The Continuously Variable Transmission (CVT) system consists of two parts: the variator (drive pulley), and the clutch (driven pulley). These are connected by the CVT belt.
The CVT system works through the changing of the distance between the plates on the two pulleys. Basically, when the plate width on the variator pulley decreases. the clutch pulley plate width increases, and vice versa. This creates an infinite number of possible gear ratios, as the transmission is altering itself on the fly to adapt to the current driving condition.
The variator is driven directly by the engine. Inside the variator are 6 rollers that are positioned in individual slots with ramps that they will move along outward when centrifugal force is applied. As the rollers move outward, they press against the ramp plate which causes the pulley plates of the variator to move toward one another, compressing the belt. This “V” shape created by the pulley plates pushes the belt outward, which draws the belt inward on the driven (clutch) side, increasing the gear ratio.
At idle, the rollers are at their innermost position, the variator pulley plates are at their farthest apart, and the CVT belt is low on the variator side and high on the clutch side (see Image 2). With increasing RPMs, the rollers move outward along their ramps applying pressure to the ramp plate, which compresses the variator pulley plates and squeezes the CVT belt outwards.
In the CVT transmission system, the rollers are actually working against the spring tension of the main torque spring on the clutch side. This is discussed in detail later in this article in the Clutch section.
Performance and racing variators have specially engineered “ramps” for the rollers. Many have teflon ramps and ramp cover plates for reduced roller friction. This means smoother transition between “gears”.
Rollers come in many different sizes and weights, depending on application.
– Lighter roller weights (7-9g) will give more climbing power at the cost of some top end speed.
– Heavier roller weights (13-15g) will allow more top speed at the serious expense of climbing ability – but are great for flat-ground racing.
– In the middle range (10-12g) is a combination of the two.
For 250cc variators:
– Lighter roller weights (15-17g) will give more climbing power at the cost of some top end speed.
– Heavier roller weights (22-25g) will allow more top speed at the serious expense of climbing ability – but are great for flat-ground racing.
– In the middle range (18-21) is a combination of the 2.
Note: Roller weights that are at the lowest recommended end of the scale are often too light to fully push the variator plate far enough out to achieve maximum speed. Similarly, the heavier weights quickly move you into a higher gear ratio at the expense of low end power. Please keep this in mind when choosing the right weights for your style of riding.
Rollers and sliders should be inspected annually (at minimum) for wear. Rollers are especially prone to developing flat spots that inhibit their ability to move smoothly. If this occurs, they should be replaced. This is one advantage of sliders, as they already have flat sides and are not affected by this wear as quickly as rollers.
The clutch in a CVT system engages when the centrifugal force of the spinning clutch overcomes the tension of the clutch arm springs and allow the clutch pads to engage with the clutch bell, creating movement.
Racing and performance clutches are made of higher quality materials, such as metal composite or kevlar clutch pads to reduce wear and heat damage. These clutches often have much larger clutch pads for better engagement with the clutch bell. Clutches can be altered with different rated arm springs to change their engagement RPMs.
The main clutch torque spring compresses the clutch pulley plates together, forcing the belt outward and acting against the variator. As the rollers compress the variator side pulley plates when RPMs increase, the belt is forced outward on the variator. Since the belt is a constant length, this causes the belt to be pulled inward on the clutch, overcoming the tension of the torque spring.
What do the different clutch springs do?
The main torque spring makes it harder for the variator to draw the belt inward on the clutch. This keeps the buggy in a lower gear ratio longer, and “downshifts” faster when decelerating so you have more power when you hit the gas again. This is especially helpful when climbing or coming out of a corner. A higher tension main torque spring downshifts you more quickly than a lower tension spring, but be aware that high spring tensions can prevent very light roller weights from ever reaching the maximum position inside the variator, sacrificing top speed.
Example: 7g roller weights and a Red 2000 RPM main spring will give you a LOT of low end, but sacrifice top speed.
Clutch arm springs control when the clutch arms and pads engage with the clutch bell. These springs are rated at 1000, 1500, and 2000 RPMs. This means that the clutch has to be spinning at this RPM speed before the centrifugal force will overcome the spring tension and allow the clutch pads to engage. This is not the same as engine RPMs, as the engine will be idling at some rate and the engagement RPM is on top of this.
Example: If the engine idles at 1000 RPMs, and the clutch arm springs are rated at 2000 RPMs, then the motor will have to rev to at least 3000 RPMs before the clutch will engage. Using higher tension arm springs can result in what seems like high motor RPMs at low speeds. The large main torque spring does not affect this engagement level.
Generally speaking, most 150cc buggy owners are very satisfied with the yellow (1500 RPM) main torque and clutch arm springs. For racing or heavy climbing purposes one might consider the heavier 2000 RPM red springs. For 250cc buggies, the red or black main torque springs are good choices when paired with the appropriate weights for your riding style.
Replacing the stock clutch main torque spring and clutch arm springs are inexpensive upgrades that can increase the responsiveness of your go kart.
The clutch bell is something that you should inspect annually at minimum, and more often if you find that you are bogging down when climbing with the engine revving high and the wheels won’t spin. This can be indicative of a “glazed” or smoothed clutch bell and/or clutch pads.
A tremendous amount of frictional heat is created when pushing the climbing limits of your buggy, and this eventually leads to smoothing of the inside edge of the clutch bell. The heat (and smoke) can turn the bell a purplish color and result in a very smooth, glazed appearance. When this happens, it’s time to replace the clutch bell and inspect your clutch pads for possible clutch replacement as well.
The CVT belt is the link between the variator and clutch drives. A good belt is necessary for peak performance, and belts should be inspected for fraying and wear and replaced if necessary.
Belts are available in regular and Kevlar varieties. Kevlar belts are stronger than regular belts and tend to last longer.