Prior to about 40 years ago, steel was pretty much the only option consumers had for bike frames. Thankfully we have many more options in 2023, including titanium (Ti), aluminum (Al), carbon fiber, as well as steel.  There are a couple of other, exotic, materials such as wood, bamboo, and magnesium but these are more curiosities and unlikely to de-throne the top-four anytime soon.  Steel is an alloy of iron and other elements such as nickel, chromium, molybdenum, and carbon.  The specific alloy "recipe" and various manufacturing techniques like heat treating and cold-working affect the properties of the various steel alloys.  Although steel is still really a great material to make a bike out of, these days it is mostly seen in cheap "grocery store" bikes or CitiBikes that require extreme durability, with little regard for total weight. However, modern lightweight, ultra-strong steel alloys are a superb material for bikes. It can be made into surprisingly light frames with excellent riding characteristics. Aluminum comes in a variety of alloys and has the advantage of very low cost. The most common alloy used in the bike industry is 6061 (Aluminum with silicone and magnesium) and is often referred to as "high grade" or "aircraft" aluminum. This is just marketing. Although this alloy is indeed used all over the aerospace industry it is not especially noteworthy or rare. Other alloys used are 7005 (wrought aluminum) and 7075 (aluminum-zinc). The first major manufacturer to really develop aluminum into widely-marketed road bike frames was Cannonade in the early-to-mid 1980s.  In my bicycle "youth" of the early and mid 1990's brands such as Klein were very sought-after.  They made really light frames (for the time) with big, round aluminum tubes that were bonded or welded together.  These frames stood out compared to the narrow steel tubes most frames had at the time, so the new aluminum bikes were very intriguing. 

Believe it or not, manufacturers were experimenting with raw, unalloyed titanium bike frames way back in the 1960's and 1970's, with the 1975 Teledyne Titan among the first. In the 1990s, titanium had its heyday as a viable, sought-after, high-end frame material. It was lighter than steel, stronger than aluminum and easier to work with than carbon fiber. The titanium bikes of that era were made from titanium-aluminum-vanadium alloy, which is still used today by titanium bike tubing manufacturers and titanium frame builders. Similar to high-end steel, titanium is rarely seen in local bike shops. It is more expensive than aluminum and does not lend itself well to mass manufacturing. However both steel and Ti are widely available from innumerable boutique and custom frame builders.

Carbon fiber came along in the late 1980's and 1990's. Craig Calfee's brand, then called CarbonFrames, and the French company Look, produced some of the first carbon fiber bikes used in professional racing. Sometimes these were painted up as a "sponsor" correct brand which was a common practice at the time. Greg LeMond, for example, rode some of the early CarbonFrames in 1991. Craig Calfee continues to make carbon fiber bikes under his eponymous brand, Calfee Design, using some of the same techniques he developed back then. Eventually, carbon fiber became ubiquitous in the industry and is mass-produced at a very large scale.  Of course it is also used for parts other the frame, such as handlebars, bottle cages, seatposts, rims, etc…

Carbon fiber - or more accurately, carbon fiber composite - is not a single material. What we call "carbon fiber" is really fibers of elemental carbon and other added materials like aramid, UHMWPE, boron, and a plasticky kind of resin – or epoxy – to hold it all together. The fibers are woven or wound into tubes or sheets, in combination with the other ingredients.  Frames can made in one piece or a couple of large sections by applying layers of sheets into a mold, or by joining pre-formed round carbon tubes.  It is very important to consider that carbon fiber composite is really strong in the direction along the length of the fibers, but really weak in the transverse direction across the grain. It also can handle a lot of tension, but does not fare as well in compression.  So in order to make a practical bike frame we need to vary the angle of the carbon fibers in multiple layers during the manufacturing process such that the stresses in the frame are distributed along the direction in which the individual carbon fibers are strongest. This is called layup, and plays a crucial role in manufacturing. Carbon fiber frame designers can add or subtract layers of material, or vary the shape of the tubes, to add extra strength where higher mechanical stresses will be encountered, or arrange the fibers to flex more for good vibration damping and comfort.  Manufacturers are also careful to prevent voids, or airspaces, which lack enough resin to hold the structure together. Most mass-produced carbon fiber bikes on the market are made by just a few very large specialty manufacturers, some even producing frames for competing companies. A few small, boutique, manufacturers weave their own carbon from scratch, like Time and Look, but this is less common.  Manufacturers really hype up the differences among the various formulations and manufacturing techniques. One manufacturer might use "super-duper-stiff" carbon, another advertises "super-uper-duper-stiff" carbon fiber while a third manufacturer claims their carbon fiber is "super-durable" and so on….  Stiffer carbon fiber is indeed stronger, which means less material is required to achieve the desired strength of the part being made. However a stiffer, lighter part can be more brittle, and may not provide as much vibration damping or durability. Higher grades of carbon fiber are also more expensive and require tighter manufacturing controls. So there are always tradeoffs between weight, ride quality, cost, etc…. It's really more about matching the characteristics of the frame to the desired qualities of the bicycle ultimately being built and sold.

Material Properties and how they compare

The chart above indicates various physical properties that engineers and framebuilders consider when they are designing and constructing our bikes. Young's Modulus (or 'E' in engineer-speak) denotes how "stiff" the material is or, in other words, how much pulling force it takes to stretch it. Yield strength denotes how much force it takes to stretch a material so much that it remains permanently deformed, without springing back to normal. One Pascal is one Newton per Meter-squared (in case you were wondering), so a gigapascal (Gpa) is quite a lot of force! Think of a wire hanger. Bend the hanger very slightly and it will spring back to its original position, this like the Young's Modulus. Bend it a lot and it will stay bent; this is the yield strength. Density gives an idea of how heavy the material is. Elongation provides an idea of how durable, or tough, a material is. Going back to our wire hanger analogy, let's pull on it instead of bending it. If we pull harder and harder the hanger will start to stretch out. How much will it stretch before it breaks? That's the elongation. A glass hanger will not stretch very much and at some point it will just crack and break – it is very brittle in other words.   On the other hand a hanger made of saltwater taffy would have a very high elongation number and will stretch a lot before it breaks, in engineering terms taffy is not especially stiff but it is tough.

You can see that steel is the heaviest in the chart, but it is wicked-strong, so we don't need a lot of it.  Aluminum is very light, but we have to use more of it because it has low strength (stiffness).  There is really no one best material.  The properties of each material are obtained in a laboratory using a solid rod or bar of the same volume and cross-sectional area for each material.  All of these materials, formed into hollow tubing of appropriate dimensions, can make a great bike… if designed and built properly.  It turns out that wide diameter tubes will have a lot more stiffness than a narrow solid bar made from the same amount of material. For example, a solid bar and an I-beam may utilize the same quantity of material, but the I-beam can take a lot more bending forces.  This is analogous to comparing a narrow solid steel bar vs a larger diameter thin-walled steel tube.  Both can weigh the same but the hollow tube is like the I-beam; it can take a lot more force without bending.  If we were to test two tubes of the same weight for stiffness, or resistance to bending, we would find that the stiffness increases with the diameter.  A 1.5 inch steel tube with a wall thickness of 0.032 inches will be 1.6 times as stiff as a 1 inch steel tube with a wall thickness of 0.049 inches.  It seems, then, that we could simply use an even larger diameter, really thin-walled steel tube, resulting in a super-light tube of adequate stiffness.  Bingo!  We're on our way to a super light 900gm steel frame!  Well, there is a practical limit to how light and thin we can make a tube due to the "beer can" effect.  If the ratio of the diameter-to-wall thickness of a tube approaches 60- or 70-to-one, the tube is much more likely to have catastrophic failure due to buckling…like a beer can.  This is true for any material, not just steel.  For this reason, steel tubes tend to be more narrow than aluminum ones.  The thinnest, lightest steel tubing (such as Columbus Spirit) has walls of only around 0.4mm thick! Such tubes are plenty stiff, but are about as light as a steel tube can be and still have resistance to buckling.  Aluminum is much less stiff than steel, carbon, or Ti so we really need to increase the diameter of the aluminum tubes otherwise we'd either have a thin-walled frame that rides like a wet noodle, or a stiff frame of narrow, thick-walled aluminum tubing that's really heavy.  Since aluminum is so much lighter, you can increase the wall thickness of larger diameter tubing to avoid buckling and still have a somewhat lightweight frame for a lot less money than carbon fiber or titanium.  Titanium frame builders tend to use tubes that are somewhere between typical diameters for steel and aluminum.  This makes sense since the properties of Ti lie somewhere between aluminum and steel.  You could push Ti towards very lightweight, thin walls and still have good durability, since it has so much toughness, but since it is less stiff than steel such a frame would be too flexible.  So most builders construct Ti frames in the "goldy-locks" zone between weight, comfort, and stiffness – which is why Ti continues to be popular.  The stiffness and comfort of carbon fiber – in general – can really be dialed-in by the manufacturer since there is such exquisite control of tube shape and layup, although this obviously comes at a high cost.

It is helpful to understand a little bit about the material properties, but what we really want to know is how the entire frame will behave as a single unit - as a system. So while the physical properties of a chosen material are important, this is really only a small part of the picture when it comes to bike design. All of these materials -  or even mixing them in a single frame - can be used to make an extremely strong, durable, reasonably lightweight bike.  We do not want a frame that is either extremely, uncomfortably stiff nor one that is so flexy the bike tire rubs on the chainstay every time we stand out of the saddle. We want comfort, but not too cushy…lively and responsive but not so stiff we feel every single divot in the road.  Each material, frame design, and construction method has certain trade-offs, like weight, comfort, cost, longevity, and ride quality.  The rider chooses which among these are most important.  Keeping this in mind, claims by the "big name" frame manufacturers should be taken with a grain of salt.  An engineer can design a bike to achieve the lightest possible weight or the greatest amount of lateral stiffness.  But the other parameters that are important to provide a good riding experience will suffer.  Bikes designed to be as stiff as possible are really joyless to ride; some manufacturers made such bikes in the past decade and they were wildly unpopular.  A well designed frame, regardless of the material, should achieve a balance of all those things we feel when we ride.  Stiff but to not too stiff, light but not so light that durability and longevity are compromised, comfortable but still retaining good road feel, etc….