Mountain sleds are also useful to access remote mountain top radio sites, therefore, this entire training will be for mountain sleds and riding techniques. While there is a limited amount of equipment and other items that can be carried on a sled, a simple scouting trip, or simple repair (replace that fuse) can be effective and timely. Also, trailering sleds are generally easier than (heaver) snowcats.
Mountain Safety
Riding in the mountains is very different than on flat roads and trails. In many cases you will be dealing with deep snow, slopes and foul weather, including winds and poor visibility. Some of the pictures and videos you might have seen in the past were "kids" (wild adults) playing in the extreme sense for recreational fun. While the techniques covered in this training bear some similarities with these "maneuvers", they are only to make you aware what modern sleds are capable of doing and not to push yourself far beyond your skill level. Some of the pictures that will be presented here show techniques beyond normal maneuvers, however, can be performed relatively safe, with sufficient practice. Some of them can be covered in the optional field training. The important point to remember is they are not what you are expected to do, rather what you can do, should the need arise to deal with a situation. It's safer to have the knowledge and practice should you need to use it to get yourself out of a potentially dangerous situation. Mountain riding can be more physically demanding from trail riding, so practice and being in shape helps greatly. More on riding, later.
Engine overview
Snowmoblies engines come in 1,2,3 or 4 cylinder, both in 2 and 4 cycle and both in air or liquid cooled operation. Most mountain sleds utilize a twin cylinder, two cycle, gasoline, liquid cooled engine. Displacement-wise they range from 300~900 cubic centimeters depending on number of cylinders and performance type. Two cycle engines do not use an oil sump like 4 cycle engines do. Therefore, they depend on the oil coming in with the air/fuel mixture for lubrication of the rings, and bearings. There are two ways used to accomplish this. While some manufactures have the operator mix oil with the gasoline, most design in an oil pump that draws from a separate tank and injects oil into the air/fuel mixture. Because this oil is burned up in the combustion process, sleds do smoke to some degree out the exhaust. During engine break-in it's recommended that the oil mix be increased (even more) by adding more oil in the gas tank, for the first tank. Details are with the owner's manual. During this period the smoke will be rather heavy. Both of these conditions are normal. The burning of oil also causes spark plug fouling, therefore, the plugs need to be replaced occasionally. The replacement interval can be as long as 6 months or shorter under constant use, but plugs can foul anytime, so spare plugs should be carried and are usually stored in the engine area, sometimes in a separate tool compartment. Snowmobile engines in general are designed to operate in ranges of 7,000 rpm, and even higher, depending on the specific model and setup of the sled. There will be more discussion on this subject later on. The left and right side of engine area are referred as the "PTO" and "MAG sides, respectively (looking forward). Another way to remember this is the fact the clutch is the "PTO" (Power Take Off) and the "MAG" is magneto (for power generation). However, as technology advances this arrangement may be different in future sleds.
Two cycle engines currently have a great power to weight advantage as compared to four cycle engines, although as technology and environmental issues continue, four cycle engines may have a part of this in the future. They are already being utilized in trail sleds, in some areas of the country. Here's a few words about bystanders and public image. Two cycle engines, at first impression, appear to be dirty, polluter, however, when you look at the big picture and look at the facts and figures, these little engines pollute far less, then compared to a 747 jet taking off, many times a day, or the millions of automobiles and trucks. We have a much large issue than these little engines. Fossil fuel issues, being a highly controversial subject, we will stop here and continue with the training at hand. The reason this is mentioned is in the event someone were to ask why we use them.
Most of this documentation will be based on this sled, being the latest model the author has researched and what's used at work. Future sleds to be purchased may have a variance in features, therefore, it's possible some notes in this training may not apply to them. While the basic engine is Suzuki brand, from Japan, most of the surrounding components and other sled parts are built, assembled and tested in the Northern part of the U.S. For this model, engine performance is controlled by the ECM (Electronic Control Module). The engine is electronically fuel injected and operates at a max RPM of 8,200. (Larger engines operate at slower RPMs). It operates on normal regular gasoline, (non-leaded, octane of 87). (Other types of engines may take higher octane). For best performance at all rpm's, this brand uses "power valves" on the exhaust system, which open at rpm's above 6900. It has oil injection from an oil reservoir which lasts around 200 miles. There is "low oil" light provided on the dash board, however, it's best to top off the oil when refueling as a precaution. Running the engine without oil will destroy it. In an emergency you can use any 2 cycle oil on a limited basis to get you home. Extended use of mixed or non-synthetic oils will eventually create problems with the power valves and other areas. If you get caught in this situation, drain the oil and start over with APV (Arctic Power Valve) oil, which Arctic Cat recommends to avoid warranty issues.
This model uses a 3-way ignition "kill" system, which is a normally open arrangement. What this means, is a short (to ground) from any of the three control points will signal the ECM stop the engine ignition. These are the key switch, the red button on the handle bar (right side) and the (optional) tether cord, that attaches to you, in the event you should fall off the sled unexpectedly. These sleds are reliable, however, once and a while an ignition problem might occur. In that event you should remember the control arrangement. One common example is the throttle limiter switch (fourth source). To prevent a runaway engine there's a switch inside the throttle lever area. Read your owner's manual for the details. The point is, in an emergency you can unplug any one of these devices to get you home, since it's a normally open system.
Engine Performance
These next several paragraphs get very deep into technical areas of the sled. If this does not apply to you, move on to the "Drive Train" section
Air/fuel mixture is very important for a two cycle engine to run at good performance levels. Altitude and temperature effect air density, and therefore, affect the mixture. A rich mixture runs the engine cooler, while a lean one runs it hotter, with an increase in power. One way to check this is by inspecting the spark plugs end color. They should be a medium dark brown. Very light or white is bad (too hot). Another way to check mixture is by piston wash, better known as "jet wash". During combustion the (injected) oil will burn and leave some carbon deposits on the piston tops. There is some variance between natural and synthetic oil deposits on the pistons, however, the effect is similar. The intake port routes the air/fuel mixture (and oil) into the combustion chambers (cylinders). Just before combustion the unburned gasoline has two (desirable) effects on the piston tops. One, it has a tendency to cool the entire pistons tops, plus, in a concentrated area (such as the intake) it further prevents additional oil from burning in those areas. Gas also does have a tendency to "clean" an oily surface, even when burnt, therefore, the gas "washes" the (previously deposited) carbon off the top of the piston top area(s) them, leaving the area shiny clean, thus, we call this side affect piston wash. We can use piston wash as a good way to determine air/fuel mixture, because of the operating temperature in the combustion chamber. For good performance, yet, protection from meltdown, the piston wash should be around 10%, meaning, 10% of the carbon deposits are washed away by the gasoline, while the remaining 90% has carbon left on the piston tops. Enriching the mixture ("fat") will increase the piston wash, which is "safer" (cooler), however, reduces engine performance and fouls the plugs. If the mixture were too lean, the carbon coverage on the piston tops would be excessive (100 %) from the heat buildup, and the pistons would eventually melt. For carburated sleds the owner/rider would need to adjust for this happy medium, by changing the jetting sizes inside each carburetor.
A jetting chart, such as the one below, shows that the numbers on the left edge are the jetting change factor depending on the relationship of altitude and temperature. After studying the chart you can see, "normal" (no temperature inversion) higher altitude has thinner air, however colder air, which counter acts the first variable somewhat. Sleds with EFI have the ECM (located in the engine area) to compensate for changing conditions and provide the correct mixture at all attitudes and temperatures. At this time it's not published how Arctic Cat does this; whether the computer changes the timing or duration each injector is open per cycle. The picture on the right, the head was removed on one cylinder, showing around 15% of jet wash is about the right combination of safe/performance.
Some larger cities supply gas with alcohol and other additives, called oxygenated fuel, distributed during the fall and winter months, in North America. Using oxygenated fuel has the same affect as leaning out the mixture. Therefore, if you use this type of fuel, you must unplug the fuel selector wire. It's on the EFI module, located under the hood, in the tool box compartment. The owner's manual will show this in better detail. This changes the EFI mapping to compensate for this type of gas. If you run oxygenated fuel without doing this you may burn up the pistons, due to a lean mixture. In the reverse situation, if you run normal fuel with the selector unplugged, the worst that can happen is you might eventually foul the spark plugs (double rich effect). The owner's manual claims running in this condition would damage the engine as well, however, after a little research and checking with a few experienced dealers, this doesn't seem to hold true and perhaps is the built-in "safety margin" that the manufacturer figures in the design. If you are concerned about warranty issues (manufacture's "blanket" disclaimer) then follow the manual. These engines are tough and went not abused last several thousand miles. Although all moving parts are subject to wear. The most engine wear items are the pistons, cylinders and bearings. More on this subject can be obtained by clicking here .
Most reputable gas pumps containing oxygenated fuel will have a label to that affect. Pumps can have a small amount (around 2%) and not required to have a label. One test to perform is put a sample of the gas in a little bottle and add a couple drops of water and give it a shake. Since water mixes with alcohol it will "disappear", showing a test "positive" for oxygenated fuel.
Advanced Information
Most Two Cycle engines use an expansion chamber in the exhaust pipe. This is to produce a very clever and highly technical "sonic super charging" function. The exhaust gases produce sonic (sound) waves that are bounced (reflected) back from the angled walls of the exhaust pipe back into the exhaust chamber at a critical time to super charge the combustion chamber. An animated picture further explains this function. Since this is a function of the speed of sound this occurs in a narrow R.P.M range thus, producing a great increase in engine horsepower curve when potting against rpm. This "Power Band" (curve) typically is 400 rpm wide, ranging from 7000-9000, depending on engine size and design. For example, most 600 engine's power band is 8000-8200 rpm. Larger engines, such as the 800s are lower, say, around 7300. Again, this depends on the brand and design of the engine. Even though the engines are capable of turning faster, out of this power band has no advantage, and even a slight disadvantage, since this would increase wear on the moving parts, such as the pistons, rings, and bearings. Some recreational or performance riders modify their engines by changing the exhaust pipe to move the power band up several hundred rpm, and increasing the horse power. Turning faster will shorten engine life and possibly void the warranty, therefore, for this training it's not recommended for any such modifications.
The rider does have reasonable control of lower rpm for maneuvering around trees, loading and other slow moving operations. During a climb or other high demanding terrain the rider typically needs maximum power, at times. Because of ever changing surface conditions it would be extremely difficult for the rider to keep the rpm within this power band. A WOT (Wide Open Throttle) condition would over-rev the engine. There is an effective way to address both these issues, with transmission/clutching.
Transmission
Our next discussion is probably the most mis-understood part of a snowmobile, which is a Continuously Variable Transmission, or CVT. The engine crank turns the primary (drive) clutch, which has movable sheaves that squeeze a "V" type belt which transfers the power to the secondary (driven) clutch. Both clutches have angled sheaves, so by moving the belt up and down the sides continuously changes the transmission ratio. The figures below illustrate this. The figure on the left shows in the idle (or off) condition, with the belt at the smallest radius on the primary clutch, and the largest radius on the secondary clutch. As the rider increases the throttle (and speed) the belt moves up on the primary and down on the secondary as shown in the right picture. In this case when the CVT is fully shifted out both clutches are turning at the same speed, (1:1) typically around 7,000-9,000 rpm (depending on engine size and design) during a WOT condition. (Wide Open Throttle).
In the primary clutch there is a coil spring keeping the sheaves apart, thus in a dis-engaged condition. There are many different spring types, with "strength" ratings calibrated in pounds of force to collapse them. Ratings given are engagement and fully collapsed condition. (Perhaps even a pressure curve could be incorporated in the future) This is accomplished by using different types of metals (perhaps alloy combinations), number of turns and size of the coil. Just in case you were wondering the picture shown is without that spring.
The primary clutch also has several fly weights, which are curved and pressed on small rollers mounted on the "spider" which is part of the other half of the primary clutch. Due to the centrifugal forces of the clutch turning, the weights move the sheaves together, thus an engaged condition, typically starting around 3500-4000 RPM. As you can see, it's a balance between the spring and weights. Several combinations of the spring and weights affect the engagement and operating RPM.
Two cycle engines have more horsepower than torque. Increasing the engine load keeps the RPM down, even under WOT conditions. Therefore, it's relatively easy to control the maximum RPM with the correct spring/weight combination. By controlling the primary shifting causes constant load on the engine, thus limiting the RPM under WOT automatically, so the rider does not need to be concerned about over revving, while steering and otherwise handling the mountain slopes.
The secondary clutch has a coil spring, similar to the spring in the primary. The sheaves are coupled to an angled set of surfaces, called the "helix". Helixs can be a single or multiple angles. Several different combinations of the spring and helix provide control of the shift-out of the secondary clutch. The secondary controls the speed and power applied to the rest of the drive train. As more power is needed when slowing down or climbing a hill the secondary down-shifts to provide that demand. One could say the primary clutch is RPM sensing (centrifugal) while the secondary clutch is torque (load) sensing. To a small degree one clutch shifting affects the other. Higher attitudes cause the engine to loose RPM (and performance). To compensate, lighter fly weights can be installed in the primary clutch, per a chart either in the manual or in the engine compartment. They come stock with mid range weights which should be fine for most riding, such as at work, for utility trips.
One more item should be made aware of concerning RPM. The discussion we just covered is based on a "load" for the sled's drive train. The drive belt does wear out relatively quicker than the other (metal) parts of the drive train. Typically you can get about 700-800 miles per belt, depending on demands and operating temperature. For example, two factors are very hard on belts, which are constant hill climbing and warm (spring conditions) operating temperature, both which create a lot of heat on both the clutches and the belt. One more factor usually from a "newbie" rider is "babying" the throttle, with the thought of creeping up through the trees or some other tight area which may be intimating to that person.
Normally when the belt fails it will starting coming apart in pieces or shreds. You may be able to hear and feel this condition, as a "thump-thump" sound or an irregular feel to the otherwise, smooth acceleration of the sled. When you experience this, the best course of action is to stop on a level spot and check under the hood. At this point it's a fairly easy task to change out the belt with the spare, usually kept on the belt guard that covers the clutches. Rarely, you will not have any warning of a complete belt failure, when the belt will break completely in half. What normally happens you'll hear a "bang" and "clank" which is the sound of the clutches disengaging violently without a belt to separate them. Also at this point there will be no load for the engine, therefore, is possible to way, over-rev it. You will lose all forward power and control so trying to continue keeping the throttle open will do no good. In fact, it's It's imperative you let off the throttle immediately . Failure to does this could destroy the engine. In some cases a belt breaking apart has enough force to crack the belly pan or even bend the (metal) foot well if they are in the line of "fire". This was witnessed by the Author. Take this into account if you are performing a steep high climb or some other demanding situation. Don't be afraid, just keep this in the back of your mind as an emergency plan on what to do. In the worse case scenario you would have to let go and jump off the sled and land in the snow. In most situations the sled would just stop in the snow, especially if it's deep, power type.
If you have a burning desire (and for maintenance) to observe this clutching action you can lift the rear end of the sled and run the engine with the clutch cover removed. Remember, with this arrangement the track and engine will NOT have the proper (snow) load, so you would be able to over-rev the engine, once shift-out occurs (1:1). You must observe safety precautions, such as a sturdy lift, a cushion on the front end and a high degree of eye protection. With things moving this fast a mishap can turn into a explosive piece of metal. Tachometer calibration can be performed with this arrangement as well, except the shield can be in place.
Drive train
The secondary clutch turns a "jack" shaft, which transfers power to the right side of the engine compartment (MAG side) to a top gear, chain and bottom gear. The chain and gears are inside a sealed case, in an oil bath. The bottom gear is much larger than the top, thus providing reduction. Typical gear pairs have 40 teeth on the bottom gear and 20 on the top for a ratio of 2:1. Another way to look at this is that the top gear makes two turns for every single turn of the bottom gear. The bottom gear turns the "drive" shaft which goes underneath and inside the track "tunnel". The drive shaft has track "drivers" on it, which pull the track around and provides forward traction. These drivers typically come in nine or ten "teeth" diameter. The entire (final)transmission ratio is based on both clutches fully shifted out (1:1) the top and bottom gear sizes, and the driver's diameter. For most mountain climbing a good final ratio is 2.15:1, however others can be used, depending on engine size (and power). This is the best compromise between ground and track speed, discussed later.
Older sleds of 1960-1990 era generally supported the entire weight of the chassis with "bogie wheels" on mini-axles, consisting of 6-8 sets. These wheels would roll on the inside of the track. Very similar to bulldozers. Recently the amount of wheels has been reduced, thus much of the chassis's weight now would rest on a set of rails as part of the suspension. Sleds later than the 1990 era only had 4 wheels with most of the weight on the inside of the track. There would be way too much friction for the (metal) rails to contact the inside of the track directly. Therefore, plastic "sliders" are mounted on the bottom side of the rails. The inside of later tracks now have metal clips. These clips come in contact with the sliders, also called "hyfax". The hyfax's depend on snow being picked up by the track lugs and being thrown into that area for lubrication.
High performance engines are liquid cooled. One or two heat exchangers are normally mounted up, inside the chassis tunnel. These exchangers depend on the loose snow being kicked up from the track for cooling. Therefore, you should try avoid riding over 20 mph and a long distance on icy or no snow conditions. Try to ride where there is some loose snow for the track to kick up into tunnel and sliders. One trick the author does is stop every so often and kick some snow into the track and tunnel by hand/foot. Another way is to install "ice scratchers" that, when lowered, provide a spray of ice flakes to do the job. Continuous runs without any snow, say, less than a quarter of mile, and at slow speeds usually will not wear the hyfax out. While it's not expensive to replace the hyfax, it is, if you let the engine over heat due to lack of (snow) cooling. Below is a (crude) picture made up of a typical sled drive train.