There’s nothing quite like a bicycle ride in the country on a nice autumn day. Not all bikes are created equal, however. Heat treatment and material selection are the key variables in bicycle frames. We will do our best to sort it out for you.
The first bicycle was actually a walking – not a riding – machine. Created in 1817 by Baron von Drais, this invention helped the Baron to more quickly navigate the royal gardens. This early cycle was made entirely of wood and never became anything more than a fad.
The velocipede was the next iteration in 1865. It was also an all-wood device with either wood or metal tires. Pedals were applied directly to the front wheel, and it was a riding machine unlike its earlier cousin. It became known as the bone shaker because of what it did to the rider on cobblestone streets. Again a fad, indoor riding academies were built in larger cities for riders of the velocipede.
Metal was first introduced to the bicycle in the 1870s with the high-wheel bicycle. Like its predecessor, the pedals were attached directly to the front wheel. But unlike the velocipede, the front wheel grew in size because the larger the wheel, the farther you could travel with one rotation of the pedals. This cycle became popular with “young men of means” because the cost was six-month’s pay for an average worker.
The bicycle craze called the “Golden Age of Bicycles” was started by the initiation of several new innovations such as John Boyd Dunlop’s pneumatic tire in 1888. Soon after this, the rear freewheel was developed, which allowed the rider to coast. This led to the 1898 invention of coaster brakes. Although they were adopted slowly, derailleur gears and hand brakes were also developed during this time.
In the last century, the bicycle frame has been the key area of development. The objective is to make the frame lighter so that the bike is easier to move with the same amount of effort. As a result, the early plain-carbon steel frames gave way to hardenable 4130 chrome-moly alloys for the high-tech bikes. While steel is still being used, many of the high-end bikes are utilizing aluminum, titanium or composite materials to further reduce the weight.
Each material has its “strengths” and weaknesses. Chrome-moly steel (4130) is commonly used because of its weldability, formability, strength, ductility and toughness. The alloy is typically heat treated at 1600°F followed by an oil quench. Tempering is performed at 750-1050°F based on the desired strength requirement. No post-weld heat treatment is necessary with this material. Although titanium is half the density of steel and aluminum is about one-third the density, a steel frame does not weigh three times as much as aluminum. One of the benefits of steel and titanium over aluminum is that they have a fatigue limit below which a repeating load will not cause failure. Aluminum does not have this endurance limit, meaning that even a very small load applied for enough cycles will result in failure.
If your bike is aluminum, it will most likely be 6061 alloy, which is heat treatable. If this alloy is welded, however, it needs to be solution heat treated and artificially aged to return it to full strength. Solution heat treatment is usually performed between 800 and 1000°F for a number of hours and quenched to room temperature. Precipitation hardening (aging) is usually done at 250-350°F for 8-36 hours. Other aluminum alloys, such as 7005, do not need to be solution heat treated after welding, but artificial aging is required.
While titanium might be the “ideal” metal for bicycle frames, it has never become popular due to its cost. Another complication with its use is that titanium is more challenging to weld. Without precise welding procedures, welds can easily become contaminated, resulting in catastrophic failures.
Even some of the more exotic frame alloys, such as aluminum lithium, require a critical heat-treatment process. If the alloy is heated for too long or at too high a temperature, the lithium can oxidize, leaving a much-too-soft pure aluminum. Carbon composites have increased in popularity for the high-end bikes, but we will limit our discussion to metallic frames.
By all means enjoy your bike ride wherever you may be. As you take in the sights and sounds, remember that thermal processing has a behind-the-scenes role in your adventure.
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