What is the Difference Between a Motorcycle Pushrod Engine and a Chain-Driven Engine?

[StMaximus]

The biggest difference between a motorcycle chain-driven engine and a pushrod engine lies in their valve systems. The pushrod engine features a lower-mounted camshaft, where the valve opening and closing are controlled by pushrods. In contrast, the chain-driven engine has an overhead camshaft, with the crankshaft driving the camshaft and rocker arms via a timing chain to operate the valves. Comparatively, chain-driven engines are quieter, produce less vibration, offer faster acceleration, and have better environmental performance than pushrod engines, making them superior. However, this design has two drawbacks: the camshaft can only transmit motion to the intake and exhaust valve mechanisms at the top of the engine through additional components (pushrods), and the camshaft in the crankcase is inconvenient to maintain. If timing adjustments are needed, the entire engine must be disassembled. To address these issues, the solution is to position the camshaft at the top of the engine, integrating it with the intake and exhaust valves to form a complete valve train. However, as the distance between the camshaft and the crankshaft increases, using simple gear meshing becomes impractical. The solution is to employ a chain or belt for transmission. The advantage of chain transmission lies in its precision compared to belt drives. The primary benefit of a chain-driven engine is its high transmission accuracy. Since the camshaft's angle and the opening and closing of the valves are defined by the crankshaft's angle, precise control of their angular relationship is essential. Chains meet this requirement perfectly and do not require frequent replacement. Although belt drives can also ensure precision, belts tend to stretch over time due to tension, necessitating regular replacement. The downside of chain drives is that they are noisier and more expensive to manufacture than belt drives, which is why manufacturers now prefer belt drives. Ultimately, it is difficult to definitively say which is better, as it depends on individual needs and preferences.