A<unk>  has a seat over three, four or more wheels, sometimes a flat area or foot plates for the feet, and handlebars or a delta-style steering arrangement in front to turn the steerable wheels. The seat may swivel to allow access when the front is blocked by the handlebars.<unk> s are usually battery powered. A battery or two is stored on board the<unk>  and is charged via an onboard or separate battery charger unit from standard electric power. Gasoline-powered<unk> s may also be available in some countries, though they are rapidly being replaced by electric models. User-powered<unk> s are propelled by a lever used in a push-pull rowing motion to provide exercise and<unk>  at the same time. The tiller, with forward/reverse directions and speed controls, is the steering column central at the front of the<unk> . The tiller may contain other features also, for example, a speed limiter, lighting controls (for nighttime use) and turn signals. A battery use indicator is also often included. Forward/reverse direction can be controlled by thumb paddles, finger controls, or a switch. There are two types of<unk> s: front-wheel drive (FD) or rear-wheel drive (RD). The front-wheel drive is usually a smaller device and is best used indoors. Rider weight capacity is a minimum of generally upwards to maximum. The rear-wheel drive is used both indoors and outdoors with rider weight capacity of . A heavy duty rear-drive can carry up to , varying by manufacturer.<unk> s come in various types: * user-powered, small, light manual<unk> s for travel, without battery or motor, propelled by a central lever drive (CLD) in a push-pull rowing motion to provide<unk>  and exercise at the same time. Some models look like large tricycles without pedals that the estimated 60% of wheelchair users who have at least some use of their legs can self-propel while stimulating muscles and exercising. * small, light<unk> s for travel, which fold or are easily disassembled into smaller parts for transport; * large, heavy<unk> s for rough outdoor terrain; * mid-range<unk> s, which are intended for both indoor and outdoor use. * slow and steady, used for shopping in stores and other places. Usually mid-range<unk> s have a speed of about .<unk> s typically have a seat that is between 16" to 20" but many of them have various seat options available during purchase.
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