Introduction to CVT Clutching

Introduction to CVT Clutching

CVT Clutching intro:

 

A CVT’s (continuously variable transmission) purpose is to be the key component in transfer of power from the engine to the wheels on the ground. Where a CVT is used, it is the key part because it is the only variable part that allows the user to cater for different conditions like acceleration from stop, slow speeds, steep grades, and all the way up to top speed. Other transmission types use complex combinations of clutches and multiple fixed gear ratios like typical 5 and 6 speed gearboxes.

 

Which is best, a CVT or a manual gearbox?

Neither are 100% efficient all of the time, and considering their different losses and areas of weakness, both produce about the same performance efficiency. CVT’s have inherent slippage and frictional losses that manual transmissions don’t have, but if correctly tuned to the task, a CVT is ‘never’ in the wrong gear, where a manual transmission is rarely ever in the ‘perfect’ gear, or cannot stay in that perfect range for most of the time like a CVT can.

A CVT can be set up to always be in the perfect gear, a manual transmission can’t. Looking at a simple comparison drag race from zero to top speed, both transmission types are about the same efficiency... but the CVT is much more consistent and easier to use. The advantages start right from the line, as a CVT operator will never stall the engine.

 

Can any CVT be improved?

All CVT transmissions are specified by the manufacturer to do the average job, all of the time, in all conditions and to meet the average of all expectations in the end user market. This specification or ‘tuning’ is very easy to alter... but it could be fair to say it requires some very technical research, understanding and practice to perfectly hit the target.

If a customer is chasing performance, the answer is just about always “yes” we can deliver more power to the ground for you.

The key to that is most efficient use of the engines peak power rpm and consideration of the type of load of driving conditions. As the name suggests a CVT can constantly change its “gear ratio” depending on the engines input (throttle position) and the sensed resistance coming back from the wheels (load).

If a customer has changed the engines performance at all, or changed any fixed gearing (like fitting bigger tyres) then the answer is always “yes” your CVT can definitely be either improved back to its expected standard relationship, or even further improved.

 

How does a CVT work?

A CVT is essentially only two pulleys, one attached directly to the engines crankshaft (Primary clutch) and the other pulley attached to the driveline at the gearbox end (Secondary clutch). Both pulleys are connected by a drive belt which cannot stretch or change in length. The Primary clutch can be seen to be ‘speed sensing’ and the Secondary clutch can be seen to be ‘load sensing’, together they are in a constant argument as to what final gear ratio they should produce. The Primary always wants to go to the highest gear possible as the rpm increases, but the Secondary keeps the Primary pulled down to the lowest gear required to fight the resistance (traction and load) that comes back from the wheels on the ground. In this constant argument the pulley sizes continually change diameters to change the CVT gear ratio and the belt moves up and down the pulleys as needed to deliver the required gear ratio result. In a “low gear” situation the Primary clutch is expanded and produces a smaller pulley, while the Secondary clutch remains compressed so that the belt runs on its largest pulley. Small to large equals low gear. As the rpm increases the Primary clutch pulls in and produces a larger pulley which overcomes the Secondary clutch which in turn has to move to a smaller pulley. Large to small equals high gear. A CVT is capable of perfectly creating an infinite number of gear ratios in between.

CVT tuning is carried out literally by ‘weighting’ one side of the pulley argument more than the other. This is mainly achieved by changing and balancing the effects of different value fly weights (levers) and/or springs.

 

For a simplified breakdown, the main components in clutch “setup” are;

 

Flyweights or ‘levers’ weight and profile.

These are the weighted levers that pivot to ‘lever’ against spring force to enlarge the pulley due to centrifugal force from increasing rpm in the primary clutch. The weight and profile (shape) of these levers can be changed or modified to determine the speed at the primary clutch end. Most aftermarket clutch kits have adjustable weights where you can add or remove weight to adjust/tune the clutch operation.

(some Primary clutches such as the Yamaha, CF moto or aftermarket CVTech use a weighted roller on a ramp instead of a lever)

 

Primary spring.

A spring in the Primary clutch that tries to open the pulley and opposes the efforts of the Flyweights. The first job the Flyweights have to do as the rpm increases is to overcome this spring force. This spring separates the pulley sheaves and basically tries to hold the primary clutch in the lowest gear (opposite to what the Flyweights want). So a stronger spring means more engine rpm... or lighter Flyweights also equals more engine rpm. Either or both can be varied to overcome the forces that hold engine rpm down.

The balancing act becomes even more complex when different primary spring weights and ‘lengths’ can be installed to produce very similar overall results but have significant other effects on important outcomes like CVT engagement rpm. This is the rpm at which the CVT starts to close and act on the belt to pull on the Secondary clutch to move the machine along (taking off rpm).

 

Secondary spring.

This is usually the strongest spring in the set up and it is used in the secondary clutch to provide initial opposition force to hold the pulleys together and keep the Secondary Clutch in its lowest gear possible. The secondary spring could be said to be most responsible for maintaining belt tension. This is best seen when the engine is turned off or even at idle rpm, the Secondary spring is the only component trying to maintain belt tension and has the CVT is waiting in its lowest gear, ready to take off. Among other things, a stronger Secondary spring generally results in higher rpm and faster acceleration.

 

Helix.

The Helix is a ramp or series of ramps in the secondary clutch that adds to the Secondary spring force and further tries to ensure that under heavier loads the CVT stays in its lowest gear possible. Think of the Helix as a wedge. While the Secondary spring force fights to keep the Secondary clutch in low gear, the Helix is like driving a wedge under that spring to increase the spring pressure. It is easiest to see the effect of the Helix when the gearbox is in neutral and the Helix is not working or essentially disconnected from the resistance or back load usually experienced from being connected to the wheels on the ground. In the neutral situation the Helix cannot sense any load and does not add to the Secondary Clutch main spring at all. By revving the engine in neutral, you will see the CVT Primary speed easily overcomes the Secondary spring force and the CVT shifts up into its highest gear with the increasing rpm. In gear, a deceleration force or any force from the back wheels that tries to drive the engine pushes the Helix efforts harder into low gear. Engine braking is something that can be tuned by varying the back side of the Helix ramps.

 

All of these components work together and there are plenty of overlapping effects and trade-offs. Generally you can’t change just one component without changing results somewhere else in the combined set up. Some results add together, and some cancel each other out. Balancing the relationships, particularly for drastic modifications is very much an art form.

There are other more complex variables that can be changed by machining or changing out parts that are almost getting away from basic tuning and more like structural modifications. Like different clutch designs, different sheave angles etc.

 

For most, a well setup clutch will have a smooth engagement, comfortable cruising pace shift out rpm, and solid performance from well placed full throttle shift out rpm around the desired peak engine power range. Well behaved backshifting and belt tension are other desirables to keep in check when tuning a CVT.

 

CVT terminology used;

 

Engagement,

Is the point at which the primary clutch starts to grip the belt and wants to move the machine when in gear. This will vary between machines, but the typical engagements might be between 1,400 and 1,900rpm.

 

Shift out rpm,

Is the engine rpm for the applied power and load. This is generally referred to under full power or hardest acceleration. It is completely machine dependent and will be different between engine sizes/models and intended use. Standard full load shift out is usually anywhere from 5,000-7,500rpm. Performance tuning ideals might sit between 7,000-9,000rpm depending on the machine setup. The easiest way to test your shift out is to do a full throttle run from a walking pace up to 80-90kmh and see where you’re rpm hovers during the acceleration stage.

 

Backshifting,

Is like changing down a gear, where the ratio changes when the Secondary clutch senses a load like a boggy soft surface or steep hill, or the engine input is reduced like when cruising at part throttle.

 

Belt grip,

Is critical to performance and belt life. Belt slip is not only lost power turned into undesirable heat, it can also destroy a belt in a very short amount of time. Belts are very sensitive to heat and accumulated heat is the number 1 cause of belt failures. More CVT air flow for cooling is a belts best friend.

 

Another thing to consider is the alignment between the Primary and Secondary clutches, this is more of a concern with Polaris clutches. It can be somewhat easily diagnosed by being hard to shift from neutral or park in to gear. If this is the case, you need to investigate further. Hard shifting can also be because of a failed Primary clutch bearing. Some Polaris SxS clutches can benefit from engine to gearbox shaft distance re-alignment as well.

 

Clutch maintenance is also very important. Regular cleaning and inspection is critical to performance and life. Realistically, every time you do a service on the engine you should also remove the belt cover and blow the clutches out with compressed air to remove dust/dirt and thoroughly inspect the belt and pulley faces. It depends on how hard machines are used, but for average conditions and use, around every 1500km or 100hours is good timing to check and service CVT transmissions. Servicing is quite simple with a CVT but does involve disassembly and inspection of all the wear parts and replacing as needed.

In extreme conditions and racing applications this interval is a lot shorter and clutch service should be part of your regular event/trip preparation.

 

Who needs a clutch kit:

Not everybody needs a clutch kit, but there are a lot of situations where improved clutch setup will greatly enhance performance and /or belt life.

 

Some machines have factory clutch setups that are better (or worse) than other factory set ups. Some are hard to improve upon if everything else is left standard. Others have known inherent problems such as too low shift out rpm or poor belt grip. In these cases even recreational or farm users will benefit from improving clutch setup to improve ride comfort, performance or extended belt life

 

A very common reason to tune is if you have changed tyre size. If you have gone for bigger wheels, you will almost certainly be bogging down the rpm more than the factory ever catered for and you will need to change your clutch setup. Changing tyre size changes the final drive ratio. For example, if you change from a 28” to a 30” tyre you are effectively trying to get the CVT to operate against an 8% higher gear ratio which will lower your initial shift out rpm. Your acceleration rpm could slip a bit too far down from the peak power curve and the machine will feel sluggish. You could then be causing excess belt slippage and more heat. The same can apply in reverse if you go to a smaller tyre size.

 

Racers or recreational users who want the most performance from their machines will benefit from an improved clutch setup in the areas of extra performance and longer belt life.

 

We sell and recommend the Dalton clutch kits. They’re well developed and work as they’re intended to. They come with recommend settings based on your application and install is easy. On some models, special tools are needed to install and we can help with supplying those tools or you can send you’re clutches in and we can install and service the clutches while they’re here.  You can find more info/prices on the Dalton Clutch kits for your model in the "Drive" sections of the ATV and SxS drop down menu in the main menu.

Make sure you also check out our upcoming Drivebelt blog for more info on drive belts

 


As usual, if you have any questions we’re here to help. Feel free to get in touch on our contact form 


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