There is another little link in the center of the suspension that could act as a limiter device in the rising rate versions of this suspension and possibly offer a small amount of additional coupling, but the vehicle does pitch some due to the inability to reduce the angle of incidence (keep the suspension rails parallel to the chassis) and it doesn't reach the ride quality of the true two way coupled designs.

One area that the rising rate SC-10 works so well at is the ability to accommodate a wide range of rider weights and still provide acceptable performance. From a 120 pound rider all the way to over 300 pounds simple preload adjustments are pretty much all that is needed to keep the unit working acceptably. Never before have we had such range and compliment Bombardier engineers for this ability.

The falling rate SC-10 units ride better than the rising rate designs do, all the way to the point of bottoming. Unless you are into plenty of tail landings you may prefer the ride quality of the less expensive suspensions. Too bad they don't have better shocks to go with the package.

(An interesting twist to the SC-10 rising rate suspensions with ACM is that AD Boivin has developed an add-on kit that provides full two way coupling. By attaching two composite pieces between the rails and the rear cross shaft, the fore and aft movement of the rear arm is limited via a slotted section around the cross shaft. Preliminary testing by SnowTech Magazine last season indicates that it improves the low speed ride comfort significantly at a very acceptable cost.)

The "slot" at the top of the AD Boivin ETS arms add two-way coupling to the SC-10.

Arctic Cat FasTrack:
Up until 1997, Arctic Cat was using a non-coupled rising rate suspension through their entire line. While excellent in terms of control and high speed performance the design has quite a reputation for not being the smoothest set up in town. Granted, they have been the non-conformists of the group by developing their own unique designs in the ETT angled tunnel, along with the Torque Sensing Link to control weight transfer. We were of the opinion that the TSL, while it may be a great control device did little for us in terms of ride quality. The greatest benefits will be noticed accelerating out of deep moguls where the TSL will resist compression of the rear arm and will actual keep enough weight on the skis to retain steering control. Rather than use up suspension travel during acceleration, the TSL makes the travel available when you crack the throttle.

The gold shaft in front of the rear arm link limits the forward movement of the arm, providing front to rear coupling to the FasTrack suspension. As the '97 production units arrived at the dealerships it became apparent that Arctic Cat had jumped in and decided to get some coupling into their suspension. In looking at the suspensions there was a new gold colored shaft bolted between the rails just in front of the lower section of the rear arm. While Arctic engineers insist that this is simply a reinforcing shaft they make no claim as to its function as a coupling device. There is no doubt that this will limit the amount of forward movement of this link, making it a front to rear coupler which will limit the angle of incidence and improve the ride quality. Those who owned a '97 Cat so equipped tend to agree.

Arctic Cat FasTrack Long Travel Suspension with Extra Travel Tunnel and Torque Sensing Link:
The problem with this kind of "add on design" is that it may not result in super coupling because of limits of the geometry. Just like the XTRA-10, the Arctic suspension is not a true parallelogram and thus is not afforded the range to tune the amount of coupling, as well as when to couple and when not to couple. But this still means that Arctic has no doubt moved into the coupled arena, and Arctic Cat riders all benefit from the improved ride comfort.

The Torque Sensing Link does provide many of the same results as a rear to front coupling device, especially when it comes to weight transfer characteristics. Rather than mount the rear cross shaft directly to the tunnel, it is mounted to a cantilevered arm which is in turn mounted to the tunnel. As the snowmobile accelerates, the Torque Sensing Link counteracts the tendency to compress the rear suspension by pushing down on the rear arm, which does help to maintain the rail angle parallel to the chassis. Instead of getting the skis to lift off of the ground, you can actually get the skis to stay planted during acceleration. An extra bonus is that this system also helps to maintain track tension.

FAST M-10:
Now that FAST has announced they will be building their own snowmobile, can we safely consider them an OEM? Regardless, the M-10 was the original, effective long travel and the first to introduce the whole coupling concept. In fact, all of the existing coupling designs owe their existence to what Gerard Karpik introduced on the M-10. The M-10 is a true parallelogram, allowing for full two way coupling. It is a falling rate design during the first seven inches of travel but uses a "crossover" spring which is unique to the M-10. This "spring inside a spring" is located on the rear shock inside of the larger diameter coil over spring. While enjoying the ride quality comfort of a falling rate design for the first seven inches of travel, the crossover spring is engaged for the final inches of travel which makes the suspension act like a rising rate unit with excellent resistance to bottoming.

The FAST M-10 has no rear arm "scissors" link. The equal length front and rear arms remain fairly parallel to each other by limiting the movement of the rear arm pivot. When you do bottom, the front and rear arms bottom together and the vehicle remains level with little kicking or pitching, which is true of the two way coupled systems. It is this combination of the falling rate comfort and resistance to bottoming that allows the M-10 to still, in our experience, to provide the best of both worlds - ride comfort as well as resistance to bottoming - in a wide range of operating conditions. Tail landings seem to be a strength in comparison to the other designs.

However, the M-10 is not without its drawbacks. As with other fully coupled designs weight transfer is rather limited. It requires a calibration that is weight specific, and when the rider weight varies by more than 25 pounds it is wise to make adjustments. Top speed is decreased slightly, which may not be regarded as important for the bump environments is was designed for, but is a consideration. (Top speed decreases of any more than a couple miles per hour with an M-10 are caused more by having the track tension too high than anything else.) Varying the points at which coupling occurs is not adjustable without replacing the coupler blocks. Even so, it has been the reference point of all other suspension designs for years.

The "biased coupler" limits the rear arm pivot on the M-10, providing true two-way coupling.

AD Boivin ETS Expert / Skidoo Formula Z 670 A.R.M.:
While these two suspensions are slightly different, they function basically the same. Skidoo purchased the right to further develop and manufacture the Expert suspension for use in their own machines, while allowing AD Boivin to continue to manufacture and produce their version of the suspension for aftermarket applications. It is perhaps the most radical departure in that it uses a single swing arm instead of the traditional two arms as found in all of our other rear suspensions. The suspension rails are linked directly to the tunnel with two "side-by-side" shock absorbers using dual rate springs.

The AD Boivin Expert has greater adjustability than the A.R.M. The rear black arm provides the front to rear coupling. A variable length limiter strap linkage keeps the rails parallel to the chassis during acceleration and trail landings, providing (in effect) rear to front coupling. By using a single swing arm the Expert provides a constant "wheelbase" between the ski spindle and the track which is said to provide greater stability. This should especially aid in cornering as the weight is not shifting from the front arm back to the rear arm, rather it is shifting to a single arm in varying proportions. In comparison, suspensions with two arms are constantly varying this "wheelbase" distance, constantly shifting from a short wheelbase to a long wheelbase.

This unit actually is a fully coupled suspension in that no matter which end of the suspension is compressed, the action of the suspension keeps the rails near-parallel to the tunnel at all times. Two separate mechanisms control movement of the rails on this design. The first is the rear rod or arm that attaches the rear of the rails to the rear cross mounting shaft. This rod provides the front to rear communication, limiting the angle of incidence. It could be argued that this "rod" is in reality the rear arm of our parallelogram, because without it true two way coupling does not exist.

The second mechanism, which is a very unique feature to this design, is what's called a "dynamic transfer regulator". This linkage varies the limiter strap length as the suspension moves through its travel, as well as compensating for varying rider weight or traction conditions. This in effect provides the rear to front communication. The beauty of this feature is that as the machine accelerates the limiter strap is lengthened, lowering the front of the suspension keeping as much track on the ground as possible while keeping the skis on the ground too. A second position lengthens the limiter strap which allows for greater weight transfer for those who want to use this suspension in deep snow applications.

While the Expert provides three lower mounting positions for the shocks, allowing rider selection of falling or rising rate shock speeds to suit the riding style and weight, the Skidoo version has selected a single mount. This not only reduces production costs, but removes confusion in getting the unit properly calibrated. Skidoo evidently feels that the typical buyer of the Formula Z 670 will fall into a 170-220 pound range, where the single mounting point appears to be calibrated for. The Skidoo selected mounting position appears to be slightly falling rate and accounts for the positioning of the suspension in more of a touring machine than in a cross country vehicle like an MX Z. Skidoo has struggled with dealers who were selling this suspension to riders who were better suited to an MX Z. It is not calibrated for an MX Z type rider, and likely will not perform up to the expectations of riders in those applications.

The Skidoo A.R.M. is based on the AD Boivin "Expert". The rails pivot on the single swing arm, limited by two separate mechanisms. This suspension is being used in the Formula Z 670, which is very stable and much smoother than the SC-10.

While our riding has been limited on this suspension design, it is an improvement in ride comfort compared to what Ski-Doo has offered in the past. Our experience on Formula Z 670s has been that body English is not really needed to get the machine to hook up or to corner. You can just sit back, squeeze the throttle and brake and turn the handlebars and the sled will go where you want it to. Tapping on the brakes and leaning far forward to corner is pretty much useless and really didn't seem to make much of a difference in how the vehicle handled. This is why we say it is far better suited as a cruiser than for aggressive riding.

 CHOOSING AN OIL!

Update: I have been hearing a lot of good press on Blue Marble oils = HP gains and real clean burn on the VES check it out at www.nulubes.com. Also Royal Purple just came out with a synthetic two stroke oil snow 2-C. www.royalpurple.com

Many snowmobilers still feel it necessary to use oil of their snowmobile brand name. Pull into a gas stop and the rider of one brand acts like it’s the end of the world if he doesn’t have his "brand" of oil. These same riders will use no-name brand belts, spark plugs, and other odd parts, but when it comes to oil they’ve been brain washed into thinking that their warranty is void if they don’t use that oil.

For many years this was sound advice, but it has turned into nothing more than a myth. The fact of the matter is this; you do not have to use the OEM oil to have your engine warranted. This does not mean that an oil related failure will be covered. It means there are many excellent quality oils to choose from, and that you are not obligated to use the OEM oil to keep your warranty in force. Regardless of what your dealer tells you, this is a fact.

Now, if and when engine damage does occur some dealers like to look at the oil in your tank and try to convince you this was the cause. Don’t buy it. Short of oil pump failure or oil line blockage, very, very few failures on engines operating at stock rpm levels are oil quality related.

Once you’ve made the decision that you might consider another oil, how do you know what’s fair, good, and the best? Maybe this is the reason so many stick with their name brand. They know it’s good and they’ve eliminated any guess work. After all, OEM oils are top quality and designed to keep a stock engine in good shape at standard RPM. If you add a well-designed exhaust system or optimum porting job to your engine, the horsepower will jump 20 to 30 percent, as well as the engine RPM.

The snowmobile’s oil injector pump can be adjusted to its maximum setting to increase oil volume, or put more oil in the tank premix, but this is substituting quantity where quality should be.

There are some really good motorcycle oils. Why not use those? Motorcycle engines never see the abuse snowmobile engines go through, and low temp pour point becomes a factor with motorcycle oil. When the temperature drops to zero or below, an injector pump may be waiting for oil that will never flow through the oil lines. Secondly, even if you happen to premix motorcycle oil, you may need the help of one of your friends to pull the starter rope. That’s because the oil down in the crank bearing has mutated into something closer to "road tar", the result from a -20 below zero night.

What kind of factors should you consider when choosing an oil for your snowmobile? Let’s cover a few.

Cost: This seems to be the biggie. Everyone wants a good product for a fair price. That’s called value. Why spend more than you have to, and why buy a better product than what you need ? What do you need? Does price indicate quality? Not really. It could, but don’t count on it.

Protection: This should be the paramount factor. Long term protection is what you desire, but how do you know ? If you buy the OEM brand, then you have peace of mind - you know it is good stuff. Since the snowmobile industry doesn’t have their own rating or certification method, we rely on the TC-W3 rating as used by the marine industry. A TC- W3 rating indicates at least a minimum performance standard has been met. But that standard is for outboards, not sleds. It is generally accepted that an oil with a TC-W3 certification will be good enough for the average engine in average conditions. Start cranking out horsepower and ride in below zero conditions and the need for a quality product increases. Anytime you modify the engine, add pipes, and produce more horsepower you should consider using a higher quality oil. In this case, quantity can not be a substitute for quality.

Pour point: This is one of the major factors to consider. Many outboard oils do not have very low pour points. When considering a TC-W3 oil, make sure it has a low pour point of at least -40 F. The pour point is a rating indicating the temperature the oil will no longer flow AT ALL. This means you can not use an oil all the way down to the pour point. An oil with a rated pour point of -40 should actually flow down to about -30 F. If you ride consistently in temps colder than that, you had better invest in some synthetic oil that pours down to temps even lower. A word of caution: One method of making a lower pour point is to increase the amount of "solvent" as a dilutent in the oil. This makes a lower pour point, but decreases the actual amount of oil per given amount. Our flow testing of 100% synthetic oils showed quite a difference when the mercury drops down to -40 in comparison to plain "outboard oil". Phillips Injex has turned out to be a favorite from the "multi-purpose" oils as it flows better at cold temps than what some of the synthetics did! We have consistently used Injex down to -30 F with no problems.

Availability/Mix-ability: Is the oil compatible with other brands, and how available is it? These two are tied together. By virtue of its TC-W3 designation it is compatible with any other TC-W3 rated oil. And for oils that don’t have this rating? We haven’t seen a "present day readily available oil" that causes any type of mixing problem in a long time. In fact, we went through something to the tune of over 50 gallons of snowmobile oil this past season, and intentionally poured some of EVERYTHING that we had (at one time or another) into one machine. Ran it down to -30. Seemed to work perfectly fine, but then none of it was junk lubricant, either. Non-scientific, but we know of plenty of riders who did the same. Draw your own conclusions.

Smoke & Smell: This is another of our considerations. We could have an oil with excellent protection with a rock bottom price, but if it smokes too much and stinks, we’ll find something else. That’s why we say it is hard to find a better smelling lubricant that Klotz Techniplate. The sweet smell of power? Image is everything, and your buddies will be impressed. Many of the synthetics have less smoke and smell better than what you’re using. We encourage you to buy a low smoke oil that does not stink so the non- snowmobiler’s can’t be offended. Seriously. A good place to be pro-active instead of reactive.

A recent study performed by the Southwest Research Institute compared a OEM petroleum based oil, a biodegradable synthetic oil, a semi-synthetic oil, and a isobutylene based synthetic oil. The oils were compared on a basis of the amount of oil smoke particulate matter in the emissions. The isobutylene based synthetic had 55% less smoke than the petroleum based oil, 90% less smoke than the biodegradable synthetic, and 31% less smoke than the semi-synthetic. So if you’re looking for an oil with less smoke, the isobutylene based synthetic is a proven winner.

Color: That’s right, color! Sounds stupid, but some people will buy oil (or not buy it) based on color. In reality, color now days has ZERO to do with performance. Dyes are added simply as a matter of identification, or to add color when premixed with gas. It is a matter of personal preference. We like purple or red oils better than green oil. (We like blondes better than red heads too.)

Carbon/Clean Burning: Oil experts say this can’t be pinned on oil alone, that there are several other factors that go into this. We do know when chunks of carbon flake off the top of the piston it is time to try a different oil. Or, when the exhaust power valve gets so gummed up that it doesn’t slide any longer a change would be wise. We have been most selective with our RAVE equipped Rotax engines for this reason. Even when running a 100% synthetic oil, removal and cleaning of the RAVE valves is a good idea every 1000 miles to keep them clean and free moving.

A lot of the decision goes into trust. Factor in what you hear and read, but make your own decisions based on your wants and needs. A name brand oil with a low pour point at a good price you can get your hands on will likely be just fine in your stock sled. If you are comfortable with the smell, smoke, price, availability, and do not find an excessive amount of carbon build up, you’re set. If you have increased the output of your engine above stock, protect the investment and run a higher quality oil. Do not let someone convince you to use their oil to maintain the warranty. While it is true this gives them one less factor to consider, it is not a requirement.

During the ‘97-‘98 season we ran Phillips Injex (petroleum based) in our stock sleds, Torco Synthetic (isobutylene based synthetic) in the Rotax sleds, and Klotz Techniplate (synthetic based) in our piped sleds. We’ve tried several others, some good, some not as good. We decided to use these three based on cleanliness of burning, pour points, availability, and (most importantly) protection for the application. We could have likely run other oils in any of the sleds and not "seem" to have a problem. Just like I could ride without a helmet and might not have a problem.

I wear a helmet.

So to prevent you from failing Spark Plug reading here is a multitude of counsellors in picture form.

Source NGK Spark Plugs hand out. I think their Hot-but-OK plug pictures are a bit too hot for me. But who am I to nay say NGK ?