There are three main types of transmissions that can be found within ATVs today. The first is the manual transmission. The manual transmission requires the operator to select the most appropriate gear for the desired situation. The lower gears are used for take-off, steep climbs, hauling heavy loads and low traction scenarios; whereas the higher gears are most desired for cruising scenarios. The manual transmission also requires the operator to disengage the engine from the transmission via the clutch lever before each shift and re-engaging them through the release of the clutch after each shift.
The next type of transmission is the semi-automatic transmission. This transmission is very similar to the manual transmission in that the operator still selects the optimum gear for the situation they are in. But they are dissimilar in that the clutch is not operated by a separate lever, but rather through the act of shifting gears. This setup frees the operator to concentrate on controlling the ATV without having to worry about clutch operation. This setup is recommended usually found within sporty utility ATVs and in transition vehicles for children who are just learning the art of gear selection, but already have experience operating an ATV.
Within the ATV world there are two main types of automatic transmissions used; both remove the need for the operator to shift gears which allows for greater concentration on balance, direction and traction. The first is the Continuously Variable Transmission better known as the CVT. The CVT is generally found in adult ATVs and works to keep the engine in its optimal rpm range. In this range the engine operates at its most efficient and produces its maximum power. The CVT also allows the operator to focus more on maintaining traction and stability in extreme conditions, and to not worry about whether they are in the correct gear for their situation. CVT's consist of two pulleys connected by a "V" shaped belt. The two pulleys act as clutches that continuously vary the drive ratio between the input shaft and the output shaft in a smooth and controlled manner. The primary clutch is connected to the engine via the input shaft and is completely disengaged from the belt at idle. As the engine speed increases, the primary clutch overcomes the spring force holding the two halves of the clutch (called clutch sheaves) apart. As the engine speed increases, the belt rides higher within the primary clutch, increasing the gear ratio between the input shaft and the output shaft. The secondary clutch works in a similar manner, although as the belt rides higher within the primary clutch, the belt is forced down, closer to the center of the secondary clutch. Its main job is to remove slack from within the belt as the primary clutch opens and closes. The secondary clutch connects to the output shaft and to the rear wheels via a chain or a drive shaft.
The other popular type of automatic transmission found within four-wheelers is much simpler and i'ts most commonly used in youth four-wheelers. This system uses a single wet clutch connected to the engine through a series of gears and engages the transmission at a certain preset RPM. The transmission on these models is a single speed with the option of reverse, but only a single forward gear. This design allows for beginning riders to concentrate on learning the handling and control of the ATV without having to worry about shifting gears.