1. Why the engine keeps running at a red light while the rear wheel stops
When a motorcycle stops at a red light, the rider puts a foot on the ground. The rear wheel comes to a complete halt, but the sound of the engine does not stop. This state, where the engine runs but the vehicle is stationary, is called idling. A motorcycle guide explains that to stop while idling or to start moving from a stop, it is essential to connect or disconnect the clutch between the engine and the rear wheel [1]. This means the engine and the rear wheel are not always directly linked. Instead, a part that can "connect or cut" the power is placed between them. Before looking at that part, let us first understand what is happening inside the running engine.
2. The four repeating piston movements inside a motorcycle engine
Inside the engine, a piston moves up and down, repeating four movements in order: intake (sucking in a mixture of air and fuel), compression (squeezing it), combustion (burning it), and exhaust (expelling the burned gas). This completes one cycle. During this cycle, the piston moves up and down twice, and the crankshaft rotates twice [2]. Motorcycle guides often describe this as "intake, compression, explosion, exhaust," but "explosion" is just a nickname. In reality, it is the movement of the mixed gas burning [3]. In the past, small motorcycles often used a different system called a two-stroke engine. Because oil was mixed with the fuel for lubrication, it produced white smoke and odors, making it difficult to meet emission regulations. Today, very few vehicles use this type [4]. So, how is the power from this continuously running engine sent to the rear wheel, and how is it cut off when needed?
3. How the clutch connects or cuts the engine's power
Inside the clutch, many discs made of different materials are stacked alternately. Normally, springs press these discs together, so power from the engine is transmitted to the rear wheel side. When the rider squeezes the lever, the springs compress, gaps form between the discs, and the power is cut [1]. In other words, the clutch is a part that disconnects power when you squeeze it. To stop, you squeeze to cut the power. To start moving, you slowly release your grip. The intermediate state, where it is neither fully disconnected nor fully connected, is called "half-clutch." In this state, the engine's power is gradually transmitted to the rear wheel [1]. On motorcycles, a "wet multi-plate clutch" with multiple friction surfaces is generally used [5]. The rider controls how much to connect based on feel. Because it is done by hand, it is a part that requires practice. So, how is the connected power changed and delivered to the rear wheel?
4. How gears change the engine's rotation and chains send it to the rear wheel
The engine has a limited range of rotations where it can produce strong power. However, the rear wheel needs a wide range of driving force, from a stopped state to high-speed running [1]. The transmission (gearbox) bridges this gap. The rider changes the combination of gears using a foot pedal to use different rotations. The power leaving the transmission is carried to the axle (rear wheel). Methods for this include roller chains, shafts, and toothed belts [5]. A chain works similarly to a bicycle chain and is one way to transmit power. To summarize the path so far: engine, clutch, transmission, chain or other transmission method, and rear wheel. When stopping, starting, or changing speed, human hands and feet are working. Examples of giving this hand work to machines are the Super Cub and the Honda E-Clutch.
5. How the Honda Super Cub removed the need to operate a hand clutch
Honda's Super Cub, born in 1958 (the first model was the C100), had a 49cc air-cooled 4-stroke OHV single-cylinder engine with 4.5 horsepower [6]. One of its features is the "automatic centrifugal clutch." There is no need to operate the clutch by hand; shifting gears is done only with the foot pedal. The pedal is shaped like a seesaw: stepping on the toe side shifts up, and stepping on the heel side shifts down [7]. This design lets a machine handle the "half-clutch" adjustment instead of the rider's hand. The detailed explanation of the clutch principle is not confirmed in the current materials, so it is not covered here. The total production of the Super Cub series reached 100 million units worldwide on October 19, 2017 [6]. Calculating this, 100 million seconds is about 3 years and 2 months. So, even if one were made every second, it would take over 3 years. This shows that the idea of reducing hand operations has been widely used.
6. How Honda's E-Clutch uses motors to operate the clutch
On November 7, 2023, Honda announced the "Honda E-Clutch," said to be the world's first for two-wheeled vehicles. This system means the rider does not need to operate the clutch lever when starting, shifting, or stopping. If the lever is squeezed, it can temporarily switch to manual operation, and the system can also be turned on or off [8]. The "world's first" claim is from Honda's announcement. The system includes two small motors, gears, a control MCU, a three-part clutch lever shaft, and an ECU that controls the engine. It uses information such as engine speed, throttle opening, gear signals, and wheel speed to operate the clutch [9]. Shifting itself is still done by the rider using the foot pedal. Like the Super Cub, the E-Clutch reduces hand clutch operations. However, it is an electronic system driven by sensors and motors. The same problem (the difficulty of connecting and cutting) was solved by different methods in eras separated by about 60 years.
7. Countersteering: why a rider turns the handlebar opposite to the turn
When a motorcycle is moving, tilting the body causes the front wheel to naturally turn to the side it is tilted toward. Guides explain that when turning, it is more important to tilt the body than to turn the handlebar largely [10]. So, how does the body start to tilt? There is a method called "countersteering." If you turn the handlebar slightly in the opposite direction to where you want to go, the body begins to tilt toward the turning side. After this initial trigger, you tilt the body inward to turn [11]. This effect is said to be less noticeable at low speeds [11]. Since this is a separate topic from how engine power is transmitted, we will keep this section brief.
8. How to convert 6,000 engine rotations per minute into rotations per second
If the engine runs at 6,000 rotations per minute, that is 100 rotations per second (6,000 divided by 60). Since the crankshaft rotates twice per cycle [2], there are 50 cycles per second (100 divided by 2). If it is a single-cylinder engine, combustion happens 50 times per second (this is a calculation example, not an actual value for a specific bike). Let us try to verify 50 times per second by tapping a desk with our fingers. Five times per second is easy. At 10 times, fingers start to tangle. The engine is moving at five times that speed. If you want to observe this outside, listen to the sound from a safe sidewalk when a bike stops at a light. Do not go into the road or get close behind the bike. The fact that the engine sound continues while stopped is the "idling" mentioned in section 1. For more details, Honda's E-Clutch announcement [8] and technical explanation [9] are available in Japanese.
Observe from a safe place like a sidewalk. Do not enter the roadway or approach the rear of the vehicle.