Understanding Bike Frame Materials
The frame is the main body of bicycle and is the framework onto which all other components are attached.
Modern bicycle frames are typically composed of multiple hollow tubes joined together to form a "double triangle" or "diamond" shape. This basic design hasn't changed much in over a century, although advances in engineering, manufacturing techniques, metallurgy and material science have provided a large number of variations on that theme. One of the most active areas of innovation, especially the past decade or so, is the material from which the frame is constructed.
Bicycle frames are usually made from steel or aluminum alloys, although more exotic materials such as carbon fiber and titanium can be found on higher-end bikes. While rare, one can find bike frames made of wood, plastic, magnesium, even bamboo.
Frame materials are a topic of intense debate and strong opinion among cyclists and there is no shortage of myth, hype and misinformation available. While there are important differences between frame materials, the fact is that the frame material matters much less than many would have you believe and it is just one of many factors that influence the strength, weight, and durability of a bike and the way it feels and performs on the road or on the trail.
How to evaluate frame materials
From a material science standpoint, there are four primary factors to be aware of: stiffness, strength, fatigue strength and weight.
Imagine fixing one end of a solid bar into a clamp so tightly that it will not budge. Now apply some force to the free end of the bar, for example, by hanging some weights from the free end. For small weights, the bar will bend slightly, but if you remove the weight the tube will spring back to being perfectly horizontal. This elastic property is the stiffness of the material. Under the same weight a stiff material will bend or flex less than an elastic one.
The stiffness or elasticity of a bike frame will have an impact on the way a bike performs and feels during a ride. Elastic frames cushion bumps and knocks, much like a suspension, while a stiff frame will transmit more of the force of each bump directly to the rider. But elastic frames also flex more under load, absorbing some of the energy you're generating to move the bike forward. For this reason cyclists interested in maximizing their performance in sprints and time trials will often prefer a stiff frame.
While different frame materials have different levels of inherent elasticity, the stiffness of a tube is mostly determined by its diameter and length. A frame's geometry also has a significant impact on the overall stiffness of the bike.
Now imagine hanging more weight from the free end of that bar. At a heavy enough load the bar will no longer spring back to its original shape--it will permanently warp (or, if the material is brittle enough, crack or break entirely). The amount of force required to crack, break or permanently deform a material is its strength. The point at which the material permanently deforms is known as the "yield strength". The point at which it breaks is known as the "breaking strength". A relatively flexible material such as steel will bend before it breaks. Brittle materials such as concrete, common glass, and more relevantly, carbon fiber, will snap before they deform--their breaking strength is less than their theoretical yield strength.
The strength of the frame material doesn't influence the performance or feel of the ride under normal conditions, but it is a significant factor in the "crash worthiness" of the frame. A strong frame can withstand an impact that may destroy a weaker frame.
3. Fatigue Strength
Under normal riding conditions each pedal stroke, bump or hop places a small amount of stress on the frame. While well below the yield strength of the frame, over a long time (hundreds of thousands of cycles of loading and unloading) these small stresses can cause cracks to develop in some frame materials, eventually leading to failure from fatigue. A material's resistance to this type of failure is known at its fatigue strength. Failure from fatigue is a very complicated and dynamic process that depends not only on the amount of force applied in each cycle and the number of repetitions but also on microscopic defects and anomalies that may be present in the material.
If you plan to keep your bicycle for decades, are heavier than average, or ride your bike hard, you may want to consider fatigue strength as a factor.
The weight, or more accurately, density, of a material is simply a measure of how much mass the material has per unit volume.
While the frame is a large component of a bike, it is easy to overestimate the importance of weight. A typical frame weighs maybe 3 to 5 pounds and represents only about one quarter of the overall weight of a bicycle. More importantly, the entire bicycle will often weigh about one tenth as much as the fully accessorized rider. Most cyclists will not notice a small difference in the weight of the frame unless they do a lot of climbing or need to lift and carry their bike often.
Frame materials in practice
Common frame materials have very different strength, stiffness and weight profiles. For the same volume of material...
- ...steel is roughly twice as strong, and nearly three times heavier and stiffer than aluminum.
- ...titanium is only half as heavy or stiff as steel, but just as strong.
- ...titanium is about 50% heavier and stiffer than aluminum, but roughly twice as strong.
- ...carbon fiber is more than seven times stronger than steel (when the force is applied in the right direction), but only one fourth as heavy.
- ...carbon fiber is more than 15 times stronger than aluminum (when the force is applied in the right direction), but only half as heavy.
But the raw material properties of these materials only tell part of the story. The statements above would be true of bike frames if frame builders built identical bikes out of every material. An aluminum frame built just like a typical steel frame would be very light, but far too weak to be of use. A titanium frame built just like a steel frame would be very strong and light, but also overly supple. Luckily frame builders are smarter than that. They make use of the unique properties of each material, tweaking parameters such as tube diameter and wall thickness to strike the right balance between weight, strength and stiffness.
For example, since aluminum is lighter but weaker than steel, manufacturers use more material to construct an aluminum frame--they build tubes of larger diameter with thicker walls to achieve the requisite strength and stiffness. In fact, due to the use of large diameter tubes, aluminum frames tend to be stiffer than steel frames, even though steel is a significantly stiffer material than aluminum.
Even within different varieties (alloys) of a given material, manufacturers apply different design criteria. For instance, high tensile steel is softer (and cheaper) than the more commonly used chromoly steel alloy. Manufacturers working with high-tensile steel must use thicker walled tubes to achieve the same strength. Although high-tensile and chromoly steel have the same weight per volume, hi-ten frames are heavier since they use more material to achieve the same strength.
Steel is the traditional choice for bike frames and remains very popular with both manufacturers and riders. Steel is strong, durable, fairly inexpensive and relatively easy to both build with and to repair. Not all steel frames are the same however. There can be significant difference between high and low quality steel frames, both in the properties of the specific alloy used and in the level of craftsmanship and quality in the construction of frame itself.
Steel is remarkably durable and among bike frame materials is relatively easy to repair. Dents or bends in a steel frame can be hammered out (or, if they are small enough, ignored). A broken steel frame could be welded and ridden again. Steel frames can withstand crashes that would total frames made of weaker materials.
Steel can rust when exposed to water, air and especially road salt. But steel frames are typically painted or treated to be corrosion resistant so in practice one need only look out for (and treat) scratches.
Unlike aluminum, steel is theoretically immune to failure from fatigue stress. As long as it is not loaded beyond its yield strength, a steel tube will not develop the microscopic cracks that lead to fatigue failure. However, although rare, it is not impossible for a steel frame to crack due to minor defects in the tubes themselves or due to weak spots in the welding. A well made steel frame can readily withstand decades of frequent use.
Steel frames are relatively elastic compared to aluminum frames. Depending upon the overall geometry, steel frames can flex a tiny bit under load. For this reason, some riders find that steel frames have "springy" or "lively" feel. This elasticity can cushion bumps a bit to provide a more comfortable ride, but also absorb some of the energy the rider generates when climbing or sprinting. Many cyclists have strong opinions (both positive and negative) about the feel of steel versus aluminum frames, but in experiments most experienced cyclists could not distinguish between steel and aluminum frames with similar geometries when the type of frame was hidden from them.
Steel is heavier by volume (that is, steel is denser) than most other frame materials. But this doesn't mean steel frames are necessarily heavier than frames made of other materials. Manufacturers often employ construction techniques that leverage steel's relative strength to reduce the weight of the frame.
One of these techniques is the use of butted tubes. Due to steel's inherent strength, light and strong steel tubes can in theory be built with extremely thin walls--down to 0.3 mm or smaller. Unfortunately, it is difficult for frame builders to work with these tubes--there's not enough material to form secure welds or attach lugs to bind the frame together. As a compromise, steel frame builders often use tubes that are thicker at the ends than in the middle, a technique known as butting. The thick ends of a butted tube add strength where it is most needed, while the thinner middle section reduces the amount of material (and therefore weight) along most of the length of the tube. Butted tubes can reduce the weight of a steel frame by 15% or more.
Steel is a combination (an alloy) of iron and other elements designed to improve upon the material properties of pure iron. The alloying elements make up a small fraction of the material by weight (iron is 97% or more of most steel alloys) and don't significantly alter the density (weight) and stiffness of the material, but they can have a significant impact on the strength, weldablity, corrosion resistance and expense of the material.
While there are dozens of steel alloys, only a few are commonly used in bike construction and only two--high tensile and chromoly steel--are used in most mass produced bicycles today.
High-Tensile (Hi-Ten) Steel
High-tensile or carbon steel is a common and inexpensive alloy comprised of iron mixed with 0.2% to 2.0% carbon. (Of note, when iron is mixed with more than 2.1% carbon it is no longer "steel", it goes by the name "cast iron".)
High-tensile steel is an inexpensive but relatively weak alloy. Although it has essentially the same density as other steels, manufacturers working with hi-ten steel are forced to use thick walled tubes to ensure adequate strength, and rarely use butting. For this reason, high-tensile frames are much heavier than their chromoly counterparts.
Today, high-tensile steel frames are primarily used for children's bikes and are sometimes found on inexpensive adult bikes.
4130 ChroMoly (CRMO) Steel
ChroMoly is a steel alloy composed of iron combined with chromium (roughly 1% by weight), molybdenum (roughly 0.2%), carbon (roughly 0.3%), silicon (roughly 0.2%), manganese (roughly 0.04%) and sulphur (roughly 0.04%). 4130 is actually just one of a family of chromoly alloys, but it is the one used for bicycle frames. ChroMoly steel is also used in the construction of airplanes, and is sometimes known as "aircraft tubing".
Chromium is the component that makes stainless steel rust proof, but the chromium level of chromoly steel is not high enough to provide corrosion resistance. (Stainless steel is 10% to 11% chromium.)
Chromoly is frequently used to build mid-to-high-range steel framed bikes. A well made butted chromoly frame is typically only marginally heavier than an aluminum frame, and quite strong and durable.
In 1953 the Reynolds Cycle Company began manufacturing a steel tube composed of proprietary manganese-molybdenum steel alloy they branded Reynolds 531. This alloy was strong and for its time, relatively light. It was once the preferred tubing for steel racing bikes (as well as British aircraft).
Over the years, Reynolds has introduced a number of branded steel tubes, the brand name indicating both the specific alloy and heat treatment but also the wall thickness and butting of the tubes. These include Reyolds 453 (a single-butted tube made of a manganese-titanium alloy), Reynolds 501, 520, 525 and 725 tubes (using 4130 chromoly steel), Reynolds 753 (high-end tubes made of a manganese-molybdenum alloy, essentially a better Reynolds 531), Reynolds 853 (4130 chromoly made stiffer by air hardening) and Reynolds 953 (a lightweight rust-proof maraging stainless steel introduced in 2006).
Reynolds steel is less common in bike frames today than it once was, but some of these alloys are still in use. The Reynolds 520 family is a well made class of 4130 chromoly tubes. You'll pay a bit more for the brand name, but you'll know you're getting a well manufactured tube. Reynolds 853 is a higher quality chromoly, made stiffer than usual by the way it is manufactured. Reynolds 953 is perhaps the best steel available for bikes today: Reynolds 953 frames are stronger than titanium, no heavier than high end aluminum and rust proof.
By volume, aluminum is inherently lighter, weaker and less stiff than steel, but it is both less expensive than steel and has a much higher strength-to-weight ratio. Using thin walled, wide diameter tubes, manufacturers can create aluminum bike frames that are lightweight, stiff, and sufficiently strong. These same properties make aluminum a common material in modern aircraft construction as well. Since the 1970s, TIG-welded aluminum bike frames have become increasing popular for certain biking applications. In addition to its use in frames, aluminum is commonly used for many other bike components, including stems, cranks and handlebars.
Aluminum bike frames have a reputation for being significantly lighter than steel, however this advantage is usually marginal in practice (and at the mid-to-high end price range, steel bike frames are often lighter than aluminum frames).
Aluminum frames are stiffer than most steel frames, by virtue of their use of large diameter tubes. This stiffness can improve handling and acceleration when sprinting or climbing, but some cyclists find that an aluminum bike frame provides a harsher or rougher ride than a steel frame.
Aluminum is corrosion resistant, but is more susceptible to failure from fatigue stress over time. Aluminum frames are both harder to repair and more a bit more likely to be damaged by a hard impact than steel, but modern alumnium frames are durable enough to provide years of reliable service.
Most aluminum frames are straight gauge (not butted), although you can find lighter weight butted aluminum frames on some bikes.
Aluminum is typically combined with (alloyed with) other elements to improve upon its material properties. The alloying elements make up a small fraction of the material by weight (typically less than 10%) but they can have a significant impact on the strength, weldablity, corrosion resistance and expense of the material.
6061 Aluminum Alloy (AA 6061)
6061 aluminum is a family of aluminum alloys combining aluminum with magnesium (roughly 1.0% by weight) and silicon (roughly 0.5% by weight), sometimes mixed with a variety of other materials including iron, copper, chromium, zinc, manganese and titanium. 6061 is a strong an easily welded alloy frequently used for bike frames (as well as aircraft, boats and more).
All 6061 aluminum alloys have the same weight (density) and stiffness, but the strength of the alloy varies slightly with the specific alloying elements and the "temper" or heat treatment used to produce the material.
There are three common tempers of 6061 aluminum:
- 6061-O is a soft annealed alloy and is the weakest of the three common tempers. It is too weak to be of use for bike frame tubes.
- 6061-T4 is a heat treated, naturally aged alloy that is roughly twice as strong as 6061-O.
- 6061-T6 is a heat treated, artificially aged alloy that is roughly 2.5 times stronger than 6061-O. 6061-T6 aluminum alloy is probably the single most popular material used in modern bike frames, and is also used extensively in the manufacture of other components such as derailleurs, stems, cranks and handlebars.
7005 Aluminum Alloy (AA 7005)
7005 aluminum is a family of aluminum alloys combining aluminum with zinc (roughly 4.5% by weight), sometimes mixed with a variety of other materials including silicon, magnesium, iron, copper, chromium, manganese and titanium. 7005 is 10% stronger and a little more brittle than 6061 aluminum.
7005 occupies a curious market position relative to 6061 frames. Unlike 6061, 7005 does not require expensive heat treatment to be strong enough to use within a bike frame. However, a non-heat-treated 7005 frame requires more material to provide its strength. This means you can find cheap but relatively heavy frames made of non-heat-treated 7005 alloy. One can also create a very strong and light frame out of heat-treated 7005 aluminum, but this adds cost to the manufacturing process. This means you can find more expensive, but very light and strong 7005 frames as well. Hence depending on the heat treatment, 6061 aluminum is comparable to 7005 at both high and low price points, but 6061 occupies the middle part of the market in addition to competing well at the high and low end. 7005 aluminum is primarily found in low-end bikes (when not heat treated) or high-end bikes (when heat treated) but rarely in the middle.
Proprietary Aluminum Alloys
Some major manufacturers use custom aluminum alloys or apply custom treatment to stock aluminum alloys in order to improve their material properties. This is as much a marketing exercise as a materials science one, but these custom treatments can produce higher quality frames.
Giant's Proprietary Aluminum Alloys
Giant is a major manufacturer of bikes with aluminum frames. (In fact, Giant sells the most bikes of any manufacturer in the world.) They have created a proprietary frame building process that they have branded "AluxX" aluminum. Giant's AluxX comes in two forms:
Giant AluxX Aluminum
Regular "AluxX" is simply 6061 aluminum, but Giant applies a custom tube construction and heat treating process to the manufacture of their AluxX aluminum frames. The result is a butted, shaped and heat treated 6061 frame. Giant cites the weight reduction benefits of this process, but doesn't tout significant improvements to stiffness or strength. At the very least, the word "AluxX" implies a well made, butted and shaped 6061 aluminum frame.
Giant AluxX SL Aluminum
Unlike plain AluxX aluminum, AluxX SL starts with a 6061 alloy but mixes in a trace amount of copper to form a stiffer alloy. This premium alloy is then butted, shaped and heat treated much like AluxX tubes. Giant claims that the resulting alloy is 18% stiffer than pain AluxX aluminum and can be used to create "the lightest and stiffest aluminum frames current available".
Trek's Proprietary Aluminum Alloys
Trek is another major manufacturer of bikes with aluminum frames. Like Giant, they have created a proprietary aluminum frame building process that they brand as "Trek Alpha". Trek's Alpha aluminum comes in three forms:
Trek Alpha White Aluminum
Alpha White is the lowest end of Trek's aluminum frame building materials. It is simply straight gauge 6061 aluminum tubing, Trek branded.
Trek Alpha Black Aluminum
Alpha Black is Trek's mid-range aluminum alloy. It is 6061 aluminum that has been shaped and butted to reduce weight and increase strength. Frames built with Alpha Black aluminum are likely on par with Giant AluxX and any other well made shaped and butted 6061 aluminum frames.
Trek Alpha Red Aluminum
Alpha Red is Trek's premium aluminum alloy. It is heat-treated 7075 aluminum that has been shaped and butted to reduce weight and increase strength.
Carbon Fiber Frames
Carbon fiber is the newest and most exotic material commonly used for bike frames. Unlike steel or aluminum, carbon is not a metal--it is composite of strands of carbon pressed together in layers with an epoxy glue, much like a high-tech version of particle board. Carbon can be shaped into interesting and aerodynamic forms, and it is common to see carbon bike frames composed of teardrop, flat or wing shaped tubes rather than the perfect cylinders used in steel bike frames.
Carbon bike frames are sometimes built as a single solid piece and they are sometimes built from individual tubes joined together with lugs much like a steel or aluminum frame.
Carbon is very lightweight and can be made very stiff. Due to the alignment of the individual fibers, carbon frames have a more distinctive "grain". This allows carbon bike frames to have different amounts of stiffness directions and to stiffen "non-linerally". That is, while a steel frame will flex twice as much under 40 pounds of pressure as it did at 10 pounds, a carbon frame tends to get "tighter" as it flexes. It will give a little bit under slight pressure, but resist more stiffly as pressure increases. Manufacturers often exploit this property to give carbon frames the stiffness of aluminum for sprinting but the springiness of steel when rolling over bumps.
Although the price is steadily dropping, carbon is by far the most expensive common bike frame material.
Like aluminum, carbon fiber bike frames are corrosion resistant, but they require a bit more tender loving care than a steel or aluminum frame. Although carbon is strong and stiff, a deep scratch or hard bump can compromise the structural integrity of a carbon frame, making it prone to catastrophic failure.