How does a bus kneel to make boarding easier?

A bus can arrive at a stop, hiss, and lower its entrance toward the curb. Its tires have not suddenly become smaller. In a bus with air suspension, controlled changes in air springs alter the distance between the body and its wheel assemblies.

An air spring is a strong flexible chamber containing pressurized air. Pressure acting across its effective area helps support the bus body. Valves connect the chamber to a supply of compressed air or to an exhaust route. Releasing some air allows the supported body to settle lower; adding air can raise it again.

For kneeling, the control system lowers selected parts of the suspension. Some buses lower mainly at the front; others lower along the boarding side. Height sensors help the controller compare the measured position with the requested one, then operate valves until the appropriate height is reached.

A ramp can bridge the remaining difference between the bus floor and the boarding surface. Lowering the entrance can reduce the ramp’s slope, helping wheelchair users and other passengers board. The exact result still depends on the curb, the bus design and where the vehicle stops.

Before ordinary travel, the suspension returns to its driving height. Air springs also help absorb road movement, while dampers control unwanted bouncing. Kneeling uses this adjustable support for a second purpose: changing the doorway’s relationship to the street. The hiss marks air leaving a system that is deliberately reshaping access.