The great majority of todays bicycles have a frame with upright seating which looks much like the first chain-driven bike. Such upright bicycles almost always feature the diamond frame a truss consisting of two triangles the front triangle and the rear triangle. The front triangle consists of the head tube top tube down tube and seat tube. The head tube contains the headset the set of bearings that allows the fork to turn smoothly for steering and balance. The top tube connects the head tube to the seat tube at the top and the down tube connects the head tube to the bottom bracket. The rear triangle consists of the seat tube and paired chain stays and seat stays. The chain stays run parallel to the chain connecting the bottom bracket to the rear dropouts. The seat stays connect the top of the seat tube at or near the same point as the top tube to the rear dropouts.
Historically womens bicycle frames had a top tube that connected in the middle of the seat tube instead of the top resulting in a lower standover height at the expense of compromised structural integrity since this places a strong bending load in the seat tube and bicycle frame members are typically weak in bending. This design referred to as a step-through frame allows the rider to mount and dismount in a dignified way while wearing a skirt or dress. While some womens bicycles continue to use this frame style there is also a variation the mixte which splits the top tube into two small top tubes that bypass the seat tube and connect to the rear dropouts. The ease of stepping through is also appreciated by those with limited flexibility or other joint problems. Because of its persistent image as a womens bicycle step-through frames are not common for larger builds.
A more recent development is the recumbent bicycle. These are inherently more aerodynamic than upright versions as the rider may lean back onto a support and operate pedals that are on about the same level as the seat. The worlds fastest bicycle is a recumbent bicycle but this type was banned from competition inby the Union Cycliste Internationale.
Historically materials used in bicycles have followed a similar pattern as in aircraft the goal being high strength and low weight. Since the late 0s alloy steels have been used for frame and fork tubes in higher quality machines. Celluloid found application in mudguards and aluminum alloys are increasingly used in components such as handlebars seat post and brake levers. In the 0s aluminum alloy frames became popular and their affordability now makes them common. More expensive carbon fiber and titanium frames are now also available as well as advanced steel alloys and even bamboo.
Since cyclists legs are most efficient over a narrow range of cadences a variable gear ratio is helpful to maintain an optimum pedalling speed while covering varied terrain.
