Material Properties

There are A LOT of different material properties. Structural properties, thermal properties, manufacturing properties, etc. But for this discussion, we're focusing on the primary structural properties that are relevant to what you feel on the bike. The chart below lists these properties for a variety of materials. Materials that may be suitable for a triple clamp are highlighted in yellow. Other materials are presented for reference. Finally, there are comparison percentages listed for each material; those are all relative to 6061-T6 aluminum as a baseline since that is a common material used in many motorcycle parts.

A summary of various material properties

Density (ρ)

This is how much the material weighs for how much of it there is in volume. A square meter of water weighs 1000 kilograms, so the density is 1000 kg/m³. Steel has a density nearly eight times that at about 7800 kg/m³, and aluminum is much lighter at about 2700 kg/m³.

Modulus (E)

Young's modulus, also called tensile modulus, is a measure of how much stress a material has for a given strain (deflection). It is directly related to the stiffness of the material. A material that has twice the modulus of another material will be twice as stiff. The material modulus of the various parts is very significant to the feel of the bike since it's one of the two attributes that contributes to the overall stiffness that you feel. An interesting thing to note here is that various alloys of the same material will have nearly the same Modulus (within a few percent). But modulus varies significantly between different materials.

Yield Strength (YTS)

The yield strength is the stress level at which a material starts to permanently deform (bend). Once the part has reached this point, it has failed (it's bent, etc.) and needs to be replaced.

Ultimate Strength (UTS)

The ultimate strength is the stress level of the material at which it completely fails: cracks, splits, etc. It isn't terribly significant because you've already yielded (bent) the part before you get to this point. But it's usually nice for the material to have a good gap between the yield stress and the ultimate stress level, so the part bends instead of completely failing.

Note that strength and stiffness are not the same thing! I've seen confusion around this in some discussion forums and posts online. Stiffness is how much something deflects with a force applied to it. Strength is how much it can deflect before it fails (bends or breaks).

Material Property Ratios and Dependencies

The ratio between properties is a useful way to compare materials, particularly stiffness-to-weight and strength-to-weight. Most relevant structural metals have a very similar stiffness-to-weight ratio. You can see from the material properties chart presented earlier (all ratios are relative to the popular 6061-T6 material for ease of comparison).

Of the yellow highlighted materials (those suitable for structural components), the stiffness-to-weight ratio is the same within 3% and most are under 1%. Isn't that interesting? If you take a steel bar and pull on it, it will have a particular tensile stiffness. If you were to find an aluminum bar of the same weight and same length as the steel bar (it will have a larger cross-section) and pull on it, it will also have the same tensile stiffness as the steel bar. And the same applies for a titanium bar or magnesium bar. This is illustrated in the image below. All four bars are the same length, weigh the same, and have the same stiffness in tension (designated by the arrows). The only difference between them (aside from the material) is the cross-sectional area.

It's worth noting that the bending stiffness of all four bars is different. Because the cross-sectional area of the magnesium bar is the biggest, it will have the highest bending stiffness. And the steel bar, despite its high modulus, will have the lowest bending stiffness. This demonstrates the importance of geometry to the resulting stiffness of a part. Because of this, the lowest density material will always be the winner here as it will have the largest cross-section. I'll get into a lot more detail on this in our next blog post on stiffness.

So, stiffness-to-weight is pretty much the same for all structural metals. But the strength-to-weight ratio is a different story… You can see from the previous chart that the strength-to-weight ratio varies significantly between materials and their different alloys. Since the stiffness-to-weight ratio is pretty consistent, the strength-to-weight ratio is a large factor in material selection.

Linear Properties

One nice thing about metals is that their stiffness and strength properties are linear within their useful range. If, for example, if you apply a force to the front end of the bike, then the triple clamps, fork tubes, frame, etc., will all flex some amount and be at a certain level of stress. Remove that force, and they go back to their original position and no stress. If you repeat that, but double the force, the flex also doubles and so does the stress. Triple it and everything triples as well.

This linear relationship remains true until you've either flexed things so much that the geometry has significantly changed or until the stress level in the material has exceeded the yield strength of the material, and it fails. Of course, there are some exceptions to this rule for unique geometries, but for most relevant metal parts on the bike, we can assume everything is linear under normal conditions.

This is referred to as Hooke's Law, which states: “the strain of the material is proportional to the applied stress within the elastic limit of that material”.

A typical stress-strain curve for metals

The image above depicts a typical stress-strain curve of a metal. The linear region (green) is the “useful range” I mentioned. This applies to all the metal parts on your bike and handlebars are a great example:

• Every bump you hit causes the bars to flex some, but they stay in the linear range and they return to their normal shape after the bump. The bars are just a spring in this sense, deflecting as you hit a bump, then returning straight after the bump.

• But if you crash, there can be enough force to bend the bars permanently by exceeding the material yield strength. This is the orange region of the curve, with stresses higher than the yield strength of the material. At this point, the bars have failed and need to be replaced.

• And if you crash really hard you can break the bars completely into two pieces. This is a stress higher than the ultimate strength of the material (red region of the curve).

Neither bending nor breaking is a good outcome! So clearly, we want to stay in the linear region where the bars continue to do their job as a spring but don't fail.

Directional Properties

Many materials have direction dependent properties. This is especially true in materials like carbon fiber where the direction of the fibers has a massive impact on the stiffness and strength properties in that direction of the material. We refer to this as having anisotropic material properties.

In the case of most metals, though, we have isotropic material properties. This means that the material properties are essentially the same in all directions. Of course there are some exceptions to this, but that's outside the scope of this post (and mostly irrelevant for the subject of materials on the bike).

A Common Misconception

In most of our blog posts I like to refute one of the (many) myths in the motocross world. And this one is everywhere: “titanium hardware makes the bike more rigid than steel hardware”. This comes up A LOT from mechanics, riders, testers, “engineers”, etc. Here's an example:

Now, I'm not trying to single anyone out here in particular; this is an industry-wide issue. You won't have to try hard to find other examples from many people saying the same thing. This myth has been perpetuated for so long and by so many, that riders are “feeling” a difference because they expect to (not because it's actually there).

But the reality is quite the opposite. And now that you have more understanding of material properties, it's pretty simple to see why: Steel hardware has a modulus of about 200 GPa and titanium hardware has one of about 114 GPa. The exact numbers and units don't really matter here, it's the simple fact that the modulus of steel is bigger than the modulus of titanium. Remember that the modulus is directly related to the stiffness? From this alone you can do the math, [(200-114)/200], to see that titanium will be 43% LESS rigid than steel, not more rigid as many claim.

Earlier I mentioned that geometry has an effect on stiffness; what if the geometry of the titanium fastener is different from the steel one it's replacing? Well, from this blog, you'll know that the titanium bolt would have to be bigger than the steel bolt to have a chance at being stiffer. But for most fasteners, there's simply no room to change the geometry: the bolt can't get bigger and still fit in the hole. For the majority of hardware on the bike, titanium bolts are just geometric copies of the steel bolt they are replacing. Those will always be less rigid.

A Practical Example:

Even when there's opportunity to change the bolt geometry, it's likely not enough to overcome the reduction in modulus of titanium vs steel. Let's look at the bolts pictured below. A while back a company claimed in their Instagram post that the stock steel KTM mount bolt "gives the front end flex and a vague feeling" because it's relieved in the middle. But their titanium bolt provides a "more responsive feel and more traction" because it's stiffer. But is it really?

Well, making the titanium bolt with a constant cross-section vs. a relieved geometry is going to increase the stiffness. But that change still doesn't offset the modulus reduction enough to make it stiffer than the stock steel bolt. You can easily calculate the stiffness difference in tension between the two knowing the differences in size. It's just the cross-sectional area multiplied by the material modulus:

So, the titanium bolt that was claimed to be stiffer than the steel bolt is actually 13% less stiff! Now that's just tension, but what about bending? Bending is a little harder to calculate due to the variance in cross-section across the length of the bolt, but we can simulate it with some basic FEA (finite element analysis):

The FEA results revel that the titanium bolt is actually about 15% more compliant in bending than the stock steel bolt. Once again, that's not stiffer!

Except in unique (and highly unlikely) circumstances, switching from steel hardware to titanium will always make the bike less rigid due to the drop in material modulus. And, if you've read my blog post on bolt torque, you'll see the material change of the fastener doesn't much matter one way or the other. For most bolted joints, the material the bolt is made from is irrelevant to the feel of the bike because all it's doing is tying two other parts together. The bolt itself doesn't have much of anything to do with the flex of the whole system, it's just the parts it's holding together that matter. So in most cases, switching from steel to titanium hardware will have no noticeable effect on feel one way or the other.

Hopefully this gives you some more understanding of the materials used in our sport. Up next, we'll dive into stiffness (flex, compliance, rigidity) in more detail. That blog post is coming soon!