Historically, the most common material 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 ultimately 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
crashworthiness). •
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 steel bicycle frame
Steel frames are often built using various types of steel alloys including
chromoly. They are strong, easy to work, and relatively inexpensive. However, they are denser (and thus generally heavier) than many other structural materials. It is common (as of 2018, in hybrid commuter bikes) to use steel for the fork blades even when the rest of the frame is made of a different material, because steel offers better vibration
dampening. 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 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 from 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 Titanium is a relatively specialised material for bicycle frames. It has many desirable characteristics including a high
specific strength, high
fatigue limit, and excellent corrosion resistance. While not as light as carbon fiber, titanium bicycles can provide a more pleasant ride quality, making the material popular among cyclists seeking comfort over performance. However, titanium has a high material cost and is more difficult to machine than steel or aluminum, which translates to relatively expensive frames compared to steel, aluminum, and carbon fiber. Welding is typically done in
inert conditions to protect the welds from oxidation. 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. In one series of tests conducted by
Santa Cruz Bicycles, it was shown that for a frame design with identical shape and nearly similar weight, the carbon frame is considerably stronger than aluminum, when subjected to an overall force load (subjecting the frame to both tension and compression), and impact strength. While carbon frames can be lightweight and strong, they may have lower impact resistance compared to other materials, and can be prone to damage if crashed or mishandled. Cracking and failure can result from a collision, but also from over tightening or improperly installing components. It has been suggested that these materials may be vulnerable to fatigue failure, a process which occurs with use over a long period of time, though this is often limited to interlaminar cracks or cracks in adhesive at joints, where stresses can be well controlled with good design practices. 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, seat stays, and chain stays.
Thermoplastic from the early 1980s.
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.
Magnesium 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.
Scandium aluminum alloy 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.
Beryllium American Bicycle Manufacturing of St. Cloud, Minnesota, briefly offered a frameset made of
beryllium tubes (bonded to aluminum lugs), priced at $26,000. Reports were that the ride was very harsh, but the frame was also very laterally flexible.
Bamboo 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.
Wood 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.
Combinations 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.
Other The
bicycle types article describes additional variations. ==Butted tubing==