Just as it isn’t remotely necessary to understand how an internal combustion engine works in order to drive a car, it isn’t necessary to know how a bicycle is made to ride one. That said, knowing a bit about the frame materials used can aid you in selecting the best bicycle for you depending on such factors as price, ride quality, and your dimensions.
The Least You Need to Know
There’s a fair amount of technical information in this chapter. Some riders will find this fascinating, but honestly, if you are a new rider, this will make more sense once you’ve had a chance to ride bicycles made from the different frame materials. When you have a chance to ride a steel bicycle and then one made from carbon fiber, you’ll probably become interested in the differences between the various materials.
There are four primary materials used in bicycle frames. They are steel, aluminum, titanium, and carbon fiber. To one degree or another all of these materials are used as tubes; different construction methods will determine how these tubes are manufactured and joined together.
Steel was the dominant material for more than 100 years and was used in top-quality racing bikes ridden in the Tour de France as recently as 1995. Aluminum began to be used in the 1970s, as did titanium, though neither gained significant popularity until the 1990s. Carbon fiber was first used as a frame material in the 1980s but has gained its widest and most popular use in the last 10 years. In terms of units sold, it is now the most popular material for top-quality racing bicycles.
The materials each offer a different blend of cost, artistry, durability, stiffness, and what is called “road feel,” which is a term for the tactile sense you get of the road surface beneath you. In broad strokes, here are each material’s top selling points:
Steel is generally the least expensive of the four materials. It excels at durability and giving artisan frame builders a platform for their artistry. Steel bikes are known for an excellent road feel.
Aluminum is also good for riders on a budget. It combines lower cost with generally less weight than found in steel. Aluminum bikes don’t often get high marks for their road feel.
Titanium combines incredible durability with reduced weight, relative to steel. Titanium is appreciated because it won’t corrode under normal use, so frame can be left bare to show a lustrous gray. Titanium bikes are on par with steel bikes for their road feel.
Carbon fiber is the material used for today’s top bikes. The best carbon fiber bikes are exceedingly expensive, stunningly low in weight, and unfortunately fragile. Carbon fiber gets high marks for its road feel but it is quite different from the other materials because it isn’t metal.
Stiffness is a quality prized because it makes a bicycle more efficient in power transfer from the pedals to the rear wheel, though a bicycle with less stiffness will also afford a rider greater comfort as he pedals. While any of the above materials can be built to make a stiff or flexible bicycle, in current practical application carbon fiber bikes are usually the stiffest. Aluminum bikes tend to be the next most stiff. Steel bikes are usually a little less stiff, while the titanium bikes, though fairly stiff, are the least stiff among contemporary road bikes.
The Bicycle Frame Defined
A bicycle frame is composed of a number of pieces of tubing. In all but rare cases, a frame will have eight tubes composing the frame and three tubes composing the fork.
The front half of the frame is referred to as the main triangle (as a result of the bicycle’s appearance in profile) even though, technically, it includes four tubes. Those tubes are the top tube, which defines the bicycle’s size; the down tube, which defines the ride quality of the bicycle; the seat tube, into which the seatpost clamps (this extendable feature allows one bicycle to fit a number of different riders); and the head tube, into which bearings are inserted and through which the fork passes and allows the bicycle to steer.
The rear half of the frame is referred to as the rear triangle. Four tubes make up this triangle. They are the chainstays and the seatstays. The chainstays and seatstays work in tandem to hold the rear wheel in place.
The fork is composed of three tubes. They are: the steerer, which passes through the frame’s head tube and headset bearings; and the fork blades, which extend down either side of the front wheel to hold it in place.
The ends of the fork blades and the junction of the chainstays and seatstays feature parts called dropouts. They give the wheels a precisely controlled location into which to be clamped.
Metallurgy for Cyclists
Steel, aluminum, and titanium share many basic properties. What makes each behave differently as a frame material has to do with the following factors:
Density: Density explains a material’s relationship to gravity. High density equals high weight. Steel is the densest of the three materials. With a weight of 495 pounds per cubic foot (lbs./cu. ft.), it is nearly twice the density of titanium (280 lbs./cu. ft.) and three times the weight of aluminum (165 lbs./cu. ft.). In an ideal world, the best frame material would have high strength with low density. This would allow the material to be drawn with very thin-walled tubes and yet enjoy great strength against damage.
Diameter: A tube’s diameter will vary according to its placement in the frame. The tubes that make up the front half of the bicycle are the largest in diameter. The tubes in the rear half of the bicycle are much smaller in diameter and taper in size as they approach the rear wheel. As a tube increases in diameter or in wall thickness, its stiffness increases. Increasing a tube’s wall thickness increases its stiffness at a roughly 1:1 ratio with its weight. Increasing a tube’s diameter, however, increases its stiffness at a cubed rate. If you double the diameter of a given tube, the new tube will weigh twice as much but be eight times as stiff as the smaller tube. This is why thin-walled, large-diameter tubing is so popular; it is possible to create an aluminum tube that is both stronger and lighter than those made from steel. As diameter goes up and thickness goes down, the ability to resist dents goes down—so there is a limit to the diameter/thickness ratio.
Elongation: This is a measure of a material’s ability to stretch, known as ductility. Chewing gum is ductile. This might not seem helpful to a bicycle, but a material with no elongation will break without ever stretching. Think glass. Glass has almost no elongation and as a result is brittle. Bicycle tubing needs to bend before failing. Some elongation is helpful. If elongation drops below 10 percent, builders tend to get concerned. Steel is usually 10–15 percent, while titanium is 20–30 percent and aluminum is 6–12 percent.
Fatigue Strength: This is the point at which tubing fails after many repeated load cycles; think of a load cycle as anything that causes the bicycle tube to flex. A basketball flexes each time it is bounced; each bounce constitutes a load cycle. The stress of the load is less than the material’s ultimate tensile strength (see below). Both steel and titanium are appreciated for having what is known as an endurance limit, that is, it is possible to apply a load an infinite number of times without the material failing. That is not the case with aluminum. Sooner or later it will break.
Stiffness: Also known as Young’s Modulus (E), the modulus of elasticity dictates the stiffness of a given material. Take two tubes drawn to the same dimensions: 35 mm in diameter, 60 cm long and with a 1 mm wall thickness. If one is made from steel and the other from aluminum they will vary in stiffness. If they are both steel, but made from different alloys, they will share the same stiffness.
Ultimate Tensile Strength (UTS): This is the absolute strength (measured in ksi, or thousands of pounds per square inch) of the tubing—the point at which it breaks. The UTS designates the force required to make a tube break. This determines how thin the wall of the tubing may be drawn. All things being equal, a thinner-walled tube is lighter and that can make a frame weigh less.
Wall Thickness: Steel tubing is made so that the ends of the tubes are thicker than the middle. This is also done with some aluminum tubes, and can be mechanically performed on titanium tubes (by lathe machining). Tubes that are thicker on the ends than in the middle are said to be double butted. This is done for several reasons. The high heat used to join metal tubes would damage the tubing if it were too thin. Typically, tubes are most stressed at the ends, so reinforcement there increases strength while limiting overall weight. Also, the thin midsection (most of the length of the tube, in fact) dissipates the transmission of vibration through the tube, making the bicycle more comfortable to ride.
Before World War II, tubes were generally straight gauge, with a 1-2 mm wall thickness. Later, when double-butted tubing was introduced, the midsection’s wall thickness was reduced to 0.7 mm. Butting figures for such a tube are expressed as 1/.7/1, indicating the thickness at each end and the middle. By the mid-1990s, wall thicknesses had been reduced to .7/.4/.7. Today, wall thicknesses on some tubes have been reduced to as little as .5/.38/.5. Aside from the physical difficulty of accurately drawing tubes of very thin walls from such strong materials, there is a reason why bicycle tubing isn’t even thinner: the beer-can factor. If a tube’s diameter-to-wall thickness rises above a ratio of 60:1, the tube is more likely to buckle under load.
Yield Strength: This is the point (measured in ksi, or thousands of pounds per square inch) at which a flexed tube prematurely bends, instead of returning to its original position after the load is removed.