Racing bicycles are built primarily for minimum weight and maximum speed on a paved road or track. Their frames may be made of ultralight carbon-fiber-epoxy composite or of titanium-aluminum alloy. They have narrow, dropped handlebars for maintaining an aerodynamic crouch and maximizing the number of hand positions; in some events the handlebars may include an aerobar on which the rider may rest the forearms. The saddle is narrow to minimize air resistance and interference with pedaling.
Racing bicycles have a short wheelbase (distance between hubs) and often 14 to 16 tightly spaced high gears to emphasize power and subtle changes in speed for handling corners and hills. They have narrow (under-1-in, or 25 mm), highly pressurized (120 to 170 pounds per square inch, or psi; 85 to 120 grams per sq meter) tires to minimize rolling friction and maximize maneuverability; their diameter is generally the so-called 700 C size (which nominally stands for 700 mm (30 in) but in reality is 666 to 686 cm (26 to 25 in), tire sizes being only partially standardized). Wheels may have light-gauge spokes and only 32 or 28 spokes instead of the standard 36; some racing bicycles are even more stripped down, not even having brakes; they are stopped by backpedaling a single fixed gear and by using the hand to apply pressure on the front tire.
Touring bicycles are built primarily for strength, durability, and stability under load, although minimizing weight is also a consideration. Commonly they have a strong, lightweight frame of a high-tensile steel alloy such as chrome-molybdenum or molybdenum-manganese steel, or even light aluminum alloys. The frame geometry and saddle are designed for comfort on long trips. Handlebars may be dropped or upright, depending on the rider's preference. The wheelbase is longer than that for a racing bicycle, and wheels may have heavy-gauge spokes to withstand rough roads; for months-long tours, riders may even install heavy-duty 40-spokes wheels. Tires for road touring are of medium width (1.1 to 1.5 inches, or 28 to 38 mm) and medium pressure (65 to 110 psi, or 45 to 75 grams per sq meter), so as to offer stability on gravel or in rain while still minimizing rolling resistance; tires for mountain-bike touring are wider for maximum traction. Tires may be either 700 C or 26 inches (650 mm) in diameter. Touring bicycles have 18 to 27 speeds with an emphasis on low gears to offer greatest leverage in climbing hills while carrying load; brakes may have wide pads to stop effectively even under load. Touring bicycles are commonly equipped with front and rear racks for carrying panniers (saddlebags) and may also be fitted with a headlight and taillight.
Mountain bicycles, or all-terrain bicycles (ATBs), are built primarily for riding off paved roads, either on "single-track" trails or across open country. Although initially their frames were designed with a touring geometry, by the late 1980s mountain bikes began to sport smaller, responsive bicycle-motocross- (BMX-) style frames, because the rider balances and controls the bike principally by throwing the hips to the rear or side to alter the center of gravity. They usually have upright, flat, relatively short handlebars. Like touring bicycles, ATBs emphasize low gears for climbing hills and surmounting rocks and logs. Tires are wide (often wider than 2 in, or 50 mm) and low pressure (35 to 60 psi, or 25 to 40 grams per sq meter) with deep, knobby tread to increase their grip on obstacles and uneven terrain, and all components are strong enough to withstand jumps and rugged handling; most mountain-bike tires are 26 inches in diameter. By the mid-1990s many ATBs were also fitted with front or rear, or both, and seat post suspension systems (hydraulic shock absorbers) to reduce the impact of cross-country stump-jumping and rock hopping on both the bicycle and its rider.
Hybrid, or cross, bicycles, as their name suggests, are a hybrid, or cross, between a touring bicycle and a mountain bike and are today the closest thing to an all-purpose bicycle for all-around riding. They have some of the features of mountain bikes (such as suspension systems), but their frames are slightly larger and have closer to touring geometry for comfort, and their tires are of medium width and pressure to minimize rolling friction. They may have either drop or upright (flat) handlebars and medium-range gearing, and they may include dropouts for racks. Sometimes hybrid bicycles are marketed as "commuting" or "city" bicycles.
Comfort or utility bicycles are commonly single-speed, three-speed, or five-speed machines, usually with upright handlebars and medium-width tires. They may also have a chain guard or coaster brakes and a wide saddle with coiled springs. Because in the late 1990s and early 2000s, there was somewhat of a revival in "retro" bicycles, with so-called comfort bikes frequently modeled on the one-speed cruisers popular in the 1950s. They may be relatively inexpensive and are thus commonly found as rental bicycles at resorts near beaches. Genuine heavier-duty utility bicycles (with an extended storage area over the back tire for carrying items) or tricycles (bikes with two wheels in back and one in front) are also found in urban areas (where they may be used to deliver anything from groceries to newspapers) and as transportation in developing countries.
Specialty bicycles encompass a wide variety of other two-wheeled vehicles that individually have relatively small niches but collectively make up a significant fraction of bicycles. Collapsible, or foldable, bicycles are useful for commuters or for frequent business travelers whose bicycles must meet size regulations on public transportation and airlines. Tandems are traditional "bicycles built for two," where the brakes and handlebars are operated by a captain (front rider) and pedals are operated by both captain and "stoker" (rear rider). Tandems are seen in racing; both riders pedaling in cadence can achieve faster times than can single riders. Tandems are also useful for touring or recreational riding for cyclists of unequal abilities, such as a parent with a child, or a captain with a stoker who has a disability.
Recumbents are bicycles on which the rider sits upright as if in a chair or even leans backward as if in a recliner, with the feet turning pedals in front and the hand steering handlebars that may be either above or below the saddle; recumbents may be singles or tandems. The popularity of recumbents, especially for touring, began increasing in the 1980s. "Adaptive" bicycles for riders having physical disabilities are of many different types, with the most usual being ones for those with paraplegia that can be pedaled by the hands instead of the feet.
Agile, single-speed BMX bicycles, modeled after motocross "dirt-bike" motorcycles, are most popular with children and teens. Their wheels are small -generally 16 or 20 inches (400 or 500 mm)- and the frame is designed to be small compared with the rider. Originally intended to be raced on short dirt courses, since the 1980s they have become the design basis for virtually all children's bicycles. Similar are "freestyle" stunt bicycles, which have particularly strong BMX-style frames and handlebars to withstand the impact of jumps; foot pegs extending from the front and rear axles are an additional feature. Stunt bikes are used for bicycle acrobatics that may include airborne somersaults. First reaching its heyday in the 1980s, freestyling is still a regular feature at the Gravity Games in Cleveland and other special meets. Both BMX and freestyle bicycles are, however, impractical for riding significant distances.
