What aluminium alloys are best for bicycle frames?

Weight and stiffness are factors that affect the performance of a bicycle. The aluminium alloy is traditionally used in the extrusion of tubes for bicycle frames, because it suits the application. But it could be possible to achieve even lighter frame weight for a lighter bike, and optimize your production process as well, with an alternative alloy.

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The alloy, which is alloyed with magnesium and silicon, is an excellent choice when welding is required, as is the case with bicycle frames. It provides stiffness, structural strength and toughness, good corrosion resistance and good machining characteristics.

Technically speaking, the alloy, in the EN 755-2 standard and T6 state, can achieve tensile strength of >260 MPa and yield strength of >240 MPa.

Removing a step from the manufacturing process of bicycle frames

Let’s look at the traditional manufacturing process of an aluminium bicycle frame with the alloy, since this alloy is most commonly used for this application.

• Extrusion of an aluminium tube. The thickness of the tube, and consequently its weight, is defined according to the geometry of the bike frame, the maximum mechanical expected loads, and the minimum mechanical properties that the alloy can achieve.
• Annealing. The extruded tubes are annealed to increase their capability to be deformed without breaks, increasing the material elongation, A, to a higher level than the extruded tubes without annealing. The extruded profiles normally have an elongation of around 10-15%, but after the annealing process, this elongation could achieve around 25%.
• Butting and/or hydroforming. The tubes of the frame, once annealed, are shaped through butting or hydroforming to obtain their final geometry. Before reaching their final geometry, these tubes may suffer up to 60% deformation, and for this reason, several production cycles of annealing/butting could be necessary. Newer bicycles have more and more complex geometries that require a material with a high grade of deformation without failure. With a higher strength alloy, the intermediate annealing processes before the butting procedure might not be necessary.
• Welding. The components of the frame are joined through welding to make the complete frame.
• Post-weld heat treatment and ageing. A result of the welding process is that mechanical properties decrease in the heat-affected zones. To recover these properties, the frame must be thermally treated. This thermal treatment must include solution annealing, quenching and ageing.
• Surface treatment. After thermal treatment, the frames are painted to obtain the desired aspect.

A lighter bicycle frame with reduced aluminium thickness

Removing aluminium to lighten the frame of the bicycle, while still achieving the strength and the mechanical properties and characteristics you desire, is probably going to require use of another alloy.

Aluminium companies are continuously developing new alloys by tailoring others, to meet the specific needs of customers in various industries, such as automotive. For manufacturers of bicycle frames, interested in optimizing their processes and lightening their products, such an alloy would require excellent formability and high strength – one designed for manufacturing processes that include an annealing phase before butting or hydroforming.

With such an alloy, the extruded tubes for the frame would have minimum elongation of 25%, which means elimination of the intermediate annealing step, thereby reducing lead time and cost.

In addition, with an optimized chemical composition and metallurgical development, the tailored alloy could offer mechanical properties that surpass the alloy. After the thermal treatment of solution annealing, quenching and ageing, it could reach 310 MPa  in tensile strength and 295 MPa in yield strength.

This would promote the design and production of bicycle frames with lower thicknesses and lower weight – a lighter bike. A bike with a frame different from the others in the market.

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Introduction: The Battle of the Bike Materials

When it comes to choosing a bike, one of the most important decisions you'll make is selecting the right frame material. And, in , if we set aside carbon which is mainly for athletes who are competitive, the two front-runners in the race are aluminium and steel. But, which one is the right choice for you? Let's dive in and compare the pros and cons of each to help you make an informed decision to end up with that perfect bike.

Aluminium: The Lightweight Contender

Aluminium Frame Bike. Image Credit: Road CC

Aluminum is a popular choice for bike frames because it's super lightweight and strong. It's also resistant to corrosion, which is a big plus for those who live in Singapore which is both coastal and humid. Another big plus, is that aluminium is easy to work with. This means that manufacturers can create a wide variety of frame designs in different shapes. On the downside, aluminum frames can be a stiffer. This is both a benefit and a drawback of aluminium. The stiffness of an aluminium frame can lead to a harsh ride and don't absorb road vibrations as well as steel frames. That said, this rigidity also translates to better transfer of power with each pedal stroke, making the bike "more effecient". If you're looking for a bike that's easy to maneuver and won't weigh you down, aluminum might be the way to go.

A good example of a versitile aluminium bike that is a workhouse for commuting would be our very own Unspokin Belt-Driven Frigate.

Steel: The Classic Heavyweight

Classic Steel Frame Fuji Bike: Image Credit Airows

Steel has been used for bike frames for decades and for good reason. It's durable, long-lasting, and has a great ride quality known for it's "vibrancy" and "Springiness". This makes it the material of choice for quality frames such as the Atala Luna and the Tokyo Bike Classic Sport. Steel frames also have a bit of "give," this will help absorb road vibrations and make for a more comfortable ride. So what is the downside? Steel is typically heavier than aluminum, which can make it a bit harder to handle. It's also more prone to rust, so if you're using these bikes as a commuter machine, you'll need to be extra careful in Singapore and keep them dry as much as possible.

Different Shapes and Stylistic Elements

Steel vs Aluminium Frame Shape: Image Credit Mission Bicycles

If Aesthetics are important to you, here's where you need to pay attention. Both aluminum and steel frames come in a variety of different shapes and styles but generally they adhere to these characteristics. Aluminum frames typically have a more aerodynamic design, which can be great for road cycling or racing or if you would like to save effort while commuting (if you would like more tips on commuting by bicycle, be sure to check out our article on 9 Top Tips for Beginner Cyclists in ). Steel frames, on the other hand, often have a more classic and vintage look, which can be great if that's what you're after. That said, steel can also be shaped to look modern while retaining a classic charm, such as in this Marin Nicasio Gravel Bike. This is because both Aluminium and Steel materials can be shaped and manipulated in different ways to achieve a specific aesthetic.

That said, Aluminum frames are more easily shaped into sleek and modern designs, while steel frames can be shaped into more traditional and classic designs.

While aesthetics are important, frame shapes and materials also impact performance. So take into account the type of riding you'll be doing (be it off-road, on road or pavements or a mixed bag) and the look you want to achieve. Which one is the Winner?

Conclusion on whether Steel or Aluminium is right for you

In the end, the debate of aluminum vs steel bike frames is like trying to compare apples to oranges. I like eating them both, and I suspect you like them both too. Both have their own unique characteristics and it's up to you to decide which one fits your needs better. If you were to ask me the question, I would like a steel bike and an aluminum bike but I understand that not everyone has the luxury of having both. So, whether you're an aluminum enthusiast or a steel loyalist, just remember to have fun and enjoy the ride!

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