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.

From a material science standpoint, there are four primary factors to be aware of: stiffness, strength, fatigue strength and weight.

1. Stiffness

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.

Testing the strength and stiffness of a material.

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.

3. Fatigue Strength

An aluminum crank arm broken by fatigue stress. The dark area is where cracks developed over time due to low load stress. The bright area is where the crank arm finally snapped once the cracks had grown large enough to compromise the strength of the component.

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.

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.

Cross section of bike frame tubes made of different materials. (A) ChroMoly Steel (B) High Tensile Steel (C) Aluminum

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.

Illustration of Butted Tubes. A straight or plain gauge tube has the same wall thickness along its entire length. A single butted tube is thicker at one end than along most of the length and is sometimes used for the seat-tube on a bike. A double butted tube is thicker at each end, and thinner in the middle. A triple butted tube uses three different wall thicknesses to further decrease the amount of material used.

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.

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.

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.