Most motorcycles use timing chains rather than pushrods for a few reasons:
Overhead Cam (OHC) Design: Many modern motorcycles use an overhead camshaft design, where the camshaft is located above the cylinder head. This configuration allows for better valve control, higher RPM capability, and improved engine performance. Timing chains are commonly used in OHC designs to drive the camshaft(s) because they offer precise and reliable timing synchronization between the crankshaft and the camshaft.
Compactness and Weight: Motorcycles, especially sportbikes, are designed to be lightweight and compact. Timing chains are more compact than pushrods and require less space within the engine, allowing for a more efficient packaging of components. Pushrod systems, on the other hand, tend to be bulkier and can take up more space, making them less suitable for motorcycles with tight engine compartments.
Higher RPM Capability: Motorcycles, particularly sportbikes, are often designed for higher engine speeds and higher RPM ranges. Timing chains are better suited to handle the increased RPMs due to their ability to maintain accurate timing and withstand the higher rotational speeds. Pushrods, being a mechanical linkage, may not be as reliable or efficient at higher RPMs and can introduce limitations to the engine's performance potential.
Reduced Friction and Maintenance: Timing chains generally have lower frictional losses compared to pushrods. This can contribute to improved engine efficiency and power delivery. Additionally, timing chains require less frequent maintenance compared to pushrods, which may need adjustments or replacements over time.
While pushrod systems can still be found in certain motorcycles, they are more commonly associated with older or specific types of engines, such as V-twin cruisers. Overall, timing chains offer advantages in terms of performance, compactness, RPM capability, and maintenance requirements, making them the preferred choice for many modern motorcycle designs.