Frame

A bicycle frame is the main component of a bicycle, onto which wheels and other components are fitted. The modern and most common frame design for an upright bicycle is based on the safety bicycle, and consists of two triangles, a main triangle and a paired rear triangle. This is known as the diamond frame.

In the diamond frame, the main triangle consists of the head tube, top tube, down tube and seat tube. The rear triangle consists of the seat tube, and paired chain stays and seat stays. The head tube contains the headset, the interface with the fork. 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 shell. The rear triangle connects to the rear dropouts, where the rear wheel is attached. It 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 (often at or near the same point as the top tube) to the rear dropouts.

Frame tubes

The diamond frame consists of two triangles, a main triangle and a paired rear triangle. The main triangle consists of the head tube, top tube, down tube and seat tube. The rear triangle consists of the seat tube, and paired chain stays and seat stays.

Head tube

Template:Main The head tube contains the headset, the bearings for the fork via its steerer tube. In an integrated headset, cartridge bearings interface directly with the surface on the inside of the head tube, on non-integrated headsets the bearings (in a cartridge or not) interface with "cups" pressed into the head tube.

Top tube

The top tube connects the top of the head tube to the top of the seat tube. In a traditional-geometry racing bicycle frame, the top tube is horizontal. In a compact-geometry frame, the top tube is sloped downward toward the seat tube. In a mountain bike frame, the top tube is almost always sloped downward toward the seat tube. See Road and triathlon bicycles for more information on geometries. Step-through frames usually have a top tube that slopes down steeply to allow the rider to mount and dismount the bicycle more easily. Alternative Step-through designs include leaving out the top tube out completely, and twin top tubes that continue to the rear dropouts as with the Mixte frame.

Control cables are routed along mounts on the top tube, or sometimes inside the top tube. Most commonly, this includes the cable for the rear brake, but some mountain bikes and hybrid bicycles also route the front and rear derailleur cables along the top tube.

The space between the top tube and the rider's groin while straddling the bike and standing on the ground is called clearance. The total height from the ground to this point is called the height lever.

Down tube

The down tube connects the head tube to the bottom bracket shell. On racing bicycles and some mountain and hybrid bikes, the derailleur cables run along the down tube, or inside the down tube. On older racing bicycles, the shift levers were mounted on the down tube. On newer ones, they are mounted with the brake levers on the handlebars.

Bottle cage mounts are also on the down tube, usually on the top side, sometimes also on the bottom side. In addition to bottle cages, small air pumps may be fitted to these mounts as well.

Seat tube

The seat tube contains the seatpost of the bike, which connects to the saddle. The saddle height is adjustable by changing how far the seatpost is inserted into the seat tube. On some bikes, this is achieved using a quick release lever. The seatpost must be inserted at least a certain length; this is marked with a minimum insertion mark.

The seat tube also may have braze-on mounts for a bottle cage or front derailleur.

Chain stays

The chain stays run parallel to the chain, connecting the bottom bracket shell to the rear dropouts. When the rear derailleur cable is routed partially along the down tube, it is also routed along the chain stay. Occasionally (principally on frames made since the late 1990s) mountings for disc brakes will be attached to the chain stays. There may be a small brace that connects the chain stays in front of the rear wheel and behind the bottom bracket shell.

Chain stays can be straight or tapered tubes. Sometimes, on higher-end bikes, they are sculpted to allow clearance for the rear wheel and cranks.

Seat stays

The seat stays connect the top of the seat tube (often at or near the same point as the top tube) to the rear dropouts. A style of seat stay that extends forward of the seat tube, below the rear end of the top tube and connects to the top tube in front of the seat tube, creating a small triangle, is called Hellenic after the British frame builder Fred Hellens who introduced them in 1923.

The expressions single seat stay, mono stay, or wishbone all refer to seat stays which merge onto one section before joining the front triangle of the bicycle, thus meeting at a single point. A dual seat stay refers to seat stays which meet the front triangle of the bicycle at two separate points, usually side-by-side. A single stay can provide stiffer mounting points for cantilever brakes.

Fastback seat stays meet the seat tube in the back instead of the sides.

There may be a bridge or brace that connects the stays above the rear wheel and below the connection with the seat tube. Besides additional bracing, this provides a mounting point for rear brakes, fenders, and racks. The seat stays themselves may also provide a mounting point for rear rim or disc brakes. Usually, no rear mount is provided on a fixed gear or track frame.

When the rear derailleur cable is routed partially along the top tube, it is also usually routed along the seat stay. One combination aluminum/carbon fiber racing frame design uses carbon fiber for the seat stays and aluminum for all other tubes. This takes advantage of the better vibration absorption of carbon fiber compared to aluminum.

Bottom bracket shell

The bottom bracket shell is a short and wide tube, relative to the other tubes in the frame, that runs side to side and holds the bottom bracket. It is usually threaded, often left-hand threaded on the right (drive) side of the bike to prevent loosening by fretting induced precession, and right-hand threaded on the left (non-drive) side. It will be over-sized, unthreaded, and possibly split in the case of an eccentric bottom bracket. The chain stays, seat tube, and down tube all connect to the bottom bracket shell.

There are a few standard shell widths (68, 70 or 73 mm). Road bikes usually use 68 mm; Italian road bikes use 70 mm; Early model mountain bikes use 73 mm; later models (1995 and newer) use 68 mm more commonly. The shell width influences the Q factor or tread of the bike. There are a few standard shell diameters (34.798 - 36 mm) with associated thread pitches (24 - 28 tpi).

External links

• Science of Cycling: Frames & Materials from the Exploratorium
• Sheldon Brown's "Revisionist Theory of Bicycle Sizing" - an explanation of the different ways of measuring frame sizes.
• Brano Meres' homemade bamboo mountain bike frame
• Metallurgy for Cyclists - discusses frame material properties in relation to suitability to frame use
• The Bicycle Forest's BikeCAD program allows you to design your own frame online.

See Also

• Forks

References

• Wikipedia's Bicycle Frame