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Discussion Starter · #1 ·
Could someone please explain to me what effects changing the weights and spring in the primary will do, and what to expect, and how changing the helix to one with different angles will accomplish, I have a clutch kit from SLP, but I don't think that its using all its potential, right now, engagement is 4800, just 600 over stock, traction is not a problem, I have 192 studs, I want more power off the line.

4,983 Posts
I am sorry how long this reply got. It's a composite of diff articles I have on file.

How do the Primary Clutch Spring and Flyweights work?
The primary clutch spring holds the clutch shive faces open with a predetermined, preload pressure. The spring is then compressed more when the flyweights push the shive faces closed, clamping the belt. The rate of this spring pressure is also measured at full compression. Also known as full shift out or high gear. These primary springs are usually referred to by color code.
For example, the Polaris silver/gold is 75 LB at 2.5 inches spring length or installed, shives open and 280 LB at 1.25 inches of spring length. or shives closed. If your sled has an engagement RPM of 4200 and full shift out at 8000, and you want to increase this to 5000 and 8000 respectively, then the simplest way to do this is to change from silver/gold to a Polaris blue spring for 120 LB at 2.5 inches and 280 at 1.25 inches. This changes the engagement RPM, but should not change the full shift out RPM.
The flyweights are responsible for closing the clutch by pushing against the preset spring pressure. By raising the engine RPM with the throttle, you raise the force of the flyweights pressing against the spring. Once the force of the weights is high enough to overcome the force of the spring then your clutch will close and engage the belt.
The position and attitude of these flyweights can be altered by several means, some of which are belt side clearance, notched
weights, cheater clips, dirty or worn pins, and bushings and rollers.
For more info clutch set up and theory check out gates web page
at or

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.
A snowmobile should be clutched to run where the engine makes the most power. A fan-cooled 440 will require a different clutch setup than an 800 cc twin simply because the two motors make different amounts of power at different rpm. In a nutshell, a fan-cooled 440 won’t need as heavy a clutch setup as an 800 cc twin.
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.
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:
Snowcross sleds rarely go over 50 km/h, but they accelerate as hard as any drag sled. A drag race setup would not work for a snowcross sled because a snowcross sled needs a quick back shift. Cam arms in a snowcross sled’s primary clutch have most of the weight at the pin and in the middle of the arm. This makes the primary clutch engage hard and run hard from engagement through midrange. However, the sled struggles on top end. The clutch also back shifts quickly when the rider lets off the gas in a corner or off a jump and the power is “right there” when the rider gets back on the gas.
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.
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.

8,842 Posts
of course your slp kit is at its full potential it like your sled is made for a viriaty of people. this is were the adjustable wieghts come in nice, you add that 1/4 gram to make the most out of your sled. you can mix your stock stuff with the slp stuff and see what happens
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