Historically, the most common frame materials for the tubes of a bicycle frame has been steel. Steel frames can be made of varying grades of steel, from very inexpensive carbon steel to more costly and higher quality chromium molybdenum steel alloys. Frames can also be made from aluminum alloys, titanium, carbon fiber, and even bamboo and cardboard. Occasionally, diamond (shaped) frames have been formed from sections other than tubes. These include I-beams and monocoque. Materials that have been used in these frames include wood (solid or laminate), magnesium (cast I-beams), and thermoplastic. Several properties of a material help decide whether it is appropriate in the construction of a 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. 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. Lateral stiffness is far more difficult to achieve because of the narrow profile of a frame, and too much flexibility can affect power transmission, primarily through tire scrub on the road due to rear triangle distortion, brakes rubbing on the rims and the chain rubbing on gear mechanisms. In extreme cases gears can change themselves when the rider applies high torque out of the saddle.
- 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.
Frame materials are listed by commonality of usage.
Steel frames are often built using various types of steel alloys including chromoly. They are strong, easy to work, and relatively inexpensive, but denser (and thus generally heavier) 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, allowing relatively light tube to be used without loss of strength. Recent advances in metallurgy (“Air-hardening steel”) have created 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, a lugged frame can be more easily repaired in the field due to its simple construction. Also, since steel tubing can rust (although in practice paint and anti-corrosion sprays can effectively prevent rust), the lugged frame allows a fast tube replacement with virtually no physical damage to the neighbouring tubes.
A more economical method of bicycle frame construction uses cylindrical steel tubing connected by TIG 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. Fillet brazing is another method of joining frame tubes without lugs. It is more labor-intensive, and consequently is less likely to be used for production frames. As with TIG welding, Fillet frame tubes are precisely notched or mitered and then a fillet of brass is brazed onto the joint, similar to the lugged construction process. A fillet braze frame can achieve more aesthetic unity (smooth curved appearance) than a welded frame.
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).
Cheaper steel bicycle frames are made of mild steel, also called high tensile steel, such as might be used to manufacture automobiles or other common items. However, higher-quality bicycle frames are made of high strength steel alloys (generally chromium-molybdenum, or “chromoly” steel alloys) which can be made into lightweight tubing with very thin wall gauges. One of the most successful older steels was Reynolds “531”, a manganese-molybdenum alloy steel. More common now is 4130 ChroMoly or similar alloys. Reynolds and Columbus are two of the most famous manufacturers of bicycle tubing. A few medium-quality bicycles used these steel alloys for only some of the frame tubes. An example was the Schwinn Le tour (at least certain models), which used chromoly steel for the top and bottom tubes but used lower-quality steel for the rest of the frame.
A high-quality steel frame is generally lighter than a regular steel frame. All else being equal, this loss of weight can improve the acceleration and climbing performance of the bicycle.
If the tubing label has been lost, a high-quality (chromoly or manganese) steel frame can be recognized by tapping it sharply with a flick of the fingernail. A high-quality frame will produce a bell-like ring where a regular-quality steel frame will produce a dull thunk. They can also be recognized by their weight (around 2.5 kg for frame and forks) and the type of lugs and fork ends used.
Aluminum alloys have a lower density and lower strength compared with steel alloys, however, they possess a better strength-to-weight ratio, giving them notable weight advantages over steel. Early aluminum structures have shown to be more vulnerable to fatigue, either due to ineffective alloys, or imperfect welding technique being used. This contrasts with some steel and titanium alloys, which have clear fatigue limits and are easier to weld or braze together. However, some of these disadvantages have since been mitigated with more skilled labor capable of producing better quality welds, automation, and the greater accessibility to modern aluminum alloys. Aluminum’s attractive strength to weight ratio as compared to steel, and certain mechanical properties, assure it a place among the favored frame-building materials.
Popular alloys for bicycle frames are 6061 aluminum and 7005 aluminum.
The most popular type of construction today uses aluminum alloy tubes that are connected together by Tungsten Inert Gas (TIG) welding. Welded aluminum bicycle frames started to appear in the marketplace only after this type of welding became economical in the 1970s.
Aluminum has a different optimal wall thickness to tubing diameter than steel. It is at its strongest at around 200:1 (diameter:wall thickness), whereas steel is a small fraction of that. However, at this ratio, the wall thickness would be comparable to that of a beverage can, far too fragile against impacts. Thus, aluminum bicycle tubing is a compromise, offering a wall thickness to diameter ratio that is not of utmost efficiency, but gives us oversized tubing of more reasonable aerodynamically acceptable proportions and good resistance to impact. This results in a frame that is significantly stiffer than steel. While many riders claim that steel frames give a smoother ride than aluminum because aluminum frames are designed to be stiffer, that claim is of questionable validity: the bicycle frame itself is extremely stiff vertically because it is made of triangles. Conversely, this very argument calls the claim of aluminum frames having greater vertical stiffness into question. On the other hand, lateral and twisting (torsional) stiffness improves acceleration and handling in some circumstances.
Aluminum frames are generally recognized as having a lower weight than steel, although this is not always the case. A low quality aluminum frame may be heavier than a high quality 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. Non-round tubes are used for a variety of reasons, including stiffness, aerodynamics, and marketing. Various shapes focus on one or another of these goals, and seldom accomplish all.
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) allows 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. Titanium 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. As titanium frames are usually more expensive than similar steel or aluminium alloy frames, the cost puts them out of reach for most cyclists.
Titanium frames typically use titanium alloys and tubes that were originally developed for the aerospace industry. The most commonly used alloy on titanium bicycle frames is 3AL-2.5V (3.5% Aluminum and 2.5% Vanadium). 6AL-4V (6% Aluminum and 4% Vanadium) is also used, but it is more difficult to weld, make tubes of, and machine. Often, the tubes are of 3AL-2.5V while dropouts and other peripheral sections are made of 6AL-4V. Experimental frames have been made with commercially pure (CP, i.e.:unalloyed) titanium, but these proved less durable for the active riding intended for frames of this cost level.
Extensive butting is also used to create low weight tubes with acceptable stiffness. The early versions of the Fat Chance Titanium (1992 and 93 versions) had tubes of different diameters welded together to create a stiffer bottom bracket area. The 1994 version had externally butted bottom tubes.
Frame tubes are almost always joined by Gas Tungsten Arc Welding (GTAW or TIG) welding, although vacuum brazing has been used on early frames. Some earlier titanium frames were made with titanium tubes bonded to aluminum lugs, as e.g. the Miyata Elevation 8000 and the Raleigh Technium Titanium.
Carbon fiber composite is an increasingly popular non-metallic material commonly used for bicycle frames. Although expensive, it is light-weight, corrosion-resistant and strong, and can be formed into 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 fiber 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. Some carbon fiber frames use cylindrical tubes that are joined with adhesives and lugs, in a method somewhat analogous to a lugged steel frame. Another type of carbon fiber frames are manufactured in a single piece, called monocoque construction.
While these composite materials can be lightweight and strong, they have much lower impact resistance than traditional materials and consequently are prone to damage or failure if crashed or mishandled. Cracking and failure can result from a collision, but also from over tightening or improperly installing components. These materials are also vulnerable to fatigue failure, a process which occurs with use over a long period of time. It is possible for broken carbon frames to be repaired, but because of safety concerns it should be done only by professional firms to the highest possible standards.
Many 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, or would be excessively heavy in other materials. While this type of frame may in fact be heavier than others, its aerodynamic efficiency may help the cyclist to attain a higher overall speed.
Other materials besides carbon fiber, such as metallic boron, can be added to the matrix to enhance stiffness further. Some newer high end frames are incorporating Kevlar fibers into the carbon weaves to improve vibration damping and impact strength, particularly in downtubes and seat- and chainstays.
Thermoplastics are a category of polymers that can be reheated and reshaped, and there are several ways that they can be used to create a bicycle frame. One implementation of thermoplastic bicycle frames are essentially carbon fiber frames with the fibers embedded in a thermoplastic material rather than the more common thermosetting epoxy materials. GT Bicycles was one of the first major manufacturers to produce a thermoplastic frame with their STS System frames in the mid 1990s. The carbon fibers were loosely woven into a tube along with fibers of thermoplastic. This tube was placed into a mould with a bladder inside which was then inflated to force the carbon and plastic tube against the inside of the mould. The mould was then heated to melt the thermoplastic. Once the thermoplastic cooled it was removed from the mould in its final form.
A handful of bicycle frames are made from magnesium which has around 64% the density of aluminum. In the 1980s, an engineer, Frank Kirk, devised a novel form of frame that was die cast in one piece and composed of I beams rather than tubes. A company, Kirk Precision Ltd, was established in Britain to manufacture both road bike and mountain bike frames with this technology. However, despite some early commercial success, there were problems with reliability and manufacture stopped in 1992. The small number of modern magnesium frames in production are constructed conventionally using tubes.
Some manufacturers of bikes make frames out of aluminum alloys containing scandium, usually referred to simply as scandium for marketing purposes although the Sc content is less than 0.5%. Scandium improves the welding characteristics of some aluminum alloys with superior fatigue resistance permitting the use of smaller diameter tubing, allowing for more frame design flexibility.
American Bicycle Manufacturing of St. Cloud, Minnesota, briefly offered a frameset made of beryllium tubes (bonded to aluminum lugs). Given the toxic nature of the material and the pricing ($26,000 for frame and fork), they never caught on. Reports were that the ride was very harsh, but the frame was also very laterally flexible.
Several bicycle frames have been made of bamboo tubes connected with metal or composite joinery. Aesthetic appeal has often been as much of a motivator as mechanical characteristics.
Several bicycle frames have been made of wood, either solid or laminate. Although one survived 265 grueling kilometers of the Paris–Roubaix race, aesthetic appeal has often been as much of a motivator as ride characteristics. Wood is used to fashion bicycles in East Africa. Cardboard has also been used for bicycle frames.
Combining different materials can provide the desired stiffness, compliance, or damping in different areas better than can be accomplished with a single material. The combined materials are usually carbon fiber and a metal, either steel, aluminum, or titanium. One implementation of this approach includes a metal down tube and chain stays with carbon top tube, seat tube, and seat stays. Another is a metal main triangle and chain stays with just carbon seat stays. Carbon forks have become very common on racing bicycles of all frame materials.