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HOW CVT CLUTCHES AND BELTS REALLY WORK

Let's start with the basics. We need to understand what CVT means. CVT means “Continuously Variable Transmission”. The CVT system in Powersports applications is made up of 3 major components. These are the Primary Clutch, the Secondary Clutch, and the Drive Belt. The power flows from the Primary Clutch through the Belt to the Secondary Clutch. Engines make peak horsepower (HP) and torque (TQ) at a specific rpm for each. The CVT’s purpose is to automatically maintain optimal rpm and optimal belt squeeze for the current load. Optimal rpm is important for fuel economy and maximum power. Optimal belt squeeze is often the most overlooked formula but also the most important. Too much squeeze will increase resistance and slightly increase heat. Too little belt squeeze is catastrophic because it will allow belt slippage that will add enormous heat to the belt and the clutches. The ideal squeeze will be right between in a golden efficiency zone. The Primary Clutch and Secondary Clutch each play a specific role to achieve the perfect balance.  

The Primary Clutch is responsible for engagement, upshift, and disengagement. The Primary Clutch has main subcomponents. The subcomponents are the Inner Stationary Sheave, Outer Moveable Sheave, Primary Spring, and the Primary weights. The Primary Spring and Primary Weights are what is usually included in the Primary portion of a clutch kit. These are what you should normally use to make adjustments to the Primary Clutch.

The Primary Spring will have 2 ratings on it. The installed force (Preload) and full shift force (Finish Rate) are often labeled like this example (140/320). This would imply the spring has a preload of 140lbs and a Finish Rate of 320lbs. The given spring force is what resists the clutch from upshifting from the Primary Weights putting centrifugal force against the Moveable Sheave. Higher Preload will raise engagement and decrease upshift force in the first portion of the shift this will likely result in higher rpm. A higher Finish Rate will decrease the upshift force in the latter part of the shift.

The Primary Weights only function is to apply upshift force. There are many adjustable weights on the market that do this but just because a clutch weight is adjustable doesn’t make it perfect. The profile, center mass value and center mass location of the weight is what really matters. By increasing mass the weight will apply more upshift force. By moving the mass you can change when the force is applied to match the needed force to stay in the golden efficiency zone the entire way through the shift. 

The Secondary Clutch is responsible for resisting upshift, maintaining belt squeeze during a steady cruise, and downshifting (backshifting). The Secondary Clutch has main subcomponents. The subcomponents are the Outer Stationary Sheave, the Inner Moveable Sheave, the Helix, and the Secondary Spring. The Helix and Secondary Spring are usually what is included in a Secondary portion of a clutch kit. These are the components you should normally use to make adjustments to the Secondary Clutch.

The Secondary Spring will have 2 ratings on it. The ratings are Preload and Finish Rate similar to the Primary Spring. The Secondary Spring Preload and Finish Rate are responsible for maintaining belt squeeze while there is no acceleration load and during the downshift.  Its force will also be added to the force applied by the helix during upshift and downshift. Belt squeeze during deceleration and coast is needed to keep the belt from contacting the out housing around the clutches and prevent belt slapping. If the Secondary Spring rates are too high it can delay upshift.

The Secondary Helix is a ramp that creates belt squeeze proportionate to load. The shallower the helix angle(lower degree number) is the more belt squeezed under load and the more it will resist upshift. A steeper angle (higher degree number) will not resist upshift as much. A vehicle with added horsepower or increased load from bigger tires needs a shallower helix than a stock. 

What you should take away from all of this clutching is a delicate balance of multiple variables. Clutching is much more than just shooting for a target rpm. Good clutching will reduce heat in your belt, increase belt life and get power to the ground more efficiently. You can make all the power in the world but if you do not get it to the ground it is all pointless.

To take the Easy way just get a good clutch kit made for your machine from a reputable company. Check out ours at www.bikemanperformance.com

 

For the latest technology in Primary Weights

Check out The Bikeman SNYPR Primary weights They are the next generation of ways to adjust the center mass value and location. This allows you to match your machine's exact needs. 

 

Any questions please call us up 715-483-3003

 

Here are some general clutching rules you should play by!

  • Primary clutch (Drive Clutch) = Front clutch off the motor
  • Secondary Clutch (Driven Clutch) = Rear clutch connected to drivetrain
  • Peak RPM = The RPM the machine sits and holds most of the time at WOT. This needs to be in your model's peak powerband for the best results. (BMP supplies you with your optimum peak RPM when purchasing clutch kits)
  • More Primary Clutch Weight = Less peak RPM & greater belt squeezing force
  • Less Primary Clutch Weight = More Peak RPM & less belt squeezing force
  • Primary Spring Starting Rate = Controls Engagement
  • Stiffer Primary Spring Starting Rate = Higher engagement RPM
  • Softer Primary Spring Stating Rate = Lower engagement RPM
  • Primary Spring Ending Rate = Controls UpShift, Back Shift & Peak RPM
  • Stiffer Primary Spring Ending Rate = Slower Upshift, Quicker Backshift, More Peak RPM
  • Softer Primary Spring Ending Rate = Faster Upshift, Slower Backshift, Less Peak RPM
  • Steeper Helix Angle = Faster Upshift, Slower Backshift
  • Shallower Helix Angle = Slower Upshift, Faster Backshift
  • Stiffer Secondary Spring Finish Rate = Slower Upshift, Faster Backshift
  • Softer Secondary Spring Finish Rate = Faster Upshift, Slower Backshift

Adding horsepower normally means you need to add clutch weight, go steeper on the helix angle, or both. Most small modifications only require adding clutch weight while larger horsepower modifications will require you to change both. Not changing clutching when you add horsepower will result in over-revving. 

On some 2-stroke snowmobiles, you must add clutch weight or RPM will be too low. 2-stroke pipes are made to work when they are hot. If you do not load the motor hard enough, not enough heat will be built up in the pipe which will result in poor running condition and lower than normal peak RPM. This is about the only case when low RPM is seen that weight should be added.

 

Tech Videos for clutching:

 

Tags: tech clutching