2-Stroke x 50cc is a promising future. Moped scooter drivetrain maintenance [1: Weight roller section].

Although there are some restrictions, such as a legal speed limit of 30 km/h and a one-person-only riding limit, the appeal of motorized bicycles is that they are easy to obtain a license and simple to ride. Although new 50cc mopeds are no longer sold in Japan due to new exhaust gas regulations, there are still many 50cc mopeds in the market. Here we focus on 2-stroke moped scooters, which can still be fully usable with care, and explain basic drivetrain maintenance over several articles.

Scooter drivetrain is simple in structure, yet has sufficient practical performance

The scooter automatically shifts gears from start to maximum speed with a twist of the throttle. Although they are less fun to operate than manual transmissions, in which the rider himself selects the gear and shifts gears, their great appeal is that anyone can drive them easily, and it is no exaggeration to say that scooters made up the majority of the 50cc class. Some say that holding the clutch lever and operating the shift pedal is the best part of riding a motorcycle, but even manual transmissions utilize a variety of electronic controls to eliminate intermittence of driving force. Some scooters with larger displacement models also allow manual shifting by the rider. Compared to these models, the basic structure of the drivetrain of 50cc moped scooters, which consists of front and rear pulleys and a drive belt, has been retained even after the transition from the 2-stroke to the 4-stroke engine. However, the drivetrain mechanism established in the 2-stroke scooter era remained valid even after the shift to 4-stroke engines. In other words, even if the engine is a 2-stroke scooter, it can be used practically on public roads today as long as the drivetrain is properly maintained. Some say that a 2-stroke scooter, with its simple engine structure and torque from low speeds, can run more briskly in town than a 4-stroke scooter. Regardless of whether it is a 2-stroke or 4-stroke scooter, there are certain parts of the drivetrain that will wear out. In the past, moped scooters had the image of being ridden down without proper maintenance, but it may be appropriate in this age of SDGs to continue riding them with proper maintenance.

The combination of pulleys and drive belts is the key to automatic gear shifting

The automatic transmission of mopeds and other scooters 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 between them. The width of the front and rear pulleys automatically changes according to the engine speed, and in conjunction with this, the reduction ratio is adjusted to change the speed by increasing or decreasing the turning radius of the drive belt that is stuck in the V-shaped groove of the pulleys. This can be thought of in terms of a bicycle transmission. Since the gear ratio on the pedal side and the rear tire side of a bicycle without a transmission is constant, the pedal speed must be changed to change the speed. However, if there are multiple gears with different numbers of teeth on the rear wheel, the bicycle can respond to a wide range of speeds while reducing the burden on the pedaler by selecting a gear with a higher number of teeth when starting off or going up a slope with a heavy load, and selecting a gear with a lower number of teeth when the speed increases. Automatic gear shifting on scooters uses weight rollers built into the drive pulleys to adjust the turning radius of the pulleys. When the engine is stopped or idling, the V-shaped groove consisting of the drive pulley and pulley face is wide and the drive belt is located at the back of the groove. Then, as engine speed increases, centrifugal force causes the internal weight roller to move outward from the pulley and closer to the pulley face, narrowing the width of the V-shaped groove and pushing the drive belt out, increasing its rotating diameter. The width of the driven pulley changes in response to this change in rotating diameter, which also changes the speed of the rear tire. The relationship between the diameter of the front and rear pulleys (rotational diameter of the drive belt) and speed is shown below.

Drive pulley Driven pulley At low speed Small Large At high speed Large Small

Inevitable weight roller wear.

The cylindrical weight roller sandwiched between the drive pulley and ramp plate rolls outward due to centrifugal force as the pulley speed increases with engine speed. In doing so, the roller is subjected to strong frictional forces, and heat is generated as it rolls overcoming the friction. The weight roller has a two-layer structure with a metal weight in the center and resin on the periphery, and the resin part wears out as the distance traveled increases and the roller deteriorates over time. Since the rollers always rotate when shifting gears, wear is inevitable, but the wear does not progress evenly, and it is common for a portion of the cylinder to wear unevenly and flatly. Uneven wear makes it difficult for the rollers to roll smoothly and may cause uneven shifting. If the situation worsens further and the resin part becomes worn and separates from the metal weight part, this can lead to major problems such as grinding the groove where the roller rolls as well as noise during gear shifting, and the cost of repair will also increase. The service manual describes the wear limit of the weight roller outer diameter, but does not describe the replacement time or distance. Rough throttle operation, frequent full-throttle driving, and riding with large changes in engine speed will increase the amount of weight roller movement, which will inevitably result in more wear. According to the website of one parts manufacturer, the recommended inspection and replacement interval is around 5,000 km. Since the weight rollers are located in places that cannot be seen from the outside, it is often thought that they do not need to be replaced, or that they will be replaced without maintenance based on the amount of time a scooter is used in general. However, like tires and brake pads, these parts wear out due to friction and need to be replaced.

Stock weight roller weight is the standard.

When replacing worn weight rollers, there is an option called roller weight. Because weight rollers are actuated by centrifugal force, the weight changes the response characteristics of the pulley. Generally speaking, a lighter weight roller will give more emphasis on acceleration, while a heavier weight roller will give more emphasis on maximum speed. A lighter weight roller is less susceptible to centrifugal force, and gear shifting begins at higher rpm, which is similar to clutch meat at higher rpm in a manual transmission. However, if the weight roller is too light, the pulley may not move sufficiently (the V-shaped groove does not narrow) and the maximum speed may not increase. Conversely, if the weight roller is made heavier, shifting will begin at lower RPMs, but acceleration may be sluggish, just as it is difficult to start in 3rd or 4th gear with a manual transmission. In this way, the weight roller can change the characteristics of gear shifting. However, when changing the gear ratio by replacing the sprockets on a chain-driven bike, the effect depends on the engine characteristics and the rider's preference and driving style, just as placing more emphasis on acceleration will cause the top speed to hit its peak and placing more emphasis on top speed will slow down the acceleration from the start. In the same way, the effect will vary depending on engine characteristics, rider preference, and driving style. If the engine and intake/exhaust systems remain stock, changing the weight roller weight with the intention of changing the shifting characteristics may not balance well with the engine characteristics, so care must be taken. If maintenance is to replace unevenly worn weight rollers, it is advisable to first run the assembly with the normal weight before making a decision. When changing the weight roller weight, it is also important to check the condition of the drive belt, which will be discussed in the next section.

Image gallery (13)