2-Stroke x 50cc is a promising future. Drivetrain Maintenance for Moped Scooters [2. Centrifugal Clutch

50cc scooters are convenient as a means of transportation to work, school, or neighborhood shopping. Now that there are no more 50cc models available as new vehicles, it is wise to keep them in good condition through maintenance. In the previous issue, we discussed the drive pulley and weight roller, which are the keys to the scooter's drive system. This issue focuses on the importance of the centrifugal clutch assembled to the driven pulley, which is turned via the drive belt.

The gearshift is stepless, but the clutch that interrupts the power is present.

The automatic transmission of scooters, including mopeds, consists of three parts: the drive pulley, which connects to the engine crankshaft; the driven pulley, which connects to the rear tire; and the rubber drive belt, which connects the pulley to the rear tire. The width of the front and rear pulleys automatically changes according to the engine speed, and the reduction ratio is automatically adjusted to change the speed by increasing or decreasing the turning radius of the drive belt, which is stuck in the V-shaped groove of the pulley in conjunction with the engine speed. When actually driving a scooter, the body will not move forward when the engine is at idle rpm, and the rear tires should start turning only when the throttle is opened and the rpm increases to some extent. The reason for this is that a clutch mechanism is built into the scooter's drive train. The clutch here refers to a device that transfers engine power to the rear tire, similar to the clutch operated by holding the clutch lever in a manual transmission vehicle. However, scooters do not have a clutch lever, and riders do not operate the clutch of their own volition. The centrifugal force generated by the rotation of the driven pulley is used for clutch disengagement on a scooter.

Centrifugal force from the rotation of the driven pulley interrupts the driving force

The drive pulley, which is directly connected to the crankshaft of the engine, is always rotating while the engine is running, and the drive belt also rotates. Then the driven pulley on which the belt is hung also rotates, but it is not transmitted to the rear tires when the engine is idling. A closer look at the components around the driven pulley shows that it can be divided into an integrated driven pulley and clutch, and a separate clutch outer. The driven pulley and clutch are parts that rotate together with the drive belt, and the clutch outer is connected to the rear tire via the final drive (gearbox). When the engine is stopped or at low rpm, the clutch shoe is closed and separated from the clutch outer, and the driving force of the engine is not transmitted. Also, when pushing the bicycle around on a parking lot, the clutch outer turns due to the rotation of the rear tire, but the drive belt does not turn because the clutch shoe is closed, and therefore neither the drive pulley nor the crankshaft turns. When the engine is started and the rotation speed of the driven pulley increases in proportion to the engine speed, the clutch shoe is opened by centrifugal force and contacts the clutch outer ring. During deceleration, the opposite is true: as the engine speed decreases, the centrifugal force applied to the clutch shoe decreases, and when the tension of the clutch spring falls below the tension of the clutch spring, it separates from the clutch outer ring and the driving force is disengaged, and the engine does not stop even when the vehicle speed reaches zero. Scooters automatically adjust the reduction ratio according to the centrifugal force applied to the weight roller inside the drive pulley according to the increase or decrease in engine speed, and centrifugal force is also utilized for the interruption of driving force. Both of these are not controlled electronically, but are controlled solely by the centrifugal force generated in a rotating object.

When disassembling the clutch, be careful of the tension of the thin nut and center spring

The outermost visible part of the driven pulley section is the clutch outer, which can be easily pulled out by removing the center nut. Once the clutch outer is removed, the inner driven pulley and the centrifugal clutch can be removed as one unit. The pulley and clutch are assembled with a thin nut on the clutch side, but extreme care must be taken when removing this nut. First of all, a tool appropriate for the nut size is required. In the case of the AF35 Honda DIO (so-called "live DIO") used as a sample vehicle for this series of work, the nut has a two-sided width of 39 mm, which is much larger than a standard wrench, eyeglass wrench, or socket wrench. Also, because the nut itself is thin, monkey wrenches and water pump pliers have a shallow fit and easily come off. Some people try to loosen the nut by tapping the corner of the nut with a flat-blade screwdriver or a hammer, but this requires experience and a knack for loosening the nut. In these situations, special tools can be relied on. Some constructors and tool stores that offer a wide lineup of tuning and maintenance parts for scooters and 4-mini models have commercialized thin wrenches dedicated to clutch center nuts. These are specialized tools and are optimized for the removal and installation of the nut. In addition, preparation before removing the nut is also important. A center spring with very strong tension is built in between the driven pulley and the centrifugal clutch, and if the nut is unintentionally removed, the clutch may come off with a great force, resulting in serious injury. To prevent such an accident, the nut can be securely attached and removed while holding the pulley and clutch with the clutch spring compressor of a special tool, but a binding band (tie wrap) can be used as a substitute. Since a thin band will break off against the spring, prepare several bands as thick as possible, pass them through the hole in the center of the pulley, tie off the clutch, and gradually loosen the nut. Since there is a risk that the spring will stretch and the binding band will break the moment the nut is released, care must be taken to prevent the jumping clutch from hitting the operator himself or those around him. Some more dexterous workers remove the nut while applying weight by stepping on the clutch with the soles of both shoes, but in either case, it is important not to underestimate the tension of the center spring.

As with drum brakes, the friction material on the surface of the clutch shoe is important

Scooter clutches are interrupted when centrifugal force causes the clutch shoe to open outward and adhere to the clutch outer. Similar to the brake shoes of a drum brake, the clutch shoe consists of a lining (friction material) affixed to the surface of a cast aluminum shoe. Therefore, as the driving distance increases and the number of times the clutch is interrupted increases, the lining wears down, and eventually the lining wears out and the clutch shoe makes direct contact with the clutch outer, the driving force is not transmitted and the clutch outer is also damaged. With mechanical drum brakes, the rider can tell when the lining is worn by the increased play in the brake lever and brake pedal, but with centrifugal clutches, the changes that the rider can feel are slight. However, as wear progresses, the clutch slips, making it harder to transmit driving force, so if you often feel that acceleration is slower than usual despite opening the throttle, it is a good idea to check the lining wear. Although we replaced the clutch assembly in this case, the clutch spring is also important as a factor that determines how well the clutch connects. The clutch spring of a centrifugal clutch determines the centrifugal force required to open the shoe and works in the direction of closing the shoe. If the spring is weak, the centrifugal force on the clutch is small = the clutch contacts the clutch outer ring at low engine speed. Conversely, if the spring is strong, the driving force will not be transmitted unless the centrifugal force is large = engine rpm is high. The idea is the same as in a manual transmission where the clutch is engaged at high rpm when the driver wants to make a strong dash for the start. The weight of the clutch shoe itself also affects the timing of clutch engagement. A lighter-than-normal shoe is less likely to be subjected to centrifugal force, so the engine revs at a higher rpm when the clutch engages, giving the driver an advantage in the dash. However, too much weight can make it difficult to handle, so when selecting parts, consider the balance of the entire drivetrain, including the engine and weight rollers.

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