Low-floor bus

A low-floor bus is a bus or trolleybus that has no steps between the ground and the floor of the bus at one or more entrances, and low floor for part or all of the passenger cabin. A bus with a partial low floor may also be referred to as a low-entry bus or seldom a flat-floor bus in some locations.

Low floor refers to a bus deck that is accessible from the sidewalk with only a single step with a small height difference, caused solely by the difference between the bus deck and sidewalk. This is distinct from high-floor, a bus deck design that requires climbing one or more steps (now known as step entrance) to access the interior floor that is placed at a higher height. Being low-floor improves the accessibility of the bus for the public, particularly the elderly and people with disabilities, including those using wheelchairs and walkers. Almost all are rear-engine, rear-wheel-drive layout.

Low-floor buses are generally divided into two major types: fully low-floor buses with a low floor throughout the length of the bus (more popular in Europe^{[citation needed]}), and low-entry buses with step-free access to only a part of the bus, most commonly between the front door and the middle door (more popular in North America). In North America, both types are commonly called low-floor, as the majority of the vehicle has a low floor, without steps at the doors.

The main reason for choosing a low-entry configuration is to allow better placement for the powertrain and other technical equipment in the raised floor section, in addition to allowing a more comfortable ride on rough roads. Some manufacturers use the initials LF or L in their model designations for fully low-floor models (or in the case of German manufacturers, NF or N, based on the German word Niederflur, which means low floor), and in North America buses that are partially low-floor are often also designated LF. In some countries, LE, short for Low Entry, is used by some manufacturers in their model designations for low-entry buses.

Most bus manufacturers achieve a low floor height by making rear-engined rear-wheel drive buses with independent front suspension, so that no axle is needed to pass under the floor of the front part of passenger compartment, or a lowered front axle. Some full low-floor buses also have a lowered rear axle, while the rear axle is not an issue on a low-entry bus.

Many low-floor buses, including the Irisbus Citelis (also in Skoda 24Tr trolleybus version), have the engine in a vertical cabinet at the rear of the bus. Van Hool have a series of "side-engine mid-drive" buses that puts the engine off to one side of the cabin longitudinally between the first and the second axle, to maximize usable cabin space. The same concept was also utilized by Volvo on their B9S articulated chassis.

For smaller buses, such as midibuses, the low-floor capability is achieved by placing the front wheels ahead of the entrance. One of the last types of buses to gain low-floor accessibility as standard was the minibus, where a similar front-wheel arrangement allows around 12 seats and a wheelchair space to be accommodated in very small low-floor minibuses, such as the Optare Alero and Hino Poncho. Accessibility was previously achieved in paratransit type applications, which use small vehicles with the fitment of special lifts. The inception of small low-floor buses has allowed the development of several accessible demand-responsive transport schemes using standard 'off-the-shelf' buses.

A disadvantage of the low floor is accommodating the bus's own wheels. With the low floor, the wheels protrude into the passenger cabin, and need to be contained in wheel pockets of waist height, and this occupies space which would otherwise be used for seating. To allow space for technical equipment, many low-floor buses have the seats mounted on podiums, making a small step up from the floor, while others are able to mount the seats directly to the floor, avoiding the step. Seating layout for a low-floor bus therefore requires careful design. Low floor configuration is also known to have poor side to side dead load distribution within the chassis due to the asymmetrical off-centre placement of driveline components - mainly engine and transmission. As a result, many of such buses require electronically controlled air suspension to compensate the lopsided configuration.