Aluminum alloys are particularly with the advent of mountain bikes from America strongly emerging. The explanation for this is not directly in the light weight. Although the density of aluminum is 3 times as low as that of steel, the rigidity is also three times as low. Also, the strength of the various aluminum alloys is about three times as low as steel alloys. A tube of an aluminum alloy have to be three times as thick at the same outer diameter as the same steel tube to achieve the same strength and stiffness. Identical frames are roughly of the same properties in terms of strength and stiffness.
The advantage of aluminum alloys mainly lies in the possibility to use oversized bicycle tubes. An aluminum tube having a diameter of 50 mm and a wall thickness of 1.2 mm is the same weight, strength and stiffness as a steel tube with a diameter of 50 mm and a wall thickness of 0.4 mm. However, the steel tube is much more vulnerable because of the very small wall thickness. The mountain bike frames are often built with oversized tube of aluminum alloy. This is an opportunity that traditional manufacturers of aluminum frames like ALAN and Vitus have not been recognized. Because they (or traditional clients from the world of racing) continued to adhere to the standard tube diameters of race frames, the benefits of aluminum alloys were not used.
As with steel strength of aluminum can be increased by the addition of various alloying elements. The main additions are copper, magnesium and zinc. Aluminum alloys are distinguished by an American designation in thousands. This figure is used to indicate the composition of the alloy. The aluminum alloys with a peak strength have a serial number in the AA7000 series and AA2000 series. The strength of many aluminum alloys is increased by giving them a specific heat treatment. Due to the aluminum alloy to be heated at a certain temperature solve the alloying elements in the aluminum, then the aluminum is cooled at a certain speed at which the alloying elements themselves during the cooling connect in a specific way with the aluminum atoms. This strengthening of aluminum process known as precipitation hardening. The temperature at which the alloying elements must be resolved and the cooling rate varies considerably from aluminum alloy.
The weldability of aluminum alloys is very different.Within the AA7000 series are easy to find weldable alloys deteriorate only 20% in strength.The precipitation-hardened alloys go much more strength back.These are then also preferably glued.It is difficult, on the basis of the indicator to give an estimate of the strength of the aluminum alloy.Welded frames made of aluminum alloys are almost all of AA 7020 or AA 7005.The yield strength of aluminum alloys is used after welding at 250 N / mm². This is similar to the specific strength of cromobyleen steel.For bonded aluminum frames are aluminum alloys used in the 5000 and 6000 series.This aluminum alloy has a yield strength of approximately 290 N / mm².AA 7075 is of aluminum alloys 5010 N / mm², the highest yield strength. This alloy is badly weldable and is only used in glued frames.
Even though aluminum oxidises more quickly than the steel most aluminum alloys do not have to be protected against oxidation.
The oxidised alumina layer which is formed on the aluminum is in fact air-tight so that the lower aluminum can not be compromised.
The protective aluminum oxide layer is extremely thin, but can be substantially thickened and strengthened by anodizing the aluminum.
With Elektrolyse the aluminum layer is in the anodizing in an artificially way made thicker.
Also, the oxide layer can be colored.
Aluminum which is welded can not be anodized.
At the weld arise color differences.
An attractive appearance is obtained by the high aluminum polishing.
By the addition of copper and zinc to deteriorate the corrosion resistance. Aluminum alloys of the AA 2000 and AA 7000 series, however, are also moderately resistant to corrosion.
Frames of the aluminum alloy will be also lacquered.
An important advantage of aluminum is the easy producibility of all kinds of parts. Aluminium can easily be extruded in all kinds of complex and simple profiles (for example, bicycle wheels, hub sleeves, seat posts, etc.) It may be injection molded under high pressure, and it can easily be forged. This makes it especially widely used in all kinds of bicycle parts such as rims, hubs, deraulleurs, brakes and cranks. As a material all aluminum alloys can be used substantially. However, the most important alloy used is AA 6061 because it is very readily deformable, and has high corrosion resistance.
A disadvantage of aluminum is that it is brittle and can break. This brittle fracture is caused by a combination of two causes:
- Aluminium has no endurance. This means that an aluminum construction, after a certain number of loads will always break. Under normal circumstances, this is not a problem. Aluminum parts are designed so that they only succumb after a load cycle which is far above that of the life of the bike.
- Aluminum is not hard which easily scratch the material originated. When the scratches can occur through corrosion large stress concentrations which the component can break with much less load changes. Many aluminum steerer tube at the clamping been broken by this. Aluminum parts must therefore be checked more critical for scratches. Also includes an aluminum frame must never be bent. By bending the aluminum can that weaken or heavily taxed fatigue which occurs at that spot. Upon confirmation of aluminum parts should also be cautious proceed. If an aluminum part is attached too tightly, that part is constantly under great tension. As a result of varying loads fatigue can occur then . Aluminum cranks should not be too tight on the bottom bracket, otherwise the tension is too high in the crank.