Standard single-rider bicycles have two common frame designs: the diamond and the step-through. The more common diamond frame is named for the diamond shape created by its front and rear triangles. The front triangle consists of the top tube between the headset (where the front fork and handlebars attach to the frame) and the saddle, the down tube between the headset and the bottom bracket (where the pedals attach to the frame), and the seat tube between the saddle and the bottom bracket. Two divergent rear triangles, one on each side of the rear wheel, share as one side the seat tube. Their other two sides are the seatstays (between the saddle and the left and right sides of the rear hub) and the chainstays (between the bottom bracket and the left and right sides of the rear hub).
Bicycle frames come in a variety of sizes, which are based on the length of the seat tube. Standard bicycles for adults normally range from 31 to 65 cm (12 to 26 in); smaller or larger sizes can also be custom-built. The frame of a road bicycle is considered to fit when the rider, with both feet placed flat on the ground, can stand astride the top tube with about 3 cm (1 in) of clearance to the crotch. A mountain bicycle frame is usually fit to have 8 cm (3 in) or more of clearance.
Since the 1970s most women, as well as men, have ridden bicycles with a diamond frame, which used to be called the menís frame. The old-style V-shaped womenís frame, which had no top tube, is no longer manufactured because of the structural weakness inherent in the absence of one arm of the front triangle. Some women and girls prefer a step-through frame over a diamond frame. In the step-through frame, the top tube may be lower and slant downward to the rear or it may be replaced by two thinner tubes that are parallel and slant downward from the handlebars all the way to the rear hub (a design known as the mixte). Both designs preserve much of the strength and rigidity of the traditional diamond frame.
The most desirable bicycle frames are both very strong and very lightweight. Frame tubes on the cheapest bicycles are made of carbon steel, which is relatively heavy. On moderately priced bicycles, the tubes are thin-walled and made of lighter, high-tensile alloys such as chromoly steel and molybdenum-manganese steel. On somewhat more expensive bicycles, tubes may be made of even lighter aluminum alloys. Aluminum is not as strong as steel, however, so aluminum-alloy tubing must have a larger diameter and sometimes an oval cross section to offer the same strength that the steel alloys provide. Some of the most expensive super-lightweight bicycle frames, usually for racing, are die-cast in one piece (known as a monocoque frame) out of carbon-fiber-epoxy composites or magnesium alloy. The lightest racing bikes of all are made of titanium-steel or titanium-aluminum alloy tubing, which may weigh half of what stainless and other alloy steels weigh.