## 2 Vibration Characteristics of Idling Vehicles
### Dependence on Gear Engagement  
Chassis vibrations when idling can sometimes indicate what gear is engaged in a vehicle, though this is not always an exact science and can depend on several factors, such as the type of transmission, engine condition, and vehicle design. When a vehicle is idling, the chassis experiences vibrations that can vary depending on the gear engaged in the transmission. The vibrations arise from the engine's operation and the interaction of various components, including the transmission, drivetrain, and suspension system. These vibrations are typically subtle but can be influenced by whether the vehicle is placed in neutral, drive, reverse, or another gear. When the vehicle is in neutral, the engine is still running and idling, but it is not connected to the wheels. The drivetrain components, such as the clutch (in manual transmission) or torque converter (in automatic transmission), are not engaged, meaning the power from the engine is not being transferred to the wheels. The chassis will experience engine vibrations that are primarily due to the engine idling. These vibrations can be transmitted to the chassis through the engine mounts. Since the drivetrain is disconnected, there is no load on the drivetrain components, so the vibrations tend to be relatively uniform and may feel softer compared to when the car is in gear. The engine operates at idle speed (usually 600-1000 rpm), so the vibration frequency is relatively low and constant, without added stress from the drivetrain. When the vehicle is in drive (automatic transmission) or in first gear (manual transmission), the engine is connected to the wheels through the transmission and torque converter (in automatic vehicles). The drivetrain components are under load, which can change the way the chassis vibrates. In automatic vehicles, when the car is in drive, the torque converter connects the engine to the wheels, transmitting power from the engine to the drivetrain. This connection can cause a slight increase in vibrations, particularly if the vehicle is idling with the transmission engaged and not moving. In a manual transmission, when the vehicle is in first gear, the engine is connected to the wheels via the clutch. At idle, the clutch may still be disengaged if the pedal is pressed, so there will not be a significant increase in vibrations. However, if the clutch is partially engaged, it can cause vibrations that feel more pronounced than when in neutral due to the partial load on the engine. When the vehicle is in reverse, the engine is connected to the drivetrain via the transmission and the torque converter (in automatics) or the clutch (in manuals), and the car is in the process of reversing (though not yet moving). The transmission in reverse gear works similarly to the drive gear but may cause slightly different vibration characteristics due to the way the power is being transferred through the reverse gear set. The reverse gear can cause a slightly different vibration frequency and pattern because it works by reversing the direction of power flow. This may cause slightly higher frequency vibrations when compared to the drive gear, due to the changes in the way the gears mesh. If the car is idling in reverse, and the engine load increases, the vibrations can feel different from when the car is idling in drive or neutral. The engine mounts and suspension may transmit these vibrations to the chassis more noticeably. In automatic vehicles, when the car is in park gear, the transmission locks the drivetrain so that the wheels are not free to rotate. The engine is still idling, but the power is not being transmitted to the wheels, much like when in neutral. The vibrations at idle in park are like those in neutral, with the engine still running and generating vibrations. However, in park gear, the vehicle is stationary, and no load is transmitted through the transmission to the wheels. The vibrations will primarily come from the engine, which can be transmitted through the engine mounts to the chassis.