When you're looking into custom parts for cars and motorcycles, you might come across "titanium bolts."
When people hear "titanium," many probably imagine:
Light.
Strong.
Rust-resistant.
Premium material.
Indeed, titanium is significantly lighter than steel and is a metal with excellent corrosion resistance and specific strength.
However,
The understanding that "titanium is always stronger than steel,"
"It's superior to stainless steel in all aspects,"
or "Replacing stock bolts with titanium will always improve performance"
is incorrect.
There are multiple types of titanium, and
Pure titanium,
Titanium alloy,
Steel,
and stainless steel
differ in strength, hardness, elasticity, friction properties, and more.
Furthermore, for bolts, you need to consider:
Material.
Strength class.
Shape.
Tightening torque.
Surface treatment.
Application location.
Mating material.
This time, we will clearly explain the advantages and disadvantages of titanium bolts, differentiating them from steel and stainless steel.
What kind of metal is titanium?
Titanium is a metal represented by the chemical symbol Ti.
Its major characteristics include:
Lightweight.
High specific strength.
Excellent corrosion resistance.
Relatively low thermal conductivity.
Lower Young's modulus than steel.
According to the Japan Titanium Society, the density of industrial pure titanium is approximately 4.51 g/cm³.
In contrast,
Steel is approximately 7.85 g/cm³.
SUS304 stainless steel is approximately 7.90 g/cm³.
This means that, when comparing the same volume, titanium weighs about 60% as much as iron or stainless steel.
This is the "lightweight" characteristic of titanium.
How light are titanium bolts?
Assuming bolts of the same shape and size are made, the difference in density directly becomes a major factor in the weight difference.
For example, if a steel bolt weighs 100g, a titanium product of an equivalent shape would weigh approximately 57-60g, based on a simple density ratio calculation.
This means that you can expect
a weight reduction of approximately 40%.
Of course, in reality, the weight will vary depending on:
The titanium alloy used.
Bolt shape.
Head shape.
Hollow processing.
Surface treatment.
However, a major advantage of titanium is that
"for a similar volume, it is considerably lighter than steel or stainless steel."
Is titanium stronger than iron?
This is a point that is often misunderstood.
The answer is,
"It depends on the type of titanium and the steel it is compared to."
Titanium includes:
Pure titanium.
Titanium alloys such as Ti-6Al-4V.
And various other high-strength alloys.
According to the Japan Titanium Society, the tensile strength of two representative industrial pure titanium types is 340-510 MPa.
In contrast, the representative titanium alloy Ti-6Al-4V is stated to have a tensile strength of 895 MPa or higher.
The strength of steel for comparison also varies greatly depending on:
General structural steel.
High-tensile steel.
Chromoly steel.
High-strength bolt steel.
Therefore, you cannot judge solely by the material name that
"titanium is stronger than iron simply because it's titanium."
Titanium's true strength lies in its "specific strength"
Crucial to understanding titanium is the concept of
"specific strength."
Specific strength is a way to look at
how much strength a material possesses relative to its weight.
While titanium has a significantly lower density than steel, titanium alloys can be engineered to have high strength.
Therefore, it has the characteristic of being
easy to make strong while remaining lightweight.
The Japan Titanium Society also lists high specific strength as a representative characteristic of titanium.
This is one reason why titanium alloys are used in applications such as aircraft and motorsports, where strength needs to be maintained while reducing weight.
Pure titanium and titanium alloy are different
Even if it says "titanium bolt," not all are made of the same material.
Broadly, there are two categories:
Pure titanium series.
Titanium alloy series.
Pure titanium possesses excellent corrosion resistance and workability, but it does not have the strength of high-strength titanium alloys.
On the other hand, one of the representative titanium alloys used in automobiles, aircraft, and machine parts is
Ti-6Al-4V.
It is also commonly known as Grade 5.
Ti-6Al-4V has higher strength than pure titanium and is widely used in parts that require light weight and high strength.
Therefore, when comparing titanium bolts, it is important to check not only that they are
"made of titanium,"
but also
what grade of titanium they are.
Advantage 1 of titanium bolts: Weight reduction
The most obvious advantage is lightness.
Many bolts are used in cars and motorcycles.
While a single bolt might only differ by a few grams or tens of grams, these differences add up when multiple bolts are replaced.
Especially in applications such as:
Motorsports.
Racing vehicles.
Aircraft.
Bicycles.
Motorcycles.
where even a few tens of grams of weight reduction is significant, titanium becomes the material of choice.
However, it is not appropriate to assume that replacing just a few bolts on a typical passenger car will
significantly change acceleration or fuel economy.
The amount of weight saved needs to be realistically assessed.
Advantage 2 of titanium bolts: Corrosion resistance
Another major characteristic of titanium is its corrosion resistance.
A highly stable oxide film forms on the surface of titanium, and this film protects the internal metal.
The Japan Titanium Society lists titanium's extremely high corrosion resistance even in seawater environments as a key feature.
Therefore, in environments exposed to
moisture,
rain,
and salt,
titanium can offer significant advantages as a material.
However, this does not mean that
"if it's titanium, corrosion problems will absolutely not occur."
The combination with dissimilar metals and the usage environment must also be considered.
Advantage 3 of titanium bolts: High specific strength
The strength of titanium alloys lies not in their absolute strength, but in their
strength-to-weight ratio.
For example, in a comparison by the Japan Titanium Society,
Ti-6Al-4V has a density of approximately 4.43 g/cm³ while its specified tensile strength is 895 MPa or higher.
Thus, a characteristic of titanium alloys is that they can
secure high mechanical strength without significantly increasing weight.
Therefore, it becomes a promising material for applications where you want to
"reduce weight but maintain the necessary strength."
Advantage 4 of titanium bolts: Appearance and surface treatment
Titanium is also a distinctive material in terms of design.
Beyond its polished metallic color, unique appearances can be created through
anodizing,
DLC,
and various other surface treatments.
In cars and motorcycles,
the engine bay,
exterior,
around the license plate,
and motorcycle frame
are places where bolts themselves are sometimes used as part of the design.
For this reason, titanium bolts are sometimes chosen not only as
functional parts
but also as
dress-up parts.
Difference from stainless steel bolts
Stainless steel is also sometimes used in cars and motorcycles due to reasons such as:
Rust resistance.
Good appearance.
Relatively easy availability.
The density of representative SUS304 is approximately 7.90 g/cm³, which is almost the same as steel.
In contrast, pure titanium is approximately 4.51 g/cm³.
Therefore, titanium has a significant weight advantage.
On the other hand,
price,
machinability,
availability,
and the variety of bolt standards and types
may make stainless steel a more suitable choice in some cases.
Also, there are many types of stainless steel, and their strength and corrosion resistance are not all the same.
Simple comparison of iron, stainless steel, and titanium
Roughly categorizing representative materials:
Steel has the following characteristics:
Relatively inexpensive.
Many varieties.
Abundant high-strength materials.
Widely used for automotive bolts.
Stainless steel has the following characteristics:
Excellent corrosion resistance.
Easy to maintain appearance.
Higher price than general steel.
Relatively heavy, similar to steel.
Titanium has the following characteristics:
Lightweight.
Excellent corrosion resistance.
Titanium alloys have high specific strength.
High price.
Difficult to process.
Requires caution during fastening.
In other words,
titanium is not superior in all aspects.
It is important to choose the right material for the purpose.
Is titanium also highly rigid?
Caution is also needed here.
"Strength" and "rigidity" are not the same.
Strength refers to
how much load a material can withstand before breaking or permanently deforming.
On the other hand, rigidity relates to
how much a material resists deformation when a load is applied.
According to data from the Japan Titanium Society, the Young's modulus is:
Pure titanium: approximately 106 GPa.
Ti-6Al-4V: approximately 110 GPa.
Steel: approximately 206 GPa.
SUS304: approximately 200 GPa.
Thus, titanium's Young's modulus is roughly half that of steel.
Given the same shape and load conditions, titanium tends to deform more elastically.
Therefore,
the understanding that "titanium is strong, so it's harder and doesn't bend as much as iron" is incorrect.
Difference in rigidity is also important for bolts
Bolts don't simply connect parts.
They fasten parts together by generating appropriate axial force.
Therefore,
the bolt's strength,
elasticity,
elongation,
tightening torque,
friction coefficient,
bearing surface,
and thread
all influence the fastened state.
It is not appropriate to replace genuine high-strength steel bolts with titanium bolts of the same dimensions solely because
"titanium seems more premium."
You need to confirm the material properties and the required fastening performance.
Titanium bolts require caution against "galling"
When using titanium bolts, it's especially important to be aware of
galling
and seizing.
When a bolt is tightened,
the male and female threads slide against each other under high pressure.
During this process, friction can damage the metal surfaces, and under certain conditions, the metals can stick together.
Nord-Lock explains that stainless steel and titanium are more prone to galling and seizing compared to steel components.
The Japan Titanium Society also cites cases of seizing between titanium bolts and nuts.
Why is titanium prone to galling?
Under conditions where thread surfaces rub against each other with high loads, titanium can experience problems due to friction and adhesion.
Regarding the friction and wear characteristics of titanium alloys, it is known that galling is a significant issue under metal-to-metal sliding conditions.
Therefore, with actual titanium bolts, countermeasures may be taken by:
Surface treatment.
Appropriate lubrication.
Combining with different materials.
Thread processing methods.
Tightening methods.
However, using lubricants changes the friction coefficient during tightening, so it doesn't mean that
you can just apply unapproved lubricants on your own judgment.
Should the tightening torque be the same as for iron bolts?
Not necessarily.
The relationship between tightening torque and the actual bolt axial force generated is affected by factors such as:
Thread friction.
Bearing surface friction.
Material.
Surface treatment.
Lubrication condition.
Even if tightened with the same torque, the resulting axial force will change if the friction coefficient is different.
Therefore, a simple substitution like
"Because the genuine iron bolt is 100 N·m, the titanium bolt of the same size is also 100 N·m"
should be avoided.
If the titanium bolt manufacturer or vehicle manufacturer specifies tightening conditions, those values should be followed.
Lubrication can sometimes lead to overtightening.
When lubricant is applied to a bolt, friction decreases.
This can lead to a greater axial force being generated even with the same tightening torque.
Therefore, caution is needed when using methods such as
"applying a large amount of lubricant to prevent galling."
For fastening components,
the appropriate tightening conditions change depending on the dry state,
lubricated state,
and surface treatment.
It is important to follow the method specified by the product manufacturer.
Are titanium bolts heat resistant?
Titanium has a high melting point.
The melting point of industrial pure titanium is approximately 1,668°C.
However, a high melting point does not mean that it
can be used anywhere at high temperatures.
For actual bolt applications, it is also necessary to consider:
Strength at high temperatures.
Creep.
Oxidation.
Thermal expansion.
Mating material.
Changes in fastening force.
Especially in areas that reach very high temperatures, such as exhaust systems, it is necessary to confirm that the material and grade are suitable for the application.
Thermal conductivity is lower than iron
Titanium is a metal with lower thermal conductivity compared to steel.
According to representative values from the Japan Titanium Society:
Pure titanium: approximately 17 W/m·K.
Ti-6Al-4V: approximately 7.5 W/m·K.
Steel: approximately 63 W/m·K.
This means that titanium has characteristics that make it less likely to conduct heat.
This can be an advantage or disadvantage depending on the application.
While it can be advantageous in "places where heat transfer is not desired,"
it also means that frictional heat generated in the fastening area is less likely to dissipate.
This characteristic is also related to the caution against galling.
Caution with combinations of dissimilar metals
In cars, multiple metals are combined, such as:
Aluminum.
Iron.
Stainless steel.
Magnesium.
Titanium.
When different types of metals are in contact and an electrolyte such as moisture or salt is present,
dissimilar metal contact corrosion
may need to be considered.
Especially since titanium has excellent corrosion resistance, it is not necessarily true that
"because the titanium bolt itself won't rust, the surrounding materials will also be fine."
When using it with aluminum parts, for example, a decision needs to be made considering:
Mating material.
Surface treatment.
Insulation.
Usage environment.
Is it okay to replace wheel bolts with titanium?
The area around the wheels is a critical safety component where
high fastening force,
repeated loads,
heat,
vibration,
and impact
are applied.
Therefore, it is not recommended to change genuine bolts or nuts solely because
the size is the same,
it will be lighter,
or it looks good.
What is needed is:
The bolt's material.
Strength.
Shape.
Seating surface shape.
Thread pitch.
Tightening conditions.
Compatibility with the wheel.
Vehicle manufacturer's specifications.
The same applies to brakes, suspension, and seat belt anchor points.
In safety-related areas, it is crucial to use parts whose compatibility has been confirmed.
"Will it make a big difference if I use it on rotating parts because it's light?"
In rotating parts such as around the wheels, there is a concept that
lightweighting equals reduced rotational inertia.
This is physically correct.
However, the extent to which the weight difference of a few bolts affects the overall driving performance of the vehicle is another matter.
Compared to the wheel,
tire,
and brake rotor,
the weight change from bolts may be small.
Therefore, it is not possible to generally state that
"replacing with titanium bolts will clearly improve acceleration performance."
It is important to actually calculate how much weight will be reduced.
What applications are titanium bolts suitable for?
Titanium bolts are particularly suitable for applications where:
Weight reduction is a priority.
Corrosive environments are present.
Appearance is important.
The specific strength of titanium is required in the design.
Strength and fastening conditions are specifically engineered for the application.
On the other hand, for applications where:
Cost needs to be minimized.
Ease of general maintenance is prioritized.
A very wide range of standards is desired.
The original design assumes high-strength steel bolts.
steel bolts may be a more reasonable choice.
Disadvantages of titanium bolts
While titanium bolts offer many advantages, they also have drawbacks.
The main ones are:
High cost.
Difficulty in processing.
Significant strength variation depending on material and grade.
Lower Young's modulus than steel.
Caution required for galling and seizing.
Attention required for tightening conditions.
Need to consider combinations with mating materials.