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Clutching For Powerby jimmypsp700, 2005-10-13
Clutching For Power Sled running like a pooch? Understanding how a clutch works is the first step in unleashing your sled’s hidden power. Last fall I had an interesting discussion with a friend of mine — a long-time snowmobiler who is brand loyal to his blue machines. His 700 was fast, but he was tired of getting smoked by smaller-displacement sleds.
“I think I need pipes,” he said.
“Pipes are good. They’re lighter and they definitely sound cool,” I replied.
But the more I talked to him, the more I realized he didn’t want pipes at all. He wanted his sled to be trailable. And he didn’t want to change jets every time the mercury varied a few degrees. I convinced him to try dialling-in the clutching instead.
We took his sled to an open field we use for testing and set up timing lights. We tried many combinations of weights, springs and helixes. In the end, we had 18 more grams of weight in the primary clutch and had dropped the sled’s 60-foot times by more than a full second. A 700 cc triple that was getting beaten by 600 cc twins was now running faster than an 800 cc twin at the 500-foot mark. The interesting thing is, most people — even veteran snowmobilers like my friend — would never guess how radical a change clutching adjustments can make.
How is this possible? To answer, we must look at how a snowmobile’s clutches transfer power. We’ll use a Black Magic clutch kit we received for a 2002 Arctic Cat ZR 800 to illustrate.
BACK TO BASICS
A snowmobile has two clutches: a primary, or drive clutch; and a secondary, or driven clutch. They are connected by a ribbed rubber belt. The primary clutch consists of a stationary sheave, a moveable sheave, a spider assembly, a spring, three (or four in some cases) cam arms, and a cover. The exception to this is a Ski-Doo primary, which doesn’t have cam arms like the other three brands. Instead, a Ski-Doo has TRA arms and ramps, though the two systems work basically the same way. The secondary clutch consists of a stationary sheave, a moveable sheave, a spring, a helix and a retainer plate.
The primary clutch bolts directly to the crankshaft of the motor. Whenever the motor is running, the primary clutch is spinning. The secondary clutch connects to the jackshaft of the snowmobile. It only spins when the primary clutch begins to shift out and grabs the belt.
Both clutches move two ways. They spin on their respective shafts and they slide in and out. At idle, the belt is at the bottom of the sheaves on the primary clutch. As rpm increases, the primary clutch senses torque input from the motor. At a certain rpm, it will turn fast enough that the cam arms will begin to move and overcome the pressure of the spring in the primary. When this happens, the moveable sheave slides in on the shaft of the fixed sheave, thereby pinching the belt and causing it to spin the secondary clutch. This is called engagement, and it’s critical to a snowmobile’s performance. If engagement rpm is too low, the sled will bog; too high, and the sled will jerk forward and damage the belt.
As rpm continues to increase, the moveable sheave on the primary clutch continues to slide in on the shaft of the stationary sheave. As this happens, the belt climbs up the sheaves. When the sheaves are all the way together, the clutches are fully engaged. This is commonly called “shift out.” At this point, the belt will be at or near the top of the sheaves on the primary clutch.
Like engagement, shift out is also critical to the performance of a snowmobile. Shift out should occur just before the peak torque point of a snowmobile’s engine. If it occurs before this point, the sled won’t be utilizing all its power. If it occurs after, the sled will over-rev and power will drop off on top end.
As rpm increases and the belt moves from the bottom to the top of the sheaves on the primary clutch, just the opposite is happening with the secondary clutch. At idle, the belt sits at the top of the sheaves on the secondary clutch. As rpm increases and the belt begins to climb the sheaves on the primary, the sheaves of the secondary are being forced open by the pressure of the belt. When the secondary clutch is fully shifted out, the belt is at the bottom of the sheaves. At this point, the snowmobile is in (or very close to) a 1:1 drive ratio.
In essence, the secondary simply reacts to input from the belt, and that input is determined by how the primary is reacting to power output from the engine. READ PART 2
Clutching For Power, continued A REAL LIFE EXAMPLE
So how do all these parts work to make a snowmobile go? A good way to explain this is to look at how a clutch kit modifies the performance of a sled's clutches. Our Black Magic kit covered all the tuning variables: it included a softer-compound belt, a new helix, new primary and secondary clutch springs, two sets of cam arms, and helix shims. Let’s look at what we are trying to accomplish with this kit and why.
One set of cam arms in the kit was two grams heavier than stock and the other was four grams heavier. The heavier weights are designed to load the motor more and use more of its power. However, if we just put the heavier cam arms in, the clutch would load the motor more but our engagement would drop because the spring would quickly get overloaded by the weights. In other words, our sled would bog off the line. (Alternately, if the cam arms were too light, the clutch would engage at too high an rpm because the cam arms would not be able to overcome the spring.) That’s why Black Magic sends a heavier primary spring with its kit — the cam arms and the primary spring work together to determine how the clutch reacts to engine rpm.
Now that we have our heavier cam arms and primary spring installed, it’s time to look at the secondary and its parts. Once the primary engages at the desired rpm and begins to pull the belt, the secondary comes into play.
The helix and the spring determine the rate at which the secondary sheaves open. A helix is basically a ramp that the secondary retainer plate rides on as the secondary’s sheaves open and close. The angle of the helix ramp determines in part how the secondary performs.
Helixes come in two basic configurations: single and dual angle. A single-angle helix has a ramp that allows the secondary to open at a constant rate. A dual-angle helix allows the secondary to begin opening at one rate then changes that rate, usually shortly after engagement.
The helix in our kit had a dual angle. A steep angle at engagement allows the secondary sheaves to shift out quickly. As the rollers travel along the ramp of the helix, they encounter the second angle, slowing down the movement.
For trail riding, this works great. We get a nice jump off the line because the belt drops quickly into the sheaves, but we also get a nice back shift when we let off the throttle because the rest of the helix ramp is at a shallower angle. This allows the secondary sheaves to come together or “back shift” quicker. If we had a steep ramp across the entire helix, we’d get good acceleration but poor back shift. In other words, when we let off the throttle (in a turn perhaps) the clutches would stay engaged to some extent. When we got back on the throttle, the sled would bog because the clutches would still be shifted out.
The secondary spring, unlike the primary spring, works two ways. It compresses and releases like the primary spring, but it also works torsionally — it twists inside the secondary as the sheaves open and close. The stiffer secondary spring in the kit will make the secondary sheaves resist opening more than the stock spring did, so the sled will grip the belt more in the secondary. The belt will have more force put into it as a result, and it will also back shift with more force.
We’ve broken down the different clutch components and what they do. We’ve also modified our clutches to make them work differently. So why isn’t this a guarantee that our sled will run better? Because there is no universal clutch setup that will work for all riders.
Snowmobiles are set up by each factory for an average rider weight, yet a heavier rider will require a different clutch setup than a lighter rider. This is because a heavier rider in a sense robs horsepower from the engine.
Seven pounds of weight equals about one horsepower. That means our Cat is making 8.5 hp less when a 220-pound rider is aboard, versus a 160 pounder. That’s the difference between a 500 cc and a 600 cc twin!
If we clutched an 800 cc Cat to optimum levels for a light rider, and then had a heavyweight get on it, it would be the same thing as if we put cam arms that were too heavy into the primary clutch. A tuner has to compensate for rider weight with weight in the cam arms.
THE CLUTCH COMPROMISE
While there is no universal clutch setup for a snowmobile, a clutch kit is a good compromise. Most make a sled run better, come with clear instructions, and are easy to install. They are a good alternative for those who don’t have a lot of time and money to spend dialling-in a sled’s clutches.
However, a complete kit isn’t always necessary; individual parts are also available. Many racers use adjustable cam arms that add and subtract weight by use of screws and washers. The advantage to this is you don’t need a box full of weights to find the right setup. Also, each sled manufacturer offers a number of spring, cam arm and helix options.
Want to make your sled perform better? Look at the clutches first.
Ever wonder how cam arms work? It’s all about weight, and where it’s placed.
• Weight near a cam arm’s pin controls engagement and backshift.
• Weight in the middle of the arm controls midrange.
• Weight near the tip controls top end.
To illustrate, let’s look at two extreme examples:
In drag racing, the objective is to get from point A to point B in the shortest amount of time, without worry about whether the sled back shifts or is “rideable.” For this reason, the clutches need to be tuned so they load the motor as much as possible for the entire length of the run. A drag engine has as stiff a spring as possible in the primary with as heavy a cam arm as the motor can handle. The secondary most likely has a steep-angle helix to get the belt low in the secondary as quickly as possible. Those who have been to a drag race may have noticed that the sleds coast quite well at the finish of the race because they don’t back shift well.
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