What Grade Titanium Do We Use In our Bikes?

By Noel Buckley

This seemingly simple question comes up fairly often from customers interested in purchasing the Cache Titanium or Tyaughton Titanium. Usually this is because there is a perception that Grade 5 Titanium (6Al-4V) is the superior choice and that Grade 9 Titanium (3AL-2.5V) is a cheaper (hence inferior) choice.

However this is not accurate, and is really based on the custom frame builder world where builders are using stock tubes provided by their suppliers and the builders are limited to what tube sets (grades and shapes) are available.

Grade 5 Titanium’s main advantage is that it is stronger (it has a higher tensile strength) than Grade 9 and hence can be used to make tube sets with thinner wall sections which reduces total frame weight slightly.  

Let’s compare two dimensionally similar tubes (ie. a top tube): one made from Grade 5 Titanium and one made from Grade 9 Titanium.

In this example these tubes are 700mm long and 37.5mm in diameter. They are butted at each end (i.e. thicker walls) and thinner in the middle section.  

You’ll notice that the table lists the strength of both tubes as “similar” but earlier we said that Grade 5 Titanium is stronger.  Why is this?  It's because our design goal for the tube’s functionality is to ensure that regardless of material, the tube needs to be able to support the forces it’s expected to experience.

This means that both tubes need to be strong enough to do their job. Grade 5 Titanium’s increased tensile strength allows it to be made with thinner walls than the Grade 9 Titanium tube to support the same load.

This is where the reduced weight comes in for the Grade 5 tube. In this example to achieve a similar strength, the Grade 9 tube requires around 370g of material whereas the Grade 5 tube needs only 300g of material due to its higher tensile strength.

This 70g of weight savings is where the perceived higher quality of Grade 5 Titanium comes from and if you are building something like a road bike with straight cylindrical tubes, using Grade 5 Titanium main triangle tubes allows for a 150-250g weight reduction in total frame weight.

Difference in Stiffness Between Grade 5 and Grade 9 Titanium

While Grade 5 Titanium does have a very slightly higher stiffness than Grade 9 (slightly higher Modulus of Elasticity), the difference is slight and the real difference in tube stiffness comes down to geometry much more so than titanium’s modulus.

A tube’s stiffness is determined by – not the tube’s diameter and not even the square of the diameter – but by the cube of the diameter. This means that even a small increase in a tube’s diameter will make a noticeable difference in its stiffness. Of course as tube diameter increases, weight also increases and you need to maintain a minimum wall thickness to avoid buckling failure.

Advantages of Forming and Tapering

Grade 5 Titanium it is best used in two configurations: the first is in billet form, and the second is in simple geometry tubes. We use Grade 5 for all of our CNC’d parts in our frames such as the dropouts, chainstay yoke, down tube cable access port, etc…

Simple tube design means tubes that are cylindrical in shape and have only minimal forming. Due to Grade 5’s Titanium’s limited cold working ability, it needs to keep its original shape and can only handle modest forming operations (minimal changes in wall thickness and perhaps a slight bi-ovaling of its outer shape). Aggressive tapering or bending are out of the question for Grade 5 Titanium as it will fracture during such forming operations.

The advantage of Grade 9 Titanium is that it can be cold worked a lot more than Grade 5 Titanium. This allows a much wider range of shapes that Grade 9 Titanium can be formed into, including aggressive butting and extensive shaping operations such as tapering, bending and forming operations.  

Trading Shape for Stiffness and Weight in Grade 9 Titanium

Where the advantage of Grade 9 Titanium really shines is in its ability to make more complicated tube shapes. Now we can design for the perfect blend of shape, weight, stiffness and even durability as well.

Let’s go back to our above simple 37.5mm diameter top tube design at the beginning of this blog post. Now, we’ll add in a tapered top tube as well. This tapered tube’s shape will be optimized for the frame and it will be larger in diameter at the head tube end (let’s say 40mm) and smaller in diameter at the seat tube end (let’s say 35mm).  

Here’s how the two tubes look in comparison to each other:

And this is a section view cut so that we can see what they look like inside, including their butting (thicker ends and thinner center section).

A performance comparison of these tubes looks something like this:

The term “stiffness units” is used to give a rough idea on how tube diameters comparatively affect stiffness. This example is designed to provide overall information, and not be an engineering treatise.

We can see that the tapered top tube achieves some performance characteristics beyond those of the two straight cylindrical top tubes.

Firstly, the tapered tube is stiffer at the top tube / head tube junction. This allows us to ensure that the bike has predictable control and that its tracking is exceptional (we do this with the down tube as well). It also ensures that the top tube is overall stronger where it needs to be and is functionally stronger than either of the cylindrical tubes.  

The top tube’s diameter tapers towards the seat tube, which allows for a bit more flex in this region and helps with compliance on rough terrain.  

Also, we see that – overall – there is a weight reduction compared to the cylindrical Grade 9 tube. We have still maintained the Grade 9 tube’s wall thicknesses, but because of the tapering of the tube’s diameter, we have reduced the weight to roughly the mid point between Grade 5 and Grade 9 Titanium. In effect, we have achieved the optimizations that we want: lighter weight than straight Grade 9 tube, optimized stiffness and compliance at each end of the tube, and weight that’s approximately in the middle of both materials.  

Grade 9’s Hidden Benefit – Toughness and Durability

One additional feature of Grade 9 Titanium is that its elongation is overall higher than Grade 5 Titanium’s. This effectively means that Grade 9 is less brittle.  This increase in toughness is beneficial if you expect your bike to crash or to handle the daily wear and tear expected of a gravel bike that’s designed to do some off-road.  

For Gravel Bikes, a Mix of Grade 5 and Grade 9 Titanium is Ideal

The age-old question of "What Grade Titanium is Better – Grade 5 or Grade 9" is
more complicated than simply one is stronger than the other. The “which material is better” question is more complicated than simply assuming that  "one is stronger than the other”. The reality is some weight savings can be gained by using Grade 5 Titanium main triangle tube sets, but this comes with a severe limitation of tube shape and durability.  With a mix of each material in a frame, you can optimize stiffness, strength, durability and bridge the gap in weight between traditional cylindrical Grade 5 and Grade 9 titanium tubes.  

The Cache's seat tube is a great example of the shaping that is possible with Grade 9 Titanium: this tube is butted, tapered, bent and formed and is unique to Knolly. Butting ensures that the tube is lightweight. The tapering and forming provide the stiffness where it's required (at the BB shell to maximize power transfer to the rear wheel under hard pedaling loads) and the elegant bend in the tube enables excellent tire clearance while providing some compliance at the saddle. This tube shape is unique among gravel bikes and a fantastic example of the tube design that Knolly invests in.

Almost all titanium frames on the market are based on the vendor’s off the shelf tube sets. Knolly is one of the few brands who develops their own tube sets from scratch.  Every tube on a Tyaughton Titanium or  Cache Titanium is made with tooling that we own. We are almost unique in this aspect and it is a key reason why our frames have legendary ride quality and feel.