In this SEVEN Tips, we share Seven Cycles’ perspective on the "types of titanium alloys" that determine the performance of a titanium frame. We hope you continue to enjoy the depth of Seven’s proud titanium technology and manufacturing philosophy.
Titanium Quality Determines Frame Longevity
The quality and strength of titanium tubing, much like steel tubing, vary widely. How and where it is manufactured significantly affects its performance. Titanium tubing produced under high-level quality control results in a long-lasting frame.
Three Factors Affecting Tubing Quality:
- Texture (grain orientation)
- Surface finish (both internal and external)
- Presence of surface defects (due to chemical reactions)
These three are critical factors that determine the fatigue strength (the ability to withstand repeated loading) of a frame. Tubes with smoother surfaces and no defects are more durable and can withstand long-term use.
Titanium, in particular, is a material more susceptible to micro-defects on the surface than iron (steel). Small scratches or chemical alterations on the surface make it prone to the onset of fatigue failure. This applies not only to the outside of the tube but also to its internal condition.
Why is the inside also important?
In bicycle titanium tubing, the internal condition is vital for fatigue durability. For example, if extremely thin tubing is used, compressive and tensile forces repeatedly occur between the outer and inner diameters, making micro-cracks prone to forming. As these cracks progress, they lead to the failure of the tube itself or its joints.
Furthermore, there have been cases where, if the internal texture (grain orientation) is coarser than the outer, failure begins from the inside.
The Only Way to Prevent Defects: Quality Control Through Machining
If there are scratches on the surface or chemical damage, the quality of the tube is decisively compromised. The only way to prevent such defects is to thoroughly perform high-precision machining and inspection, such as with a milling machine.
About Types of Titanium Alloys
The bicycle industry primarily uses the following three types of titanium materials:
- CP Titanium (Commercially Pure Titanium)
- 3AL-2.5V Titanium Alloy (commonly known as 3-2.5)
- 6AL-4V Titanium Alloy (commonly known as 6-4)
Among these, 3-2.5 titanium alloy is considered the most mainstream and superior material in current titanium frame manufacturing.
What is 3-2.5 Titanium Alloy (3AL-2.5V)?
This alloy consists of the following components:
- Aluminum: 3%
- Vanadium: 2.5%
- Pure Titanium: 94.5%
Seven Cycles believes that using high-quality 3-2.5 seamless tubing is the best choice in every respect, including durability, workability, and corrosion resistance.
What is the ASTM B-338 standard?
In the United States, 3-2.5 titanium tubing is typically manufactured according to "ASTM B-338," a standard for the aerospace industry. This is originally a high-durability standard designed for hydraulic piping, and criteria such as titanium purity and strength are strictly defined.
- Aerospace Grade: The highest standard, used for aircraft as well.
- Sports Grade: A grade that omits some processing steps to reduce costs. Its performance is slightly inferior.
Some frames are manufactured using "scrap material" that does not meet these standards, but these lack a mill test report (quality certificate). In other words, their reliability and safety cannot be guaranteed.
Quality Classification of 3-2.5 Titanium Tubing
In the United States, 3-2.5 titanium tubing is roughly classified into three grades. Here is an explanation of the characteristics and differences of each.
① AMS Standard 105 (Aerospace Grade)
This grade is a material that meets the highest level of quality standards also used in aircraft (AMS 4943 or AMS 4944). It is a highly reliable material adopted for large passenger aircraft such as the Boeing 747.
When using materials of this AMS standard, bike designers can freely specify diameters, wall thicknesses, etc., for custom orders. However, in reality, the minimum order quantity is very high and the lead time is long, so only frame manufacturers with a certain scale, like Seven, can use this material.
② Sports Grade
Sports grade is a specification that simplifies some of the processing steps of the aerospace grade to reduce costs. While this keeps the price down, it has the drawback of being prone to unevenness in the smoothness of the inner and outer surfaces and the grain alignment (texture).
It is somewhat unsuitable for applications requiring particularly high formability (the ability to bend or process the tube easily) or reliability after welding.
③ Scrap Material
This "scrap material" refers to surplus, non-conforming tube offcuts, or off-spec products produced during the production process of aerospace or sports grade materials.
- No mill test report (quality certificate) is issued.
- Chemical composition and strength are unknown.
- The material is inconsistent, making it prone to trouble during processing.
For these reasons, it is a material that should not be used to build a reliable product, but unfortunately, there is a fact that some builders use it to keep costs down.
What about Seven’s Proprietary Tubing?
Seven Cycles develops and uses proprietary standard titanium tubing that does not belong to the three classifications mentioned above.
This original tubing requires even stricter straightness and surface precision than the AMS standard, and it is manufactured and quality-controlled with performance that exceeds minimum strength requirements.
We will provide details in the next installment, the "Seven Original Tube Set Explanation Edition."
About 6AL-4V Alloy (6-4 Titanium)
6-4 titanium is a high-strength titanium alloy with the following composition:
- Aluminum: 6%
- Vanadium: 4%
- Pure Titanium: 90%
This alloy is used for small parts such as dropouts, rather than the frame body itself.
Why is it suitable for small parts?
6-4 titanium is very hard and durable, making it optimal for small parts. However, there are many problems with using it as tubing.
[Problem 1] Manufacturing costs are very high
If you try to process 6-4 titanium with the same equipment as 3-2.5 titanium, the machinery wears out severely, and manufacturing costs skyrocket.
Furthermore, it is difficult to ensure the precision (uniformity of thickness and central axis) of the inner and outer surfaces of the tube, and the finish is prone to variations.
[Problem 2] "Seamless" tubing does not exist
Currently, there is no factory in the United States that manufactures 6-4 seamless tubing.
The 6-4 tubing available on the market is solely welded tubing made by rolling a sheet of material and welding it. However, this method has two major drawbacks:
① Welding defects are prone to occurring
The long weld line is prone to minute misalignments or defects, which become points of stress concentration. This is a major risk for frames requiring durability.
② The properties of the material are not suitable for frames
6-4 titanium made in sheet form is originally intended for use as plate material, not for tube forming. Therefore, it has the problem of being weak to metal fatigue caused by repeated stress (such as pedaling).
Actual tests have proven that 6-4 titanium tubing has a shorter lifespan and is easier to break than 3-2.5.
Supplies from overseas are also limited
Recently, small quantities of seamless 6-4 titanium tubing have been entering from outside the US, but
- There are only 2–3 sizes available
- The positions of internal butts (*) are also limited
The degree of design freedom is extremely low, and it cannot accommodate diverse frame designs.
* [Butted tubing]: A structure where the center of the tube is thin and both ends are thick. Allows for both weight reduction and strength.
Moreover, current 6-4 butted tubing technology has not received clear evaluation regarding torsional rigidity or durability.
Conclusion: 6-4 is a "money pit"
Some builders claim that you can "build a better frame" by combining 3-2.5 tubes and 6-4 tubes.
In reality, however,
- There is no weight advantage
- It is impossible to balance stiffness and suppleness
- Torsional rigidity is low, negatively affecting the ride quality
For these reasons, the current reality is that the level of completion as a high-performance frame actually decreases.
What is CP (Commercially Pure) Titanium?
CP Titanium, commonly known as "pure titanium," is an industrial pure titanium material that contains no alloys. While it may appear high-quality at first glance, it actually carries many challenges as a frame material.
Why is CP Titanium unsuitable for frames?
-
Strength is extremely low:
Even at its highest estimate, it only has about half the tensile strength of 3-2.5 titanium. -
Tends to be heavy:
Because you must make the pipes thicker and wider to achieve the same strength, the frame becomes heavy as a result. -
Strength inferior even to steel:
Compared to high-quality chromoly steel, the strength of CP titanium is inferior, and it is by no means a material aimed at high-performance bikes. -
Industrial use:
In reality, CP titanium is primarily used for applications that prioritize corrosion resistance, such as chemical plants, medical instruments, and marine equipment; it is not a material developed for bicycle frames.
In Conclusion: Why 3-2.5 Titanium is the "Only Correct Answer"
The evolution of titanium alloys has developed alongside the needs of high-tech industries such as the aerospace and defense sectors. The material chosen amidst such strict requirements is precisely the 3-2.5 titanium alloy.
The fact that 6-4 titanium seamless tubing is not mass-produced at present also indicates that there is almost no industrial demand for this material.
6-4 Tubing is Not Adopted Even in High-Tech Industries
At Seven, we investigated the usage of small-diameter tubing used in cutting-edge applications such as the aerospace, military, and medical industries. As a result, we found that small-diameter 6-4 tubing is not used in any of these industries.
The reasons are as explained so far:
- Difficult and expensive to manufacture
- Lacks quality stability
- Weak to metal fatigue
This is because these fundamental problems have not been solved.
Summary
✅ 3-2.5 Titanium Alloy is the most suitable material for modern titanium frame building in terms of strength, durability, workability, and reliability.
✅ While 6-4 Titanium Alloy may be effective for some applications, it has many issues regarding practicality and reliability as tubing material, and is too costly.
✅ CP Titanium (Pure Titanium) lacks the strength required for use in bicycle frames and tends to be heavy.
Based on these facts, Seven Cycles develops proprietary, high-quality tubing that does not belong to any existing grade, and reflects this in our products.
Please continue to enjoy Seven's Titanium Technology & Manufacturing Methodology.