Bicycle frame-set (frame and fork) schematic
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
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 or cross-bar connects the top of the head tube to the top of the seat tube. In a traditional-geometry diamond frame, the top tube is horizontal (parallel to the ground). In a compact-geometry frame, the top tube is normally sloped downward toward the seat tube for additional standover clearance. In a mountain bike frame, the top tube is almost always sloped downward toward the seat tube. Radically sloped top tubes that compromise the integrity of the traditional diamond frame may require additional gusseting tubes, alternative frame construction, or different materials for equivalent strength.
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 may include leaving out the top tube out completely, as in monocoque mainframe designs using a separated or hinged seat tube, and twin top tubes that continue to the rear fork ends as with the Mixte frame. These alternatives to the diamond frame provide greater versatility, though at the expense of added weight to achieve equivalent strength and rigidity.
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 fork ends. 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 may be designed using tapered or untapered tubing. They may be relieved, ovalized, crimped, S-shaped, or elevated to allow additional clearance for the rear wheel, chain, crankarms, or the heel of the foot.
Seat stays
Example of a dual-stay seat stay system
The seat stays connect the top of the seat tube (often at or near the same point as the top tube) to the rear fork droputs. A traditional frame uses a simple set of paralleled tubes connected by a bridge above the rear wheel.
Many alternatives to the traditional seat stay design have been introduced over the years. 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 a Hellenic stay after the British frame builder Fred Hellens, who introduced them in 1923. Hellenic chainstays add aesthetic appeal at the expense of added weight.
Another common seatstay variant is the wishbone, single seat stay, or mono stay, which joins the stays together just above the rear wheel into a monotube that is joined to the seat tube. A wishbone design adds vertical rigidity without increasing lateral stiffness, generally an undesirable trait for bicycles with unsuspended rear wheels. The wishbone design is most appropriate when used as part of a rear triangle subframe on a bicycle with independent rear suspension.
A dual seat stay refers to seat stays which meet the front triangle of the bicycle at two separate points, usually side-by-side. Fastback seat stays meet the seat tube at the back instead of the sides of the tube.
On most seat stays, a bridge or brace is typically used to connect the stays above the rear wheel and below the connection with the seat tube. Besides providing lateral rigidity, this bridge provides a mounting point for rear brakes, fenders, and racks. The seat stays themselves may also be fitted with brake mounts. Brake mounts are often absent from fixed-gear or track bike seat stays.
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. There are many variations, such as an eccentric bottom bracket, which allows for adjustment in tension of the bicycle's chain. It is typically larger, unthreaded, and sometimes split. The chain stays, seat tube, and down tube all typically connect to the bottom bracket shell.
There are a few traditional 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. Some modern bicycles have shell widths of 83 or 100 mm and these are for specialised downhill mountain biking or snowbiking applications. 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).
Frame geometry
The length of the tubes, and the angles at which they are attached define a frame geometry. In comparing different frame geometries, designers often compare the seat tube angle, head tube angle, (virtual) top tube length, and seat tube length. To complete the specification of a bicycle for use, the rider adjusts the relative positions of the saddle, pedals and handlebars:
- Saddle height, the distance from the center of the bottom bracket to the point of reference on top of the middle of the saddle.
- Reach, the distance from the saddle to the handlebar.
- Drop, the vertical distance between the reference at the top of the saddle to the handlebar.
- Setback, the horizontal distance between the front of the saddle and the center of the bottom bracket.
- Standover height, the height of the top tube above the ground.
- Toe overlap, the amount that the feet can interfere with steering the front wheel.
The geometry of the frame depends on the intended use. For instance, a road bicycle will place the handlebars in a lower and further position relative to the saddle giving a more crouched riding position; whereas a utility bicycle emphasizes comfort and has higher handlebars resulting in an upright riding position.
Frame geometry also affects handling characteristics. For more information, see the Bicycle and motorcycle geometry and the Bicycle and motorcycle dynamics articles.
Commonly used measurements
Frame size
Frame size was traditionally measured from the center of the bottom bracket to the center of the top tube. Typical "medium" sizes are 54 or 56 cm (approximately 21.2 or 22 inches) for a European men's racing bicycle or 46 cm (about 18.5 inches) for a men's mountain bicycle. The wider range of frame geometries that are now made have given rise to different ways of measuring frame size; see the discussion by Sheldon Brown. Touring frames tend to be longer, while racing frames are more compact.
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