What is “Centrifugal” in the Centrifugal Clutch Typical of the Super Cub?

The centrifugal clutch, which does not move forward if the engine speed is low even if it is in gear, is a familiar gear-shifting system in business motorcycles such as the Honda Super Cub. This is a middle ground between the hand clutch, which changes gears by grasping the clutch lever, and the stepless gear shift that does not require a gear change, as in a scooter. Let's take a look at the mechanism and maintenance points.

The weight moves according to the engine speed and is intermittent.

In the early 1970s, it was not mandatory to wear a helmet and it was not unusual to ride a motorcycle in a skirt. The Yamaha Chubby was launched with the female user in mind. Later, hand clutch and centrifugal clutch versions were added to meet the needs of leisure use.

On a hand clutch vehicle, where the clutch lever is held and the shift pedal is operated, when stopping at a traffic light or for other reasons, the clutch lever must be held or the gear must be put in neutral. In contrast, scooters' automatic gear shifting has no neutral or gear steps, and can travel from idling to maximum speed without shifting gears. In contrast, the centrifugal clutch does not require the rider to hold the clutch lever when shifting gears, and the rider is still responsible for changing gears.

Today, scooter-type motorcycles for business use are becoming more common, but before scooters using V-belts became common in the early 1980s, they were popular as a gear-shifting mechanism for business and utility models such as the Honda Super Cub.

The greatest merit of the centrifugal clutch is that it enables driving without the need for clutch lever operation. This may seem obvious now that stepless speed change in scooters has become widespread, but at the time of the Honda Super Cub's introduction in the late 1950s, the only speed change mechanism available was a manual transmission with a hand clutch, so the appearance of the centrifugal clutch was truly an epoch-making event.

The centrifugal clutch would have been a breakthrough for newspaper and mail delivery drivers, who would grip the clutch lever, put the motorcycle in low gear, put it in neutral at the next house, drop it in the mailbox, grip the clutch lever, put it in low gear again, and repeat. The centrifugal clutch that does not stall even when the motorcycle is stopped in low gear must have been revolutionary and attractive. Again, the centrifugal clutch Super Cub was introduced more than 20 years before the introduction of the continuously variable speed scooter.

What makes the centrifugal clutch intermittent is the centrifugal force applied to the clutch, which is literally proportional to the engine speed. It is a simple principle that the clutch connects when the engine speed rises and disengages when it falls, and the use of centrifugal force is similar to the drive pulley of a scooter. However, the centrifugal clutch has multiple functions beyond mere power disconnection, such as transmitting the kick pedal force to the crankshaft during starting and a semi-clutch mechanism to cut the driving force during gear shifting.

In addition, in the case of scooters, the combination of front and rear pulleys and V-belt requires a certain front and rear length for the transmission unit itself, but the centrifugal clutch has the advantage of being able to be completed in almost the same space as a hand clutch. For this reason, in the 1970s, from the appearance of the Super Cub to the generalization of V-belt type scooters, the centrifugal clutch was used in many models as the drive system for business motorcycles and family motorcycle used by female users.

Clutch plate and friction disc combination is the same as the hand clutch

The centrifugal clutch features a huge clutch basket that is hard to believe for a 50/80cc family motorcycle. The Honda Super Cub has a primary clutch attached to the crankshaft, while the Chubby has a secondary clutch attached to the main axle.

The 80cc centrifugal clutch is a combination of six friction discs and five clutch plates, each of which has more discs than the hand clutch version. Fourteen steel balls are lined up at the bottom of the clutch basket, which are moved outwards by centrifugal force to adhere the discs and plates together.

The groove where the ball fits is sloped from outside to inside, and the clutch is connected when the ball moves up this groove. The early models of centrifugal clutches were cylindrical weight rollers, the same shape as those built into the pulleys of today's scooters. Super Cub weights were also cylindrical before the 1980s.

The Yamaha Chappy was a family leisure motorcycle that was introduced in 1973 with a design that was easy to get on and off and small diameter tires that were designed with female users in mind. Initially, the motorcycle debuted with a two-speed automatic transmission. Later, a four-speed model with a hand clutch suitable for male users and leisure use was added, followed by a three-speed model with a centrifugal clutch.

Unlike the Super Cub, the centrifugal clutch of the Chappie was not a primary clutch attached to the crankshaft, but a secondary clutch attached to the main axle, which was common on hand clutch vehicles. For clutch disconnection, the early centrifugal models used cylindrical roller weights similar to those used in today's scooters, while the middle and later models used spherical ball weights. Although there is a difference in shape from Honda's centrifugal clutch in this way, the principle of using the centrifugal force generated by the engine is the same.

In the case of a hand clutch, the clutch is disengaged when the lever is held, and connected when the lever is released. This is why the motorcycle will stall if you stop without holding the clutch lever while the motorcycle is in gear. On the other hand, with a centrifugal clutch, the clutch is disengaged when the motorcycle is stopped without the engine riding or idling, and is connected when the engine speed exceeds a certain level. Both hand clutches and centrifugal clutches are wet, multi-disc clutches that combine steel clutch plates and multiple friction discs, but it is interesting to note that the clutch is either connected or disengaged when the motorcycle is at a standstill, or the complete opposite.

Connecting the disengaged clutch is the centrifugal force that starts to work significantly on the clutch outer as the engine speed increases. As with the drive pulley on a scooter, the clutch plate and friction disc are crimped together as the weighted ball moves to the outside of the clutch outer to transmit power.

Disc wear and poor weighting lead to clutch failure.

The clutch spring of the hand clutch is used in the direction of pushing the clutch plate and friction disc together, but in the case of the centrifugal clutch, it is incorporated so that the plate and disc are separated so that they do not inadvertently connect at low engine speeds.

Since there is no torque like big motorcycles, step wear does not occur on clutch outer and clutch boss, but the clutch plate and friction disc may be worn by striking when the motorcycle is repeatedly started and decelerated suddenly or loaded with a heavy load for delivery work on a daily basis. In such a case, clutch disengagement may become poor and it may become difficult to shift into gear.

The damper built into the coupling between the clutch basket and the primary driven gear is important for absorbing shock during acceleration and deceleration. Both of them are fixed with clamps and cannot be disassembled, but when performing maintenance, check for excessive rattling.

Centrifugal clutches, in which the plates and discs are crimped by the expansion of the weights, can be forced into a half-clutch state by depressing the shift pedal to alleviate shock when shifting gears (in the case of Honda motorcycles). However, during normal driving when the foot is released from the pedal, it can be said that clutch slippage is difficult to occur because it is crimped by the weight.

However, if the engine is tuned to increase torque, slippage may occur due to insufficient clutch capacity. Of course, even in normal use, the friction disc may be worn out by repeated disconnection, or the clutch spring that helps the disc to stick together when the clutch is connected may be worn out. In the case of the clutch outer of the chopper with the primary driven gear integrated, the damper incorporated between the gear and the clutch may become thin with age and wear, and the shock may be generated when the clutch is disengaged.

When reconditioning a vehicle that has been immobilized for a long period of time, it is necessary to pay attention to the rust of the clutch plate. In the case of hand clutch, if rust occurs while the clutch plate and friction disc are in close contact, they will stick together and the clutch will be stuck. Then, the clutch will not disengage even if the clutch lever is held, and the motorcycle will stall in clutch meet condition as soon as the gear is put into low.

In contrast, centrifugal clutches have plates and discs that are separated when the motorcycle is stopped, so it should be unlikely that the motorcycle will suddenly start riding after putting it into gear. However, if the plate rusts during long-term storage, it may cause extra friction between the plate and friction disc when the clutch is connected, which may lead to contamination of the gear oil. The gear oil only lubricates the clutch and transmission on a 2-stroke Chapie, but on a 4-stroke, it lubricates the crankshaft, pistons, and cylinder head at the same time, so you need to be delicate about oil contamination.

When such malfunction is felt, disassemble the centrifugal clutch unit to check the wear of each part and clean it. Some owners may think that centrifugal clutches for small displacement vehicles are maintenance-free, but it is important to know that proper maintenance is required for centrifugal clutches that can be ridden easily, just as hand clutches and weight rollers for scooters also require maintenance after mileage. Just as the hand clutch and weight roller of the scooter need to be taken care of after a long-distance, it is important to know that a centrifugal clutch motorcycle needs proper maintenance.