Recumbent bicycle
A recumbent bicycle is a bicycle that places the rider in a laid-back reclining position, and often called a human-powered vehicle or HPV, especially if it has an aerodynamic fairing. 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, with four wheels a quadracycle.
Speed recumbents are generally faster than upright bicycles, but they were banned by the Union Cycliste Internationale in 1934. Recumbent races and records are now overseen by the World Human Powered Vehicle Association, International Human Powered Vehicle Association and World Recumbent Racing Association.
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.
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 models have the pedals located between the front and rear wheels; short and medium-wheelbase models have the pedals in front of the front wheel; compact long-wheelbase models have the pedals either very close to the front wheel or above it. Bikes with wheelbase less than 42 inches are classified as SWB, above 42 inches are MWB. Within these categories are variations, intermediate types, and even convertible designs 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 of an upright racing cycle. The front wheel is commonly smaller than the rear, although a number of recumbents feature dual 26-inch, 650c, 700c, or even oversize all-terrain tires. Given 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 of the same type casing construction 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 rear wheel and an ISO 406 or ISO 451 front wheel. The small front wheel helps to keep the pedals and front wheel clear of each other, avoiding the problem on a medium wheelbase recumbents called "heel strike". Pedal extenders combined with a 20-inch front wheel eliminate any heel strike issue on MWB bikes.
A pivoting-boom front-wheel drive 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 ;
- Under-seat ; or
- Center steering or pivot steering.
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 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 can be an option, and it allows for a much shorter chain. One style requires the chain to twist slightly to allow for steering.Another style, pivoting-boom FWD, has the crankset connected to and moving with the front fork. In addition to the much shorter chain, the advantages to PBFWD are use of a larger front wheel for lower rolling resistance.
The main disadvantage to all FWD designs is "wheelspin" when climbing steep hills covered with loose gravel, slick pavement, etc. Another disadvantage of PBFWD for some riders is a slightly longer "learning curve" due to adaptation to the pedal-steer effect. Beginner riders tend to swerve along a serpentine path until they adapt a balanced pedal motion. After adaptation, a PBFWD recumbent can be ridden in a straight line as any other bike, and can even be steered with the feet only. However this requires pedal force applied at an angle, resulting in a power loss greater than the long chain on MWB recumbent. Cruzbike is the only PBFWD recumbent currently in production, and features a traditional steering axis similar to most standard and recumbent bikes.
Yet another drive-train variation is on rowing cycles where the rider rows using arms and legs.
Fully suspended bikes
Modern recumbent bikes are increasingly being fitted with front and rear suspension systems for increased comfort and traction on rough surfaces. Coil, elastomer, and air-sprung suspension systems have all been used on recumbent bikes, with oil or air-damping in the forks and rear shock absorbers. The maturation of fully suspended conventional mountain bikes has aided the development of these designs, which often use many of the same parts, suitably modified for recumbent use.Fairings
Some riders fit their bikes with aerodynamic devices called fairings. These can reduce aerodynamic drag and help keep the rider warmer and drier in cold and wet weather. Fairings are also available for upright bikes, but are much less common. Fully enclosed bikes and trikes are considered velomobiles.Seats
The seats themselves are either of mesh stretched tightly over a frame or foam cushions over hard shells like the Stinger pictured, which might be moulded or assembled from sheet materials. Hard-shell seats predominate in Europe, mesh seats in the USA.Variations
Mountain bike recumbents
With the right equipment and design, recumbent bikes can be used for riding unpaved roads and offroad, just as with conventional mountain bikes. Because of their longer wheelbase and the manner in which the rider is confined to the seat, recumbents are not as easy to use on tight, curving unpaved singletrack. Large-diameter wheels, mountain gearing and off-road specific design have been used since 1999. Crank-forward designs that facilitate climbing out of the saddle, such as the RANS Dynamik, also can be used off-road.Lowracers
Lowracers are a type of recumbent more common in Europe among racing enthusiasts. These typically have two wheels or a wheel at the rear and 20-inch wheel at the front. The seat is positioned between the wheels rather than above them. The extreme reclined position, and the fact that the rider is sitting in line with the wheels rather than atop them, makes this type the most aerodynamic of unfaired recumbents.Highracers
Highracers are distinguished by using two large wheels. This necessitates a higher bottom bracket than on a lowracer so that the rider's legs are above the front wheel, and this in turn requires a higher seat. The seating position may be otherwise identical to that on a lowracer allowing similar aerodynamics. "Racer" in the name implies that this will often be the case, since these bikes strive for speed.Highracers are generally more maneuverable than lowracers since their higher center of mass make them easier to balance at lower speeds. Given the same seating position they may be faster than lowracers, since it is widely believed that rolling resistance is inversely proportional to wheel diameter. However, lowracer proponents reply that their design is faster due to aerodynamics. The reasoning is that the riders body is in line with the wheels, reducing drag.
Hip and elbow injuries are more common on highracers than on lowracers due to the greater height from which the rider can fall. However, the injuries are very rare and seldom serious.