CVT Principles

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CVT Principles - How it works, how to improve it.... (By Fabio Dougie) Have you ever wondered... what is a variator? how does a variator work?, what do rollers do? Well I must admit that some time not too long ago those were the thoughts going through my mind and that's what started me off trying to make my scooters quicker, I was fascinated by how a scooter's drive-train worked and how I could improve it.... Here are some notes that I made on the Constant Variable Transmission drive-train (I’ll call this CVT from now on) of a modern automatic scooter, I seem to be constantly amending them because of the complexity of the topic, but also because it’s a lot harder to explain about the CVT than it actually is to understand it…if you see what I mean!! By writing this article I’m trying to explain how it all works so that 1) I can follow it later when I have one of my senior moments and forget something that I shouldn’t have, and 2) it can be of use as an introduction to someone trying to understand what the CVT does and how it all works…. and perhaps more importantly how they can make modifications to it to get the most out of it… Some of the information I researched and sourced from the internet and from other members of ModernVespa, and for that I can only thank you.

A photo of a CVT showing Front Pulley Assembly/Variator and Rear Pulley Assembly/Clutch

A few definitions There seems to be several different terms for each part of the CVT and these terms seem to be interchangeable, but to make it simpler for myself I’m going to try to stick to these terms….

CVT

(also known as the transmission, or the drive-train) The whole collection of items/parts that connect the engine through to the rear wheel.

Front Pulley

(also known as the driving pulley) This is the collection of parts which includes the variator, outer front pulley half and rollers. It sits on the output shaft that is the extension of the crankshaft from the engine and provides the access to the power from the engine.

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Another photo of a CVT with the Outer Fixed Pulley removed showing the movable pulley face of the Variator

Assembly (on the left) and the Clutch bell removed from the Rear Pulley Assembly showing the clutch mechanism (on the right)

Outer Front Pulley Half

(also known as fixed pulley half, or static pulley…. or a combination of these terms) This is the outer part of the front pulley mechanism that is fixed to the output shaft of the engine and turns in time with it, its usually bolted on with a substantial nut that has to be tightened up to a precise torque, this nut is usually known as a ‘variator nut’ and on some applications can only be used once. The pulley half is fixed on the output shaft both rotationally and laterally.

Variator Assembly

This includes the moving pulley half/ramp plate, the rollers, the cover plate, and the plastic sliding bushings that keep the parts aligned. This sits on the output shaft and is free to slide along the shaft. It will slide along the shaft when the engine changes revs and by doing so will change the gearing between the engine and the rear wheels.

Photo showing a 6 roller variator (from behind), note the purple rollers sitting on the ramp-plate

and the area for movement when they are thrown outwards by centrifugal force Rollers

These sit as an integral part of the variator and roll along inclines (called ramp-plates) at certain revs of the engine to help the change in the gearing be more useable (some scooters utilise sliders instead of rollers, thinking about Dr Pulley applications)

Drive Belt

The drive belt is the connection between the front and rear pulleys and it allows for a varying gear ratio at different wheel speeds/revs of the engine.



Rear Pulley

(also known as the driven pulley) The rear pulley receives the input of power from the front pulley via the drive belt. The rear pulley comprises the clutch bell, clutch mechanism, torque controller and contra spring.

Clutch Assembly

The clutch assembly comprises the clutch plates and springs, clutch bell. This is positioned on the rear pulley shaft that turns the wheel via the final gearing in the rear hub.

A disassembled Clutch Assembly - note torque controller on left with large contra spring in place,

clutch mechanism and red clutch springs on right, clutch bell at bottom Clutch Springs

These are 3 (usually) small springs that are part of the clutch and effect its ‘bite’. Contra Spring

(also known as contrast spring, pulley spring, clutch torque spring, etc) The contra spring sits behind the clutch mechanism and maintains the gearing at the correct level at the rear pulley

Introduction One of the biggest misunderstandings is that changes to the variator, clutch and contra springs, and the weight of the rollers will increase your overall power, or top-end speed. This is incorrect, what these changes do are to make the power more useable by allowing the engine to maximise its RPM while it is accelerating, and maximise the speed the engine is revving at when it hits the highest gear ratio available. To optimise the scooter’s performance the engine needs to be running at its maximum torque output consistently while accelerating (which on most scooters is somewhere between about 6500-8500 rpm's as this is the level where the ‘power band’ is at its highest). The whole point of the CVT is to keep the engine at the most efficient RPM by making the gearing variable, the CVT adjusts the gear ratio to maintain that optimum RPMs. Top speed will be achieved when the maximum horsepower hits the limits of drag (friction and air resistance). How it works The CVT works upon the ratio of the diameter of the front and rear drive belt pulleys. Both front and rear pulleys are an assembly of moving plates. At low engine speed, the plates of the front pulley are pushed apart by the tension of the belt as it forces itself between the plates. At the same time, the plates in the rear pulley are pushed together by the spring beneath the automatic clutch. Behind the moving plate of the front pulley, there are weighted rollers, these are thrown



towards the outside edge of the variator by centrifugal force, as the rotation speed of the engine / variator increases. When the rollers move towards the outside of the variator, the inside plate is forced together towards the outer plate and this moves the drive belt towards the outside of the pulley. This effectively makes the diameter of the front pulley larger. As the belt moves out on the front pulley, the tension in the belt is increased, which overcomes the spring pressure holding the two plates together in the rear pulley. The belt moves towards the center of the rear pulley, which effectively makes the diameter of the rear pulley smaller. In this way, the ratio between the two pulleys is changed and therefore, the speed ratio between the engine and rear wheel is changed. The two diagrams below will give you the basic idea of how the gearing ratios change as the road speed/engine rpm changes.

A moving diagram showing front and rear pulley settings at high and low speeds

Another diagram showing front and rear pulley settings at high and low speeds -

Note in the sectional diagrams at the left and right that you can see the belt and roller positions

Front Pulley

Starting with the front pulley, this is where the rollers and ramp plate are located. As the revs of the engine increase, centrifugal force pushes the roller weights outward and up/onto the ramp-plates surface. This causes the sliding variator to move toward the outer fixed pulley half. As the variator slides closer to the front pulley it applies pressure to the drive belt that sits in between the angled faces of the front pulley half and the variator so that it forces the belt out to a higher gear ratio. This is the basic operation of the front pulley. Faster engine speeds cause the belt to go outward changing the gearing.



Rear Pulley

Moving onto the rear pulley, the rear pulley has a large and powerful spring (known as the contra spring) holding the halves of the rear pulley together. The front half of the rear pulley is placed under pressure by the contra spring to make sure that it holds the correct tension to ensure the best gearing. There are angled grooves that the pulley travels on. As torque is applied, this limits the belt from travelling in too quickly hence under/over effecting the gearing ratio.

Clutch Assembly

The rear pulley spins under the power of the engine via the front pulley and the drive belt, and when it gets to certain preset revs the clutch plates spin out and ’grab’ onto the clutch bell which in turn spin the wheel (via the preset final gearing in the hub, where applicable)

Modifications There are several modifications that can be made to a scooter’s CVT drive-train and these are best done in a balanced way….

Roller WeightsThe first is perhaps to change your rollers, or more precisely their weight…. Rollers can get worn with time and its not uncommon to find ‘flat-spots’ at fairly low mileages (this can be sometimes to do with riding styles). By lowering the weight of the rollers that you are using it allows the scooter to hold onto a lower gear longer (think about a car where you don’t change gear till later to get a little more power). By using lighter rollers you are allowing the engine to rev a little higher than with heavier rollers and helping it to hit a higher rev band before it ‘changes gear’ (it doesn’t actually change gear but you’ll see what I mean). Having heavier rollers make the scooter change up a gear too soon, again using the car analogy, it’s a bit like driving along in second gear then changing to third gear way too soon, the engine doesn’t have enough cadence to keep the gearing good! Lighter rollers may be needed in situations where the stock exhaust has been replaced by a performance exhaust as the freer flowing exhaust breathes better and takes the usable power band higher up the rev range, using lighter rollers allows the engine to rev up easier to reach that power range more effectively. Also for a similar reason, a heavier rider may benefit from slightly lighter rollers to allow the scoot to stay in a 'lower gear' for a touch longer. Don't be tempted to go too light though... this can lead to a reduction in top-end speed, I've recently found that by changing my lightened rollers for ones that are only a touch heavier (but still lighter than stock) that I added almost 5mph top-end, the very light rollers that I was originally using (although accelerating very well indeed) had 'robbed' the scooter of its maximum speed. It can be very much a trade off between acceleration and max speed.. you have to consider your riding style and the type of roads that you use.

Weighing in! - 6 rollers weighing only 39 grammes in total, 6.5g per roller

Performance Variator Assemblies

The main effect of performance variators is to change the angle/curvature of the ramp plate so that the rollers deliver a better spread of acceleration throughout the rev range. If you are thinking about getting a new variator, consider this.. it will probably not have any effect on your overall top speed. What it will do is give you a more steady acceleration.

Contra spring

If when you approach a small hill the scooter slows drastically you may have too light a contra spring. The contra spring helps push the rear pulley back together when torque is applied to the rear pulley. In addition, slowing down from speed and then struggling to accelerate up again also indicates too light a spring because when you slow down the spring is supposed to push the rear pulley together into a lower gear ratio. If you go with a heavier contra spring, you may need to go to a slightly higher roller weight. Likewise, if you go to a lower spring, you may need to go to a lighter weight.

Torque Controller

There are angled grooves that the rear pulley travels on. As torque is applied, this limits the belt from travelling in too quickly hence under/over effecting the gearing ratio. Replacements can be made of this part of the rear pulley with a performance part that has differently angles grooves.

Clutch springs

The initial ‘take-off’ of your scooter is mainly controlled by the clutch-springs, these are 3 small springs that allow the clutch to bite.. as the clutch spins the centrifugal force pulls against the springs to allow the clutch plates to ‘grab’ the clutch bell. By replacing these springs with performance ones you can make the clutch grab at higher revs hence allowing the take-off to be more ‘snappy’. The downside of this is that in heavy traffic the revving up of the engine to get the clutch to engage can be tiresome.



Replacing original clutch springs with Red 2000rpm springs to give a racier 'bite'

Drive Belt

It is widely recognised that the genuine piaggio belt does the best job, replacement 'performance' belts tend not to last too long (comparatively), however replacing a worn belt with a genuine belt will give you an increase in performance. The top end speed of you scooter may be affected by a worn/thin belt as the belt struggles to reach the outer limit of the driven pulley and hence not reaching 'top gear'.



New belt versus old worn belt, these two belts were originally the same thickness!

{this shows width rather than thickness] De-restriction Some smaller CC scooters may be restricted by placing a washer in-between the faces of the front pulley half and the variator, this stops the belt from reaching the outermost position on the front pulley hence not allowing the scooter to hit ‘top-gear’. It is a fairly simple task to remove the washer to de-restrict the CVT.

This is the restriction washer that may need to removed on smaller CC scoots

...and finally..... here's a video that explains more about how a CVT works....



. Last Updated Sat Oct 15, 2011 9:04 am

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