Which factor can contribute to excessive vibration in a motorcycle?

Excessive vibration in a motorcycle can often be attributed to unbalanced wheels. When wheels are unbalanced, it means that the weight distribution around the wheel is not even. This can lead to the wheels wobbling or oscillating as they rotate, which results in increased vibration felt throughout the motorcycle. Proper wheel balancing is crucial for smooth operation, as it ensures that all components of the wheel and tire assembly work together harmoniously.

In contrast, while low tire pressure can affect handling and ride quality, it typically does not cause excessive vibration in the same way unbalanced wheels do. An overfilled fuel tank might lead to weight distribution issues but does not directly cause vibration. Similarly, worn brake pads can lead to noise when braking or affect stopping power but are not usually a primary cause of excessive vibration during normal operation. Thus, among the given options, unbalanced wheels are the most significant factor contributing to excessive vibration in a motorcycle.