There are a wide variety of types of<unk> s, differing by propulsion method, mechanisms of control, and technology used. Some<unk> s are designed for general everyday use, others for single activities, or to address specific access needs. Innovation within the<unk>  industry is relatively common, but many innovations ultimately fall by the wayside, either from over-specialization, or from failing to come to market at an accessible price-point. The iBOT is perhaps the best known example of this in recent years. Manual self-propelled<unk> s Folding chair and stackable rigid chairs for visitors in NÄL hospital, Sweden<unk> s allow access for tourists with disabilities, like this child in Egypt. A self-propelled manual<unk>  incorporates a frame, seat, one or two footplates (footrests) and four wheels: usually two caster wheels at the front and two large wheels at the back. There will generally also be a separate seat cushion. The larger rear wheels usually have push-rims of slightly smaller diameter projecting just beyond the tyre; these allow the user to manoeuvre the chair by pushing on them without requiring them to grasp the tyres. Manual<unk> s generally have brakes that bear on the tyres of the rear wheels, however these are solely a parking brakes and in-motion braking is provided by the user's palms bearing directly on the push-rims. As this causes friction and heat build-up, particularly on long downslopes, many<unk>  users will choose to wear padded<unk>  gloves. Manual<unk> s often have two push handles at the upper rear of the frame to allow for manual propulsion by a second person, however many active<unk>  users will remove these to prevent unwanted pushing from people who believe they are being helpful. Everyday manual<unk> s come in two major varieties, folding or rigid. Folding chairs are generally low-end designs, whose predominant advantage is being able to fold, generally by bringing the two sides together. This is an advantage for people who need to store the<unk>  frequently or to put it in a small vehicle. Rigid<unk> s have permanently welded joints and many fewer moving parts. This reduces the energy required to push the chair by eliminating many points where the chair would flex and absorb energy as it moves. Welded rather than folding joints also reduce the overall weight of the chair. Rigid chairs typically feature instant-release rear wheels and backrests that fold down flat, allowing the user to dismantle the chair quickly for storage in a car. A few<unk> s attempt to combine the features of both designs by providing a fold-to-rigid mechanism in which the joints are mechanically locked when the<unk>  is in use. Many rigid models are made with ultralight materials such as aircraft-grade aluminium and titanium, and<unk> s of composite materials such as carbon-fibre have started to appear. Ultra lightweight rigid<unk> s are commonly known as 'active user chairs' as they are ideally suited to independent use. Another innovation in rigid chair design is the installation of shock absorbers, such as "Frog Legs", which cushion the bumps over which the chair rolls. These shock absorbers may be added to the front wheels, to the rear wheels, or both. Rigid chairs also have the option for their rear wheels to have a camber, or tilt, which angles the tops of the wheels in toward the chair. This allows for more mechanically efficient propulsion by the user and also makes it easier to hold a straight line while moving across a slope. Sport<unk> s often have large camber angles to improve stability. Rigid-framed chairs are generally made to measure, to suit both the specific size of the user and their needs and preferences around areas such as the "tippyness" of the chair - determined by the distance between the center of gravity and the rear axle. Experienced users with sufficient upper-body strength can generally balance the chair on its rear wheels, a "wheelie", and the "tippyness" of the chair controls the ease with which this can be initiated. The wheelie allows an independent<unk>  user to climb and descend curbs and move more easily over small obstacles and irregular ground such as cobbles. The rear wheels of self-propelled<unk> s typically range from in diameter, and commonly resemble bicycle wheels. Wheels are rubber-tired and may be solid, pneumatic or gel-filled. The wheels of folding chairs may be permanently attached, but those for rigid chairs are commonly fitted with quick-release axles activated by depressing a button at the centre of the wheel. All major varieties of<unk> s can be highly customized for the user's needs. Such customization may encompass the seat dimensions, height, seat angle, footplates, leg rests, front caster outriggers, adjustable backrests and controls. Various optional accessories are available, such as anti-tip bars or wheels, safety belts, adjustable backrests, tilt and/or recline features, extra support for limbs or head and neck, holders for crutches, walkers or oxygen tanks, drink holders, and mud and wheel-guards as clothing protectors. Light weight and high costs are related to the manual<unk>  market. At the low-cost end, heavy, folding steel chairs with sling seats and little adaptability dominate. Users may be temporarily disabled, or using such a chair as a loaner, or simply unable to afford better. These chairs are common as "loaners" at large facilities such as airports, amusement parks and shopping centers. A slightly higher price band sees the same folding design produced in aluminium. Price typically then jumps from low to mid hundreds of pounds/dollars/euros to a four figure price range, with individually custom manufactured lightweight chairs with more options. The high end of the market contains ultra-light models, extensive seating options and accessories, all-terrain features, and so forth. The most expensive manual chairs may rival the cost of a small car. Manual attendant-propelled<unk> s Boardwalk<unk> s, early 20th century An attendant-propelled<unk>  is generally similar to a self-propelled manual<unk> , but with small diameter wheels at both front and rear. The chair is maneuvered and controlled by a person standing at the rear and pushing on handles incorporated into the frame. Braking is supplied directly by the attendant who will usually also be provided with a foot- or hand-operated parking brake. These chairs are common in institutional settings and as loaner-chairs in large public venues. They are usually constructed from steel as light weight is less of a concern when the user is not required to self-propel. Specially designed transfer chairs are now required features at airports in much of the developed world in order to allow access down narrow airliner aisles and facilitate the transfer of<unk> -using passengers to and from their seats on the aircraft. Powered<unk> s An electric-powered<unk> , commonly called a "powerchair" is a<unk>  that additionally incorporates batteries and electric motors into the frame and that is controlled by either the user or an attendant, most commonly via a small joystick mounted on the armrest, or on the upper rear of the frame. Alternatives exist for the traditional manual joystick, including head switches, chin-operated joysticks, sip-and-puff controllers or other specialist controls, which may allow independent operation of the<unk>  for a wider population of users with varying motor impairments. Ranges of over 10 miles/15 km are commonly available from standard batteries. Powerchairs are commonly divided by their access capabilities. An indoor-chair may only reliably be able to cross completely flat surfaces, limiting them to household use. An indoor-outdoor chair is less limited, but may have restricted range or ability to deal with slopes or uneven surfaces. An outdoor chair is more capable, but will still have a very restricted ability to deal with rough terrain. A very few specialist designs offer a true cross-country capability. Powerchairs have access to the full range of<unk>  options, including ones that are difficult to provide in an unpowered manual chair, but have the disadvantage of significant extra weight. Where an ultra-lightweight manual chair may weigh under 10 kg, the largest outdoor power-chairs may weigh 200 kg or more. Smaller power chairs often have four wheels, with front or rear wheel drive, but large outdoor designs commonly have six wheels, with small wheels at front and rear and somewhat larger powered wheels in the centre. A power-assisted<unk>  is a recent development that uses the frame and seating of a typical rigid manual chair while replacing the standard rear wheels with wheels of similar size which incorporate batteries and battery-powered motors in the hubs. A floating rim design senses the pressure applied by the user's push and activates the motors proportionately to provide a power assist. This results in the convenience, and small size of a manual chair while providing motorised assistance for rough/uneven terrain and steep slopes that would otherwise be difficult or impossible to navigate, especially by those with limited upper-body function. As the wheels necessarily come at a weight penalty it is often possible to exchange them with standard wheels to match the capabilities of the<unk>  to the current activity. Mobility scooters Mobility scooters share some features with powerchairs, but primarily address a different
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