Aluminium Frame Tech Explained

Alloy bikes are incredibly popular and much of this popularity stems from their relatively low cost when compared to their carbon, steel and titanium counterparts. But, have you ever looked at bikes online and wondered just exactly what the descriptions of a frame’s tubing actually refer to? Read our Aluminum Frame Tech Explained blog to find out more.

The development of alloy

Throughout much of the 20th century, Steel was the only option when it came to buying a new bike. The status quo was about to be changed forever when in the mid-’70s strange new bikes started to appear on the market. Considerably lighter than the steel counterparts they created quite the storm in cycling circles. This was, of course, the arrival of aluminium and it was even then being recognised as a viable lightweight alternative to the trusty steel push-hog.

These early examples of alloy frames were constructed along the lines of the contemporary steel frames of the era. Sections of straight gauge tubing were ‘glued and screwed’ together. The result was a bike that had a very comfortable ride ‘feel’ but would also flex alarmingly, particularly when descending or sprinting.

By the latter stages of the 20th century, the next evolution in aluminium frame design was starting to take shape. The next-generation of alloy frames were constructed of lightweight, large diameter, straight gauge aluminium tubes. The resulting ride was the complete opposite of the earlier attempts.

Rather than a frame that rode comfortably with too much flex, these new designs were lighter but bone-shakingly stiff. Because of this aluminium frames gained a reputation for being a harsh ride over rougher surfaces. A reputation that they have not quite shaken off today, even when refinements in frame construction have added compliance for a much more comfortable ride quality.

The classic 7005 Audax was one of the first alloy frames that Ribble released way back in the late ’90’s. Such was the regard for this machine it remained on sale for nigh on 20 years with minimal design updates.

Tig Welding

The ’80s and ’90s saw the major cycling manufacturer’s make the first forays into the realm of TIG (Tungsten Inert Gas) welding. TIG welding is a process that utilises 3 component parts;

  • a non-consumable tungsten electrode is used to produce the weld.
  • Inert shielding gas is used to prevent contamination.
  • A filler material is used to fill the weld.

Due to the extra stresses applied to the welds, most high-quality alloy frames will have smooth welds (also known as seamless welds) to avoid stress concentration.

High-quality aluminium frames such as this CGR AL have seamlessly smooth welds for a much cleaner appearance and improved strength.

Tube Shaping

The CGR AL has a double-butted downtube for aditional strength at the bottom bracket and headtube junctions.

Thanks to its low-density, aluminium is a relatively simple material to work with and can be easily shaped to suit different ride disciplines. A typical example would be an alloy TT bike with oversized tubing for improved aerodynamics. The tube profiles will normally be achieved through hydroforming. A process where a tube ‘blank’ is placed inside a die. Fluid is then forced through at incredibly high pressure which forces the tubing to assume the shape of the mould it is sat within.

As well as shaping of the tube, it is also possible to vary the thickness of the walls of the tubes to improve strength. This process is known as ‘butting’, there are 3 types of butting which are;

  • Single butted – One end of the tube is thicker for increased strength at a join. A prime example would be the seat tube where the strength is required at the bottom bracket junction.
  • Double-butted – The tubes are thicker at each end to provide additional strength at the junctions but thinner in the middle to save weight. The down tube is most commonly double-butted.
  • Triple butted – Tubes will have 3 different wall thicknesses.
This simple graphic highlights the differences between the different types of butting.

Aluminium today

Compared to steel, aluminium alloys have a lower density and lower strength. You would, therefore, think that steel would be far superior, right? Well, not quite, you see, alloys actually have a superior strength to weight ratio. This allows more scope for experimentation with the thickness of the tube walls. The walls can be strengthened simply by increasing their thickness yet still remain lightweight. The end result is a frame that is much stronger and stiffer whilst also remaining lighter than their contemporary steel counterparts.

A modern alloy frame will commonly have a tube wall thickness twice the diameter of a comparable steel frame, use twice as much material in its construction and the overall tube diameters will be approximately 20-30% larger to provide the required level of stiffness. The two most common alloys in use today are 6061 and 7005. Throughout the Ribble range, you will see 6061 feature heavily, or more specifically the T6 variant.

The T6 designation refers to the tempering process and contains magnesium and silicon as the major alloying elements. This precipitation-hardened (heat-treated) aluminium alloy has a very high tensile strength which results in a very tough, yet light frame.

Today alloy bikes remain very stiff but advances in materials and construction have led to many improvements in their ride feel. One of the biggest of which is the dropped seat stay’s that feature on the Ribble range. The extra length of seat-tube that this creates allows for a little bit of flex which helps to absorb vibrations form rough surfaces.

Aluminium bikes like the Endurance AL Disc continue to be as popular today as in their 90’s heyday but bow benefit from additional compliance that improves comfort.

The incredible story of 2 best friends taking on the Himalayas on the Ribble Adventure 725. Read about it here.

Ribble Cycles were established in1897! Read about the intriguing journey here.

  1. Thanks for the article.
    It is really interesting to read about the history and development of aluminum frames.
    The explanation of the double and triple butted sections would suggest that each tube would need to be made bespoke for each frame size rather than cutting off the shelf tubes?
    It adds another level of respect to the frame builders skill, and a greater insight as to how my CGR Al was designed and built.
    Thanks again.

  2. Excellent article. I’ve been buying bits and pieces from Ribble since the mid 1980s, but recently I have lost touch so it’s good to bring myself up to speed.

    My only criticism is that you didn’t discuss why you use carbon forks. Could this be added to the article?

    Many thanks

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