Primary Clutch Springs Explained

4 wheelers and ATV's

Primary Clutch Springs Explained (With Charts!)


Your ATV or UTV’s Polaris primary clutch spring is a core part of your ride’s continuously variable transmission (CVT). While you might be more used to seeing springs in mattresses or trampolines, it turns out this simple mechanism is a key part of your transmission. But what is a primary clutch spring, and what’s a clutch spring chart? Harvey’s ATV parts has the answers.

What is a primary clutch spring?

The primary clutch, or “drive clutch,” is the forward mechanism of a CVT attached to your engine's output shaft. It consists of two tapered plates or “sheaves” on a self contained shaft. One sheave is mobile and one stable, with an end of the drive belt looped between. The mobile sheave is capped with a “spider” plate and a series of flyweights that, when spun, push the assembly inward toward the stationary sheave. This engages the belt, driving it up the sheave faces to create a higher gear ratio. The secondary clutch mirrors, its sheaves forced open to provide belt slack.

The primary clutch spring is held between the mobile sheave and the clutch cap. It’s an essential part of the CVT assembly, and comes in a variety of color-coded compression and expansion rates. This seemingly simple part of the Polaris primary clutch is crucial to proper transmission operation and tuning.

What does a Primary Clutch Spring do?

The most important function of a drive clutch spring is to control the transmission’s shift speed. Without primary clutch springs, the sheaves would slam closed together when the engine accelerates, and likely toss your belt when decelerating. Not exactly the smooth escalation of gear ratio that we look for from a CVT. Primary clutch springs are cylindrical spiral compression springs, meaning they’re made of one wound piece of metal with consistent diameter in each spaced loop. 

Put simply, this type of spring wants to stay “open”, and will push back against force trying to “close” it. There are two stages of compression in these springs: engagement (or preload), and full shift (aka ending rate).

Engagement is the point at which the spring gives and collapses, allowing the sheaves to contact and engage the drive belt (about halfway compressed, one inch). Higher preload rates make for more acceleration required to shift out of idle.  

Full Shift is when the spring is totally compressed. This portion of the spring requires stronger force to squeeze together, and is responsible for pushing back against the flyweights and determining peak RPM. 

Example: Polaris Primary Clutch Spring Chart

As there are a number of different rated primary clutch springs, Polaris has helpfully color coded and labeled each variation. Each one is optimized for different builds, so making sure your clutch is balanced with the proper spring is crucial to your ride’s best operation.You can also find a Polaris primary clutch spring chart with spring color, wire diameter, length, force lbs, and load rate organized into columns. We’ve provided an example table below (circa 2009).

Some official documents like this one even provide line graphs to help visualize the relationship between the spring length and force required for compression.

What do the numbers represent?

There are two numbers printed on every Polaris primary clutch spring (ex. 120-300). 

The first number represents the starting rate, the pounds of force per inch required to start the first phase of collapse and engage the drive belt. 
The second number is the full shift out, the force (PSI) required to fully compress the spring and reach peak RPM. 

Wrapping up

Clutch springs may seem like a relatively simple piece of the CVT, but their effect on your ride’s performance can’t be overstated. But you don't need to worry about whether you’ve got the right rating when you can buy a Harvey’s Mother Clutcher OEM primary clutch, pretuned and balanced by Harvey himself

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