Aluminium Grades for Bicycle Components

Do you ever glance over the specifications of cycling related products and notice material codes such as 7075-T6 Aluminium, 304 stainless steel, Grade 5 Titanium or T700 carbon fiber?

These codes refer to the grades, specifications, chemical makeup and performance characteristics of the material. Not all grades of materials are created equally, and it's important to note that just because something is made from carbon fiber , it might not have the performance you may expect it to have. Each grade of material has properties best suited for different applications, however sometimes corners are cut for manufacturing efficiencies and costs.

I have put together this handy guide explaining some of the differences in materials, their properties, what applications are best suited for them and what to look out for in purchasing new parts for your bike.

In this article we are looking at Aluminium alloys.

Aluminiums alloys are man-made material made up of various elements from the periodic table (predominantly aluminium (element 13) hence the name Aluminium alloys). Pure aluminium on its own is a very soft and weak material, and isn't very suitable for many industrial applications. To achieve certain requirements such as corrosion resistance, strength in certain applications, weight and malleability (how easily it is deformed) Silicone and iron is added to it to improve its performance. This combination of elements is what we call an alloy.

There are hundreds of different grades of aluminium alloys, each with a different set of properties designed for certain tasks. For example, a marine grade aluminium has great corrosion resistance against salt from the ocean because of its high Magnesium content, where if a grade with a low Magnesium content were to be subdued to the same conditions it would deteriorate much quicker in the same environment. 

The two most common aluminium alloys you will find in the bicycle industry are 6061 and 7075 grades. 

Composition

The first difference between 6061 and 7075 can be found right away just by looking at their number designations. 6061 is in the 6XXX series of aluminum alloy grades and 7075 is in the 7XXX series. Knowing this, without even digging into their individual material data sheets, it can be deduced that 6061 will have a higher amount of silicon, and that 7075 will have a much higher amount of zinc.

Workability

6061 nearly always has the edge over 7075 when it comes to fabricating the two aluminum alloy types. This is mostly due to 6061 having a lower hardness and tensile strength. The lower hardness allows it to be machined more easily than 7075. The lower tensile strength means that 6061 is easier to form than 7075. While both materials can be joined by soldering, brazing or adhesives, 6061 is weldable and 7075 is generally considered not weldable. Even though 6061 is considered weldable

Aluminium Alloy Heat Treating and Ageing

Many grades of Aluminium alloys can be hardened through two process called tempering and aging. This is done by heating up and cooling the material through a controlled process to strengthen the bonds between its molecules. The process makes the material harder and stiffer. A secondary aging process helps microscopic crystals grow within the material to fuse its structure together.

T6 is a grade of tempering commonly found in aluminium components in the cycling industry. 

The T6 temper is usually achieved by homogenizing the cast aluminium alloy at 450°C for several hours, quenching, and then ageing at 120°C for 24 hours. 

Check out this Video by the BBC giving a brief introduction into the heat treating and aging process for aluminiums.

Hardness

Material hardness is how resistant a material is from deformation when pressing into its surface. In the context of chainrings, the harder a material it is, the less energy is lost as the chain presses into each tooth

Hardness is often measured using the Brinell scale (named after a Swedish dude called Johan August Brinell). 

Yield strength and Tensile strength

These two units are very important metrics for a high stress component. 

The yield strength of a material is how much stress the material can take before its deformation can no longer bounce back to its original position. Any deformation that occurs as a result of stress higher than the yield strength is permanent - i.e bent cranks or chainrings have exceeded the yield strength. 

Yield strength is increasingly important with greater torques being applied to components such as long stems and large chainrings. The greater the distance the component is experiencing the force from, the more likely the part is to have permanent deformation.

Tensile strength is the maximum tensile load a material can withstand prior to fracture. It is a measure of a material's resistance to failure under tensile loading. Materials with a higher tensile strength can withstand greater forces without catastrophic failure - Such as that seen at the Tokyo2020 Olympics with the Australian Pursuit handlebars snapping immediately after a high intensity standing start.

Mechanical Properties

When analysing the properties between 6061-T6 and 7075-T6, several noticeable differences are observed. 

7075-T6 compared to 6061-T6 aluminium

• 78% stiffer (Yield Strength)
• 81% stronger (Tensile Strength)
• 61% harder

Data source: https://www.makeitfrom.com/

6061-T6

In my experience, 6061 aluminium is one of the most common aluminium alloys for general purpose use. If you go down to a hardware store and see aluminium tubes or extrusions for sale, it will most probably be 6061. The grade has a great balance for many applications. It is affordable, easily machined, great to weld, and anodizes easily. When tempered to T6, 6061 grade aluminium’s tensile strength increases by 139% from 130Mpa to 310MPa

6061-T6 remains quite a soft material in comparison to other available options for aluminium alloys. Although it has good applications for bike components that are not subjected to massive forces or thread torque ratings, I would not recommend the alloy for use as high stress components such as chainrings or drive components. 

Recomended applications:

• Non high stress components or areas that require extra deformation - such as dropouts for wheel nut bite.
• Budget bicycle components 

Pros:

• Cost Effective
• Easily sourced around the world
• Easily worked
• Light weight

Cons:

• Soft
• Low durability 
• Prone to fatigue or catastrophic failure in high stress applications on a bike
• Low efficiency in wearing components

7075-T6

7075-T6 aluminium is one of the most common materials you will see spec'd for mid-high end bike components. 

When tempered to T6, 7075 grade aluminium’s tensile strength increases by 133% from 240MPa to 560MPa

Although near double the price of 6061-T6, it is also near double the tensile strength. This makes it one of the best performing alloys available in terms of its strength to weight ratio. 

7075-T6 is quite a difficult alloy to source. Due to its scarcity and high demand it is about twice as expensive than 6061-T6, so if you see 7075-T6 spec components going for cheap prices it is highly plausible that it isn't actually 7075-T6 grade. 

One major downside of 7075-T6 is that it is much more difficult to work with than the softer alloys. It is hard to machine cleanly and doesn't anodise easily. Much more time and care is required to manufacture products from it.

We use 7075-T6 aluminium for our Track Chainrings, Elite Track Chainrings and Elite Longboi Stems or anything that has high stress requirements or threaded torque ratings. We source it from the USA as we don't have the smelting industries here in New Zealand to produce these higher grade materials locally.  

Recommended applications:

• High stress applications such as chainrings, stems and cranks

Pros:

• Great strength to weight ratio

Cons:

• Difficult to machine
• Difficult to anodise and coat
• Expensive
• Hard to source

Related Articles:

Carbon Fiber Grades for Components (Coming Soon)

Titanium Grades for Bicycle Components (Coming Soon)

Stainless Steel Grades for Bicycle Components (Coming Soon)

Coatings for Bicycle Component Efficiencies and Durability (Coming Soon)