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In-depth analysis of motorcycle movement and system
When the natural frequency of a valve spring matches or is an integer multiple of the valve's opening and closing cycle, resonance occurs. In motorcycle engines, this resonance can be extremely damaging. It not only increases engine noise but also causes the torsional shear stress in the valve spring to rise significantly—often several times the normal level. This leads to large fluctuations in spring force, which can result in gaps in the valve mechanism, causing impact and abnormal valve movement. In severe cases, it may lead to spring failure and even serious safety issues.
To prevent resonance, modern valve springs are typically designed with a high natural frequency. Additionally, many engines use a dual-spring system, where two springs are placed inside and outside the valve. This distributes the load based on the principle of equal strength, reducing the overall spring height. If one spring breaks, it doesn’t immediately damage other components, thus minimizing the risk of sudden accidents.
Valve springs must have sufficient rigidity to ensure that their pressing force exceeds the maximum inertial force generated by the valve train. This force is calculated as the product of the mass of the valve train and its maximum acceleration. However, during operation, vibrations caused by harmonic effects from the cam profile can distort the valve motion law, leading to continuous spring vibration. As engine speed increases, the amplitude of these vibrations may grow, significantly increasing the spring’s instantaneous load. In practice, the actual load on the spring is often twice the inertial force of the valve. Therefore, when designing the spring, a safety margin must be considered, especially for high-speed engines.
Key dimensional parameters of a spring include the mean diameter, wire diameter, pitch, number of active coils, and total number of coils. Based on the position of the valve when it is closed and fully open, the pre-load height and maximum tension height are determined. These values help calculate the spring load at different stages, allowing for an initial design assessment.
At high speeds, the valve opens and closes rapidly, and the spring vibrates at a high frequency. However, the spring does not compress or extend uniformly; instead, the active end compresses or extends first. To minimize vibration, the natural frequency of the valve spring should ideally be at least twice the engine speed. Another solution is to use variable-pitch springs, which adjust stiffness based on valve lift, changing the natural frequency and preventing resonance. Using inner and outer springs with different frequencies can also dampen each other’s vibrations. Some engines employ external damping plates to further reduce vibration.
The material used for valve springs must meet strict requirements. Typically, they are made from high-quality spring steel wire, such as ZHOUBA, YIN, or YIN alloy, and are oil-quenched and tempered. High-carbon spring steel wire offers superior mechanical properties, including excellent fatigue resistance and anti-relaxation performance. During manufacturing, the wire undergoes two rounds of rolling and polishing to improve surface quality, making it suitable for high-stress applications. After shot peening, soft nitriding is often applied to further enhance fatigue strength. Recent advancements like composite treatments and refined soft nitriding processes have shown promising results in improving performance and reliability.
Additionally, since engine oil may contain moisture, it’s important to coat the spring’s outer surface with oil to prevent corrosion and ensure long-term durability.