Heat treatment and material selection are key variables in bicycle frames. In this article, I explained the metals used as frame materials and their heat treatments.
From its invention until today, the bicycle frame has been the main area of development, with the aim of making the frame lighter and thus easier for the bicycle to move faster with the same amount of effort.
When it was first invented, the bicycle was a vehicle without pedals, made of wood, and with wooden wheels. Created by Karl Freiherr von Drais in 1817 , this invention helped Von Drais, a forest officer, navigate the royal gardens more quickly. The role assigned by the European press and its inventor for this invention, called the Drezin , was to replace horses.
The Velocipede was the dreseller's cousin, born in 1865. It was also an entirely wooden vehicle with wooden or metal tires. The pedals were attached directly to the front wheel and, unlike its previous cousin, it was a type of riding machine. He became known as the bone shaker for what he did to drivers on cobbled streets. Closed riding academies were also built in major cities for "Velocipede" riders. In its current state, it was difficult to use and could not be a tool that the public could use.
Metal was first used in the high-wheel bicycle in the 1870s.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 further you could go with one turn of the pedals. This new invention became popular among income-earning young men, as the cost was six months' wages for the average worker.
The bicycle craze, called the "Golden Age of Bicycles", began in 1887 with the introduction of several innovations such as John Boyd Dunlop 's pneumatic tire. Shortly after this, the ruble rear wheel was developed. This led to the invention of counter pedal brakes (coaster brakes) in 1898. Derailleur gears and parking brakes were also developed during this time, although they were slowly adopted.
In the last century, the bicycle frame became the main area of \u200b\u200bdevelopment. The goal is to make the frame lighter, making it easier for the bike to move faster with the same amount of effort. As a result, the first plain carbon steel frames gave way to hardenable 4130 chrome-moly alloys for high-tech bikes. While steel is still used today, most high-end bikes are constructed from aluminum, titanium or carbon fiber to further reduce weight.
Every material has its strengths and weaknesses. Chromium-moly steel (4130) is widely used due to its weldability, formability, strength, ductility and toughness.
Watch out for corrosion!
The alloy is typically heat treated at 870°C and then quenched in an oil bath. Tempering is carried out at 400-600°C depending on the desired strength (hardness) requirement. Post-welding heat treatment (stress relief ) is not required for this material. Since their resistance to corrosion, i.e. rust, is almost non-existent, they must always be protected with a protective layer (paint or coating).
Although the density of steel materials is about two times that of titanium and about three times that of aluminum, a steel frame does not weigh three times that of aluminum. Frame tubes with thinner cross-sections can be used due to their good strength.
Aluminum gets tired!
One of the advantages of steel and titanium over aluminum is that they have a fatigue behavior where a repetitive load below the fatigue limit will not cause damage. Aluminum does not have this ability, meaning that even a very small load applied for a sufficient number of cycles will cause damage. I would also like to remind you that this number of cycles should be around a million.
Two important issues enable the development of all bicycle frames made of metallic materials: Developments in pipe production and welding technology.Aluminum frames, which were first used in the 1970s, were manufactured more widely with the development of TIG (tungsten inert gas) welding.
If your bike is aluminum, it will most likely be 6061 alloy, which can be heat treated. However, if this alloy is welded, it must be solution heat treated and artificially aged to reach full strength. Solution heat treatment is usually carried out at 430 to 540° C for several hours and then ends with cooling to room temperature. Precipitation hardening (aging) is usually done at 120-180°C 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.
It is possible to find bicycle frames made of titanium alloys in the mid-1990s.Although titanium is the "ideal" metal for bicycle frames, it has never found wide use due to its cost and difficulties in manufacturing methods. The biggest obstacle to its production is that titanium is more difficult to weld. Without precise welding procedures, welds can easily become corrupted (contaminated), causing catastrophic damage during use.
The two most popular alloys used in bicycle frames are
3AI - 2.5V [UNS R56320]
6AI - 4V [UNS R56400]
Both alloys must be subjected to solution and aging during the pipe production phase.
More exotic frame materials such as aluminum lithium are also used on a limited basis. These alloys 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 very soft pure aluminum.
Likewise, some frame parts made of magnesium alloys can also be found. It should not be overlooked that heat treatment of magnesium alloys is very laborious and risky.
In this article, I tried to explain the metallic materials used in bicycle frames with their development throughout history, leaving aside aspects such as usage and comfort. Steel, aluminum and titanium are frame materials that have been used for many years. As a cycling enthusiast and material engineer, I cannot distinguish one from the other. The goal should be to have one of each.
Comments