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.
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.
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.
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.
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.
The chain stays run parallel to the chain, connecting the bottom bracket shell to the rear fork ends or 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 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.
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 dropouts. A traditional frame uses a simple set of paralleled tubes connected by a bridge above the rear wheel. When the rear derailleur cable is routed partially along the top tube, it is also usually routed along the seat stay.
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 seat stays add aesthetic appeal at the expense of added weight. This style of seat stay was popularized again in the late 20th century by GT Bicycles (under the moniker “triple triangle”), who had incorporated the design element into their BMX frames, as it also made for a much stiffer rear triangle (an advantage in races); this design element has also been used on their mountain bike frames for similar reasons.
Recently, a variation of the traditional seat stay that bypasses the seat tube and connects further into the top tube was patented by Volagi Cycles. This frame element added length to the traditional design of seat stays, making a softer ride at the sacrifice of frame stiffness.
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.
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).
On some gearbox bicycles, the bottom bracket shell may be replaced by an integrated gearbox or a mounting location for a detachable gearbox.
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.
- stack, the vertical distance from the center of the bottom bracket to the top of the head tube.
- reach, the horizontal distance from the center of the bottom bracket to the top of the head tube.
- bottom bracket drop, the distance by which the center of the bottom bracket lies below the level of the rear hub.
- handlebar drop, the vertical distance between the reference at the top of the saddle to the handlebar.
- saddle 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.
- front center, the distance from the center of the bottom bracket to the center of the front hub.
- 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.
Frame size was traditionally measured along the seat tube 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. Touring frames tend to be longer, while racing frames are more compact.
Road and triathlon bicycles
A road racing bicycle is designed for efficient power transfer at minimum weight and drag. Broadly speaking, the road bicycle geometry is categorized as either a traditional geometry with a horizontal top tube, or a compact geometry with a sloping top tube.
Traditional geometry road frames are often associated with more comfort and greater stability, and tend to have a longer wheelbase which contributes to these two aspects. Compact geometry road frames have a lower center of gravity and tend to have a shorter wheelbase and smaller rear triangle, which give the bike quicker handling. Compact geometry also allows the top of the head tube to be above the top of the seat tube, decreasing standover height, and thus increasing standover clearance and lowering the center of gravity. Opinion is divided on the riding merits of the compact frame, but several manufacturers claim that a reduced range of sizes can fit most riders, and that it is easier to build a frame without a perfectly level top tube.
Road bicycles for racing tend to have a steeper seat tube angle, measured from the horizontal plane. This positions the rider aerodynamically and arguably in a stronger stroking position. The trade-off is comfort. Touring and comfort bicycles tend to have more slack seat tube angle traditionally. This positions the rider more on the sit bones and takes weight off the wrists, arms and neck, and, for men, improves circulation to the urinary and reproductive areas. With a slacker angle, designers lengthen the chainstay so that the center of gravity (that would otherwise be farther to the back over the wheel) is more ideally repositioned over the middle of the bike frame. The longer wheelbase contributes to effective shock absorption. In modern mass-manufactured touring and comfort bikes, the seat-tube angle is negligibly slacker, perhaps in order to reduce manufacturing costs by avoiding the need to reset welding jigs in automated processes, and thus do not provide the comfort of traditionally made or custom-made frames which do have noticeably slacker seat-tube angles.
Road racing bicycles that are used in UCI-sanctioned races are governed by UCI regulations, which state among other things that the frame must consist of two triangles. Hence designs that lack a seat tube or top tube are not allowed.
Triathlon- or time-trial-specific frames rotate the rider forward around the axis of the bottom bracket of the bicycle as compared to the standard road bicycle frame. This is in order to put the rider in an even lower, more aerodynamic position. While handling and stability is reduced, these bicycles are designed to be ridden in environments with less group riding aspects. These frames tend to have steep seat-tube angles and low head tubes, and shorter wheelbase for the correct reach from the saddle to the handlebar. Additionally, since they are not governed by the UCI, some triathlon bicycles, such as the Zipp 2001, Cheetah and Softride, have non-traditional frame layouts, which can produce better aerodynamics.
Track frames have much in common with road and time trial frames, but come with rear-facing, horizontal fork ends, rather than dropouts, to allow one to adjust the position of the rear wheel horizontally to set the proper chain tension. Also the seat tube angle is steeper than on road racing bikes.
For ride comfort and better handling, shock absorbers are often used; there are a number of variants, including full suspension models, which provide shock absorption for the front and rear wheels; and front suspension only models (hardtails) which deal only with shocks arising from the front wheel. The development of sophisticated suspension systems in the 1990s quickly resulted in many modifications to the classic diamond frame.
Recent mountain bicycles with rear suspension systems have a pivoting rear triangle to actuate the rear shock absorber. There is much manufacturer variation in the frame design of full-suspension mountain bicycles, and different designs for different riding purposes.
Roadster bicycles traditionally have a fairly slack seat-tube and head-tube angle of about 66 or 67 degrees, which produces a very comfortable and upright “sit-up-and-beg” riding position. Other characteristics include a long wheelbase, upwards of 40 inches (often between 43 and 47 inches, or 57 inches for a longbike), and a long fork rake, often of about 3 inches (76mm compared to 40mm for most road bicycles). This style of frame has had a resurgence in popularity in recent years due to its greater comfort compared to Mountain bicycles or Road bicycles. A variation on this type of bicycle is the “sports roadster” (also known as the “light roadster”), which typically has a lighter frame, and a slightly steeper seat-tube and head-tube angle of about 70 to 72 degrees.
Diamond frame variations
There are many variations on the basic diamond frame design.
- The elevated chainstay bicycle was popular in the early 90s. It featured a rear triangle with elevated bottom frame stays, negating the need for the chain to be drawn through the rear frame. This allowed for a shorter wheelbase and improved handling during technical ascents, at the cost of compromised integrity and resultant increased bottom bracket flex (unless reinforced) compared to a frame with traditional chainstays.
The cycle types article describes additional variations.
It is also possible to add couplers either during manufacturing or as a retrofit so that the frame can be disassembled into smaller pieces to facilitate packing and travel.