The 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 is made of two triangles, a main triangle and a paired rear triangle. This is known as 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.
Unless otherwise specified, the remainder of this article focuses primarily on the diamond frame.
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 top tube connects the head tube to the seat tube at the top. In a mountain bike frame, the top tube is almost always sloped. In a traditional-geometry racing bicycle frame, the top tube is horizontal. In a compact-geometry frame, the top tube is sloped. See Road and triathlon bicycles for more information on geometries.
Control cables are routed along mounts on the top tube. Most commonly, this includes the cable for the rear brake, but some mountain bikes and hybrid bicycle 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 standover height.
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. On older racing bicycles, the shifters were mounted on the down tube. On newer ones, they are integrated with the brake levers on the handlebars.
Bottle cage mounts are also on the down tube. 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 carry bottle cage mounts.
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. The chain stays provide a mount for rear rim brakes and disc brakes.
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. When the rear derailleur cable is routed partially along the top tube, it is also 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.
A single seat stay refers 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.
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 usually specifies:
- saddle height, the distance from the center of the bottom bracket to the point of reference on top of the saddle 13cm from the rear 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 saddle reference point and the center of the bottom bracket.
The geometry of the frame depends on the intended use. For instance, a road bicycle will place the rider in a lower, more crouched position; whereas a utility bicycle emphasizes comfort and has an upright seating position. Geometry also affects handling characteristics. Frame geometries in which the wheelbase is shorter are quicker in cornering but harder to balance. In some instances frame geometries can contribute to high-speed wobble .
Frame size was traditionally measured from the centre of the bottom bracket to the top of the seat tube. Typical "medium" sizes are 21 or 23 inches (approximately 53 or 58 cm) for a European men's racing bicycle or 18.5 inches (about 46 cm) 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.
Road and triathlon bicycles
A road 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 contribute to these two aspects. Compact geometry road frames have lower center of gravity and tend to have shorter wheelbase and smaller rear triangles, which make the handling to be quicker. Compact geometry also allows the top of the head tube to be above the top of the seat tube, increases standover clearance, and has lower center of gravity.
Road bicycles for racing tend to have steeper seat tube angle, measured from the horizontal plane. Touring and comfort bicycles tend to have more slack seat tube angle. The slacker angle forces the rear wheel to be further behind the rider, thus contributing to shock absorption.
Road racing bicycles are governed by UCI regulations, which state among other things that the frame must consist of two triangles. Hence the designs which include the absence of a seat tube, or the absence of a top tube are not allowed in UCI-sanctioned road races.
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. The reason for this is 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 handlbar.
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.
There are other variations on the basic diamond frame design. Historically, women's 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. This was to allow the rider to dismount while wearing a skirt or dress. This is also known as a step-through frame.
The cycle types article describes additional variations.
Historically, the tubes of the frame have made of steel. Steel is still used, newer frames can also be made from aluminum alloys, titanium and carbon fiber. Several properties of a material help decide whether it is an appropriate in the construction of bicycle frame:
- Density (or specific gravity) is a measure of how light or heavy the material per unit volume.
- Stiffness (or elastic modulus) can in theory affect the ride comfort and power transmission efficiency (but in practice, because even a very flexible frame is much more stiff than the tires and saddle, ride comfort is in the end more a factor of saddle choice, frame geometry, tire choice, and bicycle fit).
- Yield strength determines how much force is needed to permanently deform the material. (for crash-worthiness).
- Elongation determines how much deformity the material allows before cracking (for crash-worthiness).
- Fatigue limit and Endurance limit determines the durability of the frame when subjected to cyclical stress from pedaling or ride bumps.
Tube engineering and frame geometry can overcome much of the perceived shortcomings of these particular materials.
Steel is stiff, strong, easy to work, and relatively inexpensive, but more dense than many other structural materials.
A classic type of construction for both road bicycles and mountain bicycles uses standard cylindrical steel tubes which are connected with lugs. Lugs are fittings made of thicker pieces of steel. The tubes are fitted into the lugs, which encircle the end of the tube, and are then brazed to the lug. Historically, the lower temperatures associated with brazing (silver brazing in particular) had less of a negative impact on the tubing strength than high temperature welding, alowing relatively light tube to be used without loss of strength. Recent advances in metallurgy have created ("air hardening") tubing that is not adversely affected, or whose properties are even improved by high temperature welding temperatures, which has allowed both TIG & MIG welding to sideline lugged construction in all but a few high end bicycles. More expensive lugged frame bicycles have lugs which are filed by hand into fancy shapes - both for weight savings and as a sign of craftsmanship. Unlike MIG or TIG welded frames, lugged construction facilitates more easy field repair of a damaged frame. To clarify, when cycling in third world countries, a lugged frame can generally be repaired due to its familiar and simple construction. Also, since steel tubing can rust, the lugged frame allows a fast tube replacement with virtually no physical damage to the neighboring tubes.
A more economical method of bicycle frame construction uses cylindrical steel tubing connected by TIG welding or brazed (fillet) welding, which does not require lugs to hold the tubes together. Instead, frame tubes are precisely aligned into a jig and fixed in place until the welding is complete. The fillet braze welding process of joining frame tubes is more labor intensive, and consequently is less likely to be used for production frames. Some custom frame builders and their customers prefer a fillet braze welded frame for aesthetic (smooth curved appearance) reasons.
Among steel frames, using butted tubing reduces weight and increases cost. Butting means that the wall thickness of the tubing changes from thick at the ends (for strength) to thinner in the middle (for lighter weight). Modern tubing is made of special steel alloys (generally chromium-molybdenum, or "chromoly" steel alloys) chosen for their combinations of strength and lightness. One of the most successful older tube types was manganese alloy tube such as Reynolds "531". Reynolds and Columbus are two of the most famous manufacturers of bicycle butted tubing.
Aluminum alloys are very lightweight, but react badly to repetitive flexing. If too much flexion is demanded of aluminum, it will fail through a process called fatigue (i.e. the tube will crack, then fracture); unlike some steel alloys, failure can occur completely and without warning. Also, in contrast to some steel and titanium alloys which have a fatigue endurance limit, aluminum has no fatigue endurance limit, and if cracking is not identified even the smallest repeated stresses will eventually cause failure if repeated enough. However, alloying, mechanical design and good construction practice helps to extend the fatigue life of aluminum bicycle frames significantly.
The most popular type of construction today uses aluminum alloy tubes that are connected together by Tungsten Inert Gas (TIG) welding. Aluminum bicycle frames started to appear in the marketplace only after this type of welding become economical until the 1970s. Comparing equal tube sizes, aluminum is less stiff than steel, but it is also lighter. In order to raise aluminum’s stiffness, the tubing diameter is increased beyond that of steel and thus known as oversized tubing. The greater diameter generally results in a frame that is significantly stiffer than steel. This is not always a benefit, since the flex of a compliant steel frame feels more comfortable to many riders compared to an aluminum frame.
Aluminum frames are generally recognized as having a lower weight than steel, although this is not always the case. An inexpensive aluminum frame may be heavier than an expensive steel frame. Butted aluminum tubes -- where the wall thickness of the middle sections are made to be thinner than the end sections -- are used by some manufacturers for weight savings. Other innovations include the shaping of the cross-section of the tubes, such as in a oval or teardrop shapes, for optimizing stiffness and compliance in different directions as well as reducing wind resistance.
Titanium is perhaps the most exotic and expensive metal commonly used for bicycle frame tubes. It combines many desirable characteristics, including a high strength to weight ratio and excellent corrosion resistance. Reasonable stiffness (roughly half that of steel) allow for many titanium frames to be constructed with "standard" tube sizes comparable to a traditional steel frame, although larger diameter tubing is becoming more common for more stiffness. As many titanium frames can be much more expensive than similar steel alloy frames, cost can put them out of reach for many cyclists. Many common titanium alloys and even specific tubes were originally developed for the aerospace industry.
Titanium frame tubes are almost always joined by Tungsten inert gas welding (TIG), although vacuum brazing has been used on early frames. It is more difficult to machine than steel or aluminum, which sometimes limits its uses and also raises the effort (and cost) associated with this type of construction.
Carbon fibre, a composite material, is the only non-metallic material commonly used for bicycle frame tubes. Although expensive, it has light weight, corrosion resistance and high strength, and can be formed in almost any shape desired. The result is a frame that can be fine-tuned for specific strength where it is needed (to withstand pedaling forces), while allowing flexibility in other frame sections (for comfort). Custom carbon fibre bicycle frames may even be designed with individual tubes that are strong in one direction (such as laterally), while compliant in another direction (such as vertically). The ability to design an individual composite tube with properties that vary by orientation cannot be accomplished with any metal frame construction commonly in production.
Simple carbon fibre frames are assembled using cylindrical tubes that are joined with adhesives and lugs, in a method somewhat analogous to a lugged steel frame. More exotic carbon fibre frames are manufactured in a single piece, called monocoque construction. While these composite materials provide light weight as well as strength, they have much lower impact resistance and consequently are prone to damage if crashed or mishandled. It has also been suggested that these materials are vulnerable to fatigue failure, a process which occurs over a long period of time.
Many specialty racing bicycles built for individual time trial races and triathlons employ composite construction because the frame can be shaped with an aerodynamic profile not possible with cylindrical tubes. While this type of frame may in fact be heavier than others, its aerodynamic efficiency may allow an individual cyclist to attain maximum speed and consequently outweigh other considerations in such events.
- 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.
- Homemade bamboo mountain bike frame