Aluminum frame bikes are among the best selling models and according to statistics they represent the segment with the biggest sales volume on the market.
It is quite natural to be so, since poor quality steel is an obsolete material (however the story is radically different with hi-quality steel), carbon can be expensive sometimes and titanium is not so easy to find. Below you can read an extensive presentation of the two aluminum types widely used in cycling industry, 6061 and 7005, and of the various technologies enhancing their performance.
There are two main reasons why 6061 and 7005 are the most used aluminum alloys in cycling industry: their availability and their properties. Two other alloys, 6066 and 6069 are also used, but on a smaller scale, at least nowadays. Even scandium frames were actually aluminum frames alloyed with a small amount of scandium, and still remained too expensive. There are also other types of aluminum, such as 7075 and 2014 which are very strong, but almost impossible to weld.
6061 is an alloy that consists of aluminum, magnesium and silicone and is considered to be superior to 7005, made of aluminum and zinc, although the latter appears to be more resistant. 7005 aluminum has a resistance to failure of 51,000 psi, compared to the 45,000 psi recorded for 6061 aluminum while also being more resistant to squeeze. However, these are just laboratory obtained data, after tests performed on different aluminum tubes. When it comes to bicycles frames, the differences between the levels of resistance are also influenced by factors such as welding quality, tube shaping and thickness.
Experience has proved that there are indeed differences between 6061 and 7005 alloys but they are hardly to be taken into account. It’s very unlikely that a usual cyclist will ever put his frame to such pressure that the limits of the two alloys will come into play.
But then, why is 6061 considered superior to 7005? Because the 6061 alloy is easier to handle, reducing frames production costs. Basically, 6061 aluminum allows manufacturers to obtain lighter frames which are also better designed. And last but not least technologies and machinery needed to process 6061 alloys have become cheaper in recent years, which has some obvious implications.
Furthermore, it is necessary to clarify another two terms we often meet for aluminum frames: butted and tapered.
What do single butted, double butted and triple butted really mean?
The term butted refers to the thickness of tube wall. If tubes have only one dimension along them, we refer to the frame as being butted, if it they have two dimensions (being thicker at the tube ends and thinner along the middle) then we are talking about double butted frames. Finally, if tubes have three different wall thicknesses along their length, then then frame is described as being triple butted. This technology is successfully applied to reduce the amount of material used for a frame, decreasing its weight by 15%.
Tapered refers to the conical shape of head tube (the part of the frame housing the fork’s steering tube). To put it as simply as possible, the top of the head tube has a smaller diameter than its bottom. Thus, manufacturers obtain a higher resistance in this area of the frame, a higher rigidity and a better stability of the bicycle.