Why Do Buses Vibrate More When Stopped?

Buses are known for their occasional vibrations, but have you ever wondered why your ride feels more shaky when the bus is idling at the stop? This phenomenon can be explained by a combination of factors related to the bus's engine, suspension system, and overall structure. In this article, we will explore the reasons behind the increased vibrations when a bus is stopped.

Engine and Systems Running

Even when a bus is stopped, the engine and several auxiliary systems such as the air conditioning or heating are operational. These systems continue to run and can generate vibrations that travel throughout the vehicle. For example, the engine's movement and the operation of the alternator can create minor vibrations that are more noticeable when the bus is stationary.

Suspension System

The complexity of a bus's suspension system is its primary role when the vehicle is in motion. However, when the bus is stopped, the suspension may not be fully engaged, resulting in an amplification of vibrations from the engine or other components. The suspension is designed to isolate the vehicle from road bumps and vibrations, but it's less effective when the bus is stationary, leading to a more noticeable shake.

Weight Distribution

Buses are heavy vehicles, and when they are stationary, the weight distribution can cause certain parts to settle or shift slightly. This settling or shifting can lead to vibrations being more prominent and noticeable. When the bus is stationary, the weight distribution can create a different balance, making the vibrations more apparent.

Surface Conditions

The surface on which a bus is parked can also play a role in the increased vibrations. If the bus is parked on an uneven or rough surface, any minor movements or vibrations from the engine can be amplified. Even small imperfections in the road or uneven parking lots can compound these minor vibrations, making them more perceptible to passengers.

Resonance Phenomenon

A fascinating concept in physics known as resonance can further explain why vibrations are more pronounced when a bus is stopped. Resonance is the tendency of a system to vibrate at greater amplitude at certain frequencies of its forcing function. In the case of a bus, the combination of the bus frame, engine block, and other components forms a driven oscillator. When the bus is idling, the frequency of the engine vibrations is closer to the natural resonance of the bus structure, leading to enhanced vibrations.

At higher speeds, the frequency of vibrations from the pistons and firing strokes is much higher, and these frequencies are not aligned with the natural resonance of the bus. This is why passengers often feel less vibration at high speeds. However, as the engine slows down to idle, the frequency of engine vibrations comes closer to the natural resonance of the bus structure, causing the vibrations to become more noticeable.

Another contributing factor is the flywheel and power stroke. At high engine RPM, the flywheel maintains a nearly constant speed, but at lower RPM, the speed changes significantly during the compression and expansion phases. The energy stored in the flywheel is proportional to the square of the angular velocity (iw^2), while the momentum is proportional to the angular velocity (iw). As the engine slows down, the amount of momentum transferred to the bus increases, leading to more noticeable vibrations at lower RPM and a maximum at the stall.

In conclusion, the increased vibrations when a bus is stopped are a result of several interconnected factors related to the engine, suspension system, weight distribution, surface conditions, and resonance. Understanding these factors can help explain the sometimes unsettling ride of a stationary bus.

Keywords: bus vibration, idling engine, suspension system