Why Are Underbone Motorcycles Fuel-Efficient?
4 Answers
The reasons for the fuel efficiency of underbone motorcycles are as follows: Body Specifications: The vehicle is lightweight with narrow tires. Underbone motorcycles inherently have a smaller self-weight and narrower tires, resulting in lower rolling resistance and reduced fuel consumption; they utilize a dual-clutch system. Dual-Clutch System: Typically, underbone motorcycle engines employ a dual-clutch system that retains gear shifting but features an automatic clutch. Compared to the CVT system of scooters, this offers significantly higher transmission efficiency. Automatic Clutch: Thanks to the automatic clutch, power delivery follows the increase in RPM, ensuring smoother acceleration. Consequently, the carburetor can be tuned to operate on a leaner mixture, thereby saving fuel.
Speaking of the secret to the fuel efficiency of underbone motorcycles, I repaired quite a few of them in my younger days. The key lies in their lightweight and compact frame, with an empty weight of just over 100 kilograms—easy to push around. The engine is also simplified; a 125cc single-cylinder air-cooled engine is sufficient, unlike high-displacement bikes that guzzle fuel. The belt drive design is clever, reducing power loss by 20% compared to chain drives, which is especially noticeable in stop-and-go city traffic. Plus, the narrow tires mean less contact with the ground and reduced friction, making the ride feel like gliding on ice. With a fuel tank capacity of only 4 liters, manufacturers are forced to prioritize fuel efficiency. I’ve seen the most fuel-efficient ones cover 200 kilometers on a single tank as if it were nothing.
From a physical principle perspective, the fuel efficiency of underbone motorcycles is scientifically sound. The belt drive system boasts over 92% efficiency, 7 percentage points higher than gear drives. With the vehicle's center of gravity positioned below knee level, inertial losses during acceleration are minimized. The 105kg curb weight (equivalent to three middle school students) creates an advantageous power-to-weight ratio. The narrow tires feature a rolling resistance coefficient of 0.008 - half that of automobile tires. The precisely balanced crankshaft design eliminates vibration even at 20,000 rpm, ensuring all kinetic energy is channeled into propulsion. These meticulous details maximize the utility of every drop of fuel.
I used to ride a step-through bike for long-distance trips, and the fuel consumption was indeed a pleasant surprise. The credit largely goes to the CVT transmission, which maintains engine speed without dropping during automatic shifting. You can see that the speed remains steady regardless of how much you twist the throttle, unlike geared bikes that constantly require clutch engagement and waste fuel. The riding posture is also well-designed—keeping the body upright reduces wind resistance. The engine tuning is clever, reaching maximum torque at 6,000 RPM, which aligns perfectly with the efficient range for everyday speeds. Even the plastic body panels are lightweight, and the mirrors are hollowed out. Once, I disassembled the engine and noticed the piston rings were a third thinner than those on a standard motorcycle, reducing friction losses and naturally improving fuel efficiency.
