Frame Materials 101

Which frame material is the best?

There is no all-out winner for best material for a bicycle frame: Each of the four commonly used materials has its own personality. Also, WHAT a frame is made of is probably not as important as HOW the material is used to make the frame. The goal of nearly all frame builders is to make a frame that is stiff laterally and compliant vertically, as well as being as light and strong. Stiff laterally so that when pedaling, energy is transferred directly to the tires when accelerating or climbing. Compliant vertically for a comfortable ride. 

The stiffness, weight, and strength of a bike frame depend on many factors, only some of which involve the material.  The optimum frame design for one material will be different for another, because materials vary widely in strength, stiffness, and density (weight). The best aluminum frames have fat, thin-wall tubes and don't deflect much side to side when you sprint.  The best steel frames have smaller-diameter, thin-wall tubes, and flex noticeably in a sprint.  Titanium and carbon frames fall in between.   Experienced cyclists often divide into camps, with steel riders carping about the excess stiffness of aluminum and aluminum zealots decrying the spongy feel of light steel frames. 

Most bike frames are made from either hi-tensile steel, cromoly steel, aluminum, carbon fiber, or titanium. Alloys of steel are the traditional frame material, but the use of aluminum is starting to overtake steel even on entry level bikes. Manufacturers of aluminum frames have evolved their designs to be very durable and with an excellent stiffness-to-weight ratio. Steel has not stood still, with tubing manufacturers becoming more sophisticated in working steel into new alloys and shapes that make it a good choice at any price level. 

All steels have the same inherent stiffness and weight, regardless of strength - Reynolds superb 853 is no stiffer than 1010 (mild steel).   Adding a tiny bit of chromium and Molybendum make it strong enough to "butt" or thinned down in the middle, thus lighter. This alloy is generically called chrome-moly, chromoly, CrMo, Cromo, Chromo, and most quality steel frames use a variant of chrome-moly.

This principle of engineering frames to use less of a stronger, stiffer material is true for all frame materials. The materials themselves are not lighter, it is the way they are used that allows the builder to use less material to build a stronger frame. At one time nearly all high quality frames were made from Chromoly. The recent development of very high strength "air-hardened" steels (like Reynolds 853 or True-Temper's OX-Platinum that gain rather than lose strength as they cool from welding) has made for frames that have a strength to weight ratio equal to titanium frames.

The strength of any type of steel allows builders to engineer a certain amount of flex by using thinner tubes which translates into what riders call a "lively" feel or springiness, something builders using aluminum can't do because if aluminum flexes it will fatigue and eventually fail. Steel frames are also relatively easy and cheap to repair, and the technology has been around for a long time.

Aluminum is the second most common material, and promising to overtake steel as the most popular frame material. It is 12% stiffer and 20% lighter than steel in it's most common bike tubing configurations. Aluminum is rustproof and it dampens vibration 50% faster than steel. It's stiffness (lateral) gives frames a real fast feeling because the transfer of pedaling force is so immediate, but some complain that the same (vertical) stiffness make for an unforgiving, harsh ride . This effect is ameliorated by the now common use of carbon fiber forks or suspension to soak up road shock.

In order to guarantee strength, 6000 series aluminum must be very precisely thermally treated after being welded, then quenched, and then artificially aged (7000 series is more forgiving). An aluminum frame can be made stiffer and lighter than steel because it is not nearly as dense. This is done by increasing a tube's diameter while maintaining the wall thickness, making a tube that is eight times as stiff, but only twice the weight. This "oversizing" of tubing runs the risk of a "beer can" effect if the tube walls are thinned too much. Aluminum's affordable lightness and stiffness make it the first choice these days for bikes with any kind of suspension.

Titanium has an excellent balance of properties for framebuilding, combining durability and lightweight. Titanium alloys are half as stiff as steel, but also half as dense.  The strongest titanium alloys are comparable to the strongest steels.  Stiff titanium frames need larger-diameter tubes than comparable steel frames, but not as big as aluminum.  Titanium is very corrosion resistant, and very light frames can be made stiff enough and strong enough for bigger riders.  Most Ti frames are the 3Al/2.5V alloy (3% aluminum/2.5% vanadium), though a stronger alloy, 6Al/4V (6% aluminum/4% vanadium) is increasingly used. 

Titanium is expensive not only because of the material costs, but because welding must be done meticulously to avoid contamination, and machining must be done precisely.

Carbon fiber is a material made up of non-metallic graphite fiber cloth that is layered together with high strength epoxy resin to form a matrix. Originally used in the aerospace industry, it can have a high strength to weight ratio, but it is quite expensive.

Individual fibers of carbon are tremendously strong and stiff, but they are useless unless arranged in a strong pattern, and held together with a strong "glue" (usually epoxy).  Unlike metals, in which strength and stiffness properties are nearly the same in all directions, carbon fiber composites can be tuned to orient the strength where it's needed (for instance, stiff laterally and compliant vertically).  This is the ultimate frame material for unconventional frames and shapes, as it can be molded and tuned more than any metal.

Metal Matrix is like carbon fiber, but it uses ceramic fibers and the metal (aluminum) acts like the epoxy.

All metals used in bicycle frames are alloyed, or small amounts of other metals are added to give the metal different properties. However, bike people usually refer to alloy as aluminum alloy.

The strength of any type of alloy is not just a function of the alloying, but the also the heating, cooling and mechanical working (rolling, forging, extruding, etc.) to which a material has been subjected. Depending how much a material is heated and how fast it is cooled, it might become harder and more rigid or brittle and soft. This is because the more you heat a metal, the more you change the alignment of the crystalline structure. It is almost always better to use less heat and mechanically work the metal, like in cold forging. The best tubing materials are subjected to optimal heat and mechanical treatments, are very strong.

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STEE           L
The Good
- Best steel alloys are very strong
- Best stiffness overall
- Long-lasting
- Air-hardened alloys make ultra-high strength affordable welded frames
- Steel is the most recycled metal - good for the planet.
- Over 100 years of use in bicycles.

The Bad
- Can be heavy - not the materials for big, light frames
- Rust-prone

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ALUMINUM
The Good
- One-third the density of steel, allowing the forming of large diameter tubes     (resulting  in excellent stiffness)
- Easily formed into aero shapes
- Even cheap frames can be light
- Makes a light frame for a big rider
- Evolved designs have a proven track record.
- Doesn't rust!
The Bad
- One-third to one-half the strength of best steels and titanium (can break)
- One-third the stiffness of any steel, requiring larger diameter tubes
- Modest fatigue strength
- Not easily repaired or straightened
- Big, thin tubes means easy crash damage
- Refining aluminum

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TITANIUM
The Good
- Half as dense as steel, making very light, tough and resilient frames
- As strong as most steels
- Wont rust - no paint needed
- Good fatigue strength
- Makes a light frame for a large rider
The Bad
- Half as stiff as steel (and known to be somewhat flexy)
- Difficult to repair
- Expensive

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CARBON FIBER COMPOSITES
The Good
- Lightest & Strongest
- Excellent shock absorbtion
- Won't rust
- Excellent fatigue strength
- Virtually unlimited design possibilies
The Bad
- Very Expensive
- Strength and Stiffness are design dependent
- Technology still evolving
- Molded styles have limited sizes due to molding costs

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A German magazine, EFBe (Engineering For Bikes) did fatigue tests on several top brands of frames, with some unexpected results. The "cycles to failure" test measured how many times the frame withstood flexing from a machine. If it made it 200,000 times, it did not break. Cannondale's CAAD3 6061-T6 Aluminum frame did not break, outlasting the Columbus SLX tubing bikes. Perhaps the least expected is Merlin's Team Road Titanium braking way before Schmolke's Russian Titanium did. Making Titanium stiff and durable seems to be a challenge. Carbon proved its strength, with Trek's carbon frame surviving and the Time frame the last to fail.