A recumbent bicycle is a bicycle that places the rider in a laid-back reclining position. Some recumbent riders may choose this type of design for ergonomic reasons: the rider's weight is distributed comfortably over a larger area, supported by back and buttocks. On a traditional upright bicycle, the body weight rests entirely on a small portion of the sitting bones, the feet, and the hands.
Others may choose a recumbent because some models also have an aerodynamic advantage; the reclined, legs-forward position of the rider's body presents a smaller frontal profile. Recumbents are much faster than upright bicycles, but they were banned by the Union Cycliste Internationale (UCI) in 1934. Recumbent races and records are now overseen by the World Human Powered Vehicle Association (WHPVA) and International Human Powered Vehicle Association (IHPVA).
Recumbents are available in a wide range of configurations, including: long to short wheelbase; large, small, or a mix of wheel sizes; overseat, underseat, or no-hands steering; and rear wheel or front wheel drive. A variant with three wheels is a recumbent tricycle.
Description
Recumbents can be categorized by their wheelbase, wheel sizes, steering system, faired or unfaired, and front-wheel or rear-wheel drive.
Wheelbase
Long-wheelbase (LWB) models have the pedals located between the front and rear wheels; short-wheelbase (SWB) models have the pedals in front of the front wheel; compact long-wheelbase (CLWB) models have the pedals either very close to the front wheel or above it. Within these categories are variations, intermediate types, and even convertible designs (LWB to CLWB) – there is no "standard" recumbent.
Wheel sizes
The rear wheel of a recumbent is usually behind the rider and may be any size, from around to the 700c (or 27″ on some older models, as on upright road bikes of that time) of an upright racing cycle. The front wheel is commonly smaller than the rear, although a number of recumbents feature dual 26-inch (ISO 559), 650c (ISO 571), 700c (ISO 622), or even 29 × 4″ oversize all-terrain tires. Given the higher rolling resistance of the smaller front wheel, loss of steering and control are somewhat more likely attempting sharp or quick changes of direction while crossing over patches of loose dirt, sand or pebbles. Larger diameter wheels generally have lower rolling resistance but a higher profile leading to higher air resistance. High-racer aficionados also claim that they are more stable, and although it is easier to balance a bicycle with a higher center of mass, the wide variety of recumbent designs makes such generalizations unreliable. Another advantage of both wheels being the same size is that the bike requires only one size of inner tube.
One common arrangement is an ISO 559 (26-inch) rear wheel and an ISO 406 or ISO 451 (20-inch) front wheel. The small front wheel and large rear wheel combination is used to keep the pedals and front wheel clear of each other, avoiding the problem on a short wheelbase recumbent called "heel strike" (where the rider's heels catch the wheel in tight turns). A pivoting-boom front-wheel drive (PBFWD aka moving bottom bracket recumbent) configuration also overcomes heel strike since the pedals and front wheel turn together. PBFWD bikes may have dual wheels or larger.
Steering
Steering for recumbent bikes can be generally categorized as
over-seat (OSS) or above seat steering (ASS); under-seat (USS); or center steering or pivot steering. OSS/ASS is generally direct—the steerer acts on the front fork like a standard bicycle handlebar—but the bars themselves may extend well behind the front wheel (more like a tiller); alternatively the bars might have long rearward extensions (sometimes known as Superman or Kingcycle bars). Chopper-style bars are sometimes seen on LWB bikes.
USS is usually indirect—the bars link to the headset through a system of rods or cables and possibly a bell crank. Most tadpole trikes are USS.
Center steered or pivot steered recumbents, such as Flevobikes and Pythons, may have no handlebars at all.
In addition, some trikes such as the Sidewinder have used rear-wheel steer, instead of the more common front-wheel steer. They can provide good maneuverability at low speeds, but have been reported to be potentially unstable at speeds above .
Drive
Most recumbents have the cranks attached to a boom fixed to the frame, with a long drive chain for rear wheel drive. However, due to the proximity of the crank to the front wheel, front wheel drive (FWD) can be an option, and it allows for a much shorter chain. One style requires the chain to twist slightly to allow for steering.