What Are Bicycle Tires Made Of? A Step-by-Step Exploration of the Manufacturing Process
In this guide, we’ll take a high-level look at what bicycle tires a made of and how everything comes together.
While it might seem like a simple rubber hoop, bringing a bike tire into the real world involves high-tech machinery, the latest manufacturing techniques, and labor-intensive craftsmanship.
Here, we’ll walk you through the process step-by-step.
Step 1: Selecting Rubber Compound Ingredients
What bicycle tires are made of largely depends on which purposes they’ll be used for.
Choosing a Balance of Characteristics
As such, after modeling its design using 3D software, the manufacturer will create a rubber formula that displays different characteristics. These include:
- Aerodynamics, running performance, and minimal rolling resistance for a road tire
- Elasticity for a mountain bike tire
- Durability and longevity for a touring tire
- Surface adhesion for maximum wet weather performance
- Density and dampening for a commuting tire
It’s a balancing act, though. Why? Because improving one trait typically reduces another’s performance. To illustrate this, increasing rubber hardness boosts a tire’s longevity and reduces rolling resistance, but also decreases its wet weather adhesion, while also delivering a harsher ride.
Although the specific designs and rubber mixes used by manufacturers are closely guarded secrets, Bicycling.com explains the mix of ingredients is generally the same:
Selecting the Right Rubber Polymers
When it comes to what bicycle tires are made of, there are four main types of rubber used.
The tread and sidewalls are typically constructed from natural rubber, which is derived from the bark of the Hevea Brasiliensis tree. Synthetic rubber versions, such as butyl rubber and halogenated butyl rubber, are usually extracted from crude oil and used in the tire’s casing.
Adding Curatives, Fillers, & Vulcanization Agents
Carbon black helps to reinforce and strengthen modern bike tires, as well as improve their abrasion resistance, maximize traction, and provide their dark color. In years past, sulfur provided many of these same benefits.
Silica (silicon dioxide), on the other hand, determines traits like flexibility and wet weather performance.
Bicycle tires aren’t made from just one type of carbon black or silica, though. Different versions are used depending on the section of the tire being constructed, along with the manufacturer’s specific performance requirements.
Common Softeners & Antidegradants
Sulfur also acts as a softener during the process, along with other industrially refined ingredients like:
- Macro and micro synthetic crystalline waxes, including paraffin
- Aromatic amines
- Zinc oxide
- Various pigments and dyes
As a tire is used, these waxes and the beneficial components they contain naturally migrate to its surface, thereby providing protection from exposure to ozone (antiozonants), oxidation (antioxidants), and heat.
To give you an idea of commonly used ingredient ratios, Schwalbe tells us their tires typically contain 40% to 60% rubber and 15% to 30% filler, with 20% to 35% remaining ingredients.
Step 2: Mixing & Extruding
These ingredients meet inside industrial machinery known as an internal mixer. (The Maxxis website references a specific model, known as a Banbury mixer.)
The Mixing Process
Here, bales of rubber are kneaded inside a large chamber by two rotors, which move at variable rotations per minute. They’re simultaneously heated to temperatures as high as 338°F (170°C), while other ingredients are added as needed to achieve desired traits like elasticity, durability, surface adhesion, dampening, and so forth.
Depending on the size of the mixer, manufacturers can combine between 40 kg and 400+ kg of rubber and other ingredients per session, and in less than five minutes. They draw multiple samples during this relatively short process to ensure the desired properties are present.
The Extrusion Process
Once mixing is complete, doughy rubber exits the machine through a discharge hatch at the bottom and is sheeted out like a pastry into a thick, continuous layer by powerful rollers.
At this point, Maxxis tells us it’s referred to as a ‘slap.’
Step 3: Tread & Sidewall Formation
Slap slated for sidewalls is covered with plastic sheeting and rolled further. Tread slap (the part that comes into direct contact with the ground) is cut into long, narrow strips.
These strips are then fed into an extruder and heated again. Once the rubber becomes doughy, a revolving screw applies shear force and sends it through a die, forming a profile that’s thicker in the center and thinner on the sides. Together, this helps improve resistance to wear.
To cool and set its shape, tread rubber is subsequently submerged in water. It’s also dipped in an anti-tack compound like metallic stearate or stearic acid, powdered mica, talc, or different water-based formulas, to prevent sticking. Then, it’s wound onto spools with a fabric separator between each layer to further prevent unwanted adhesion.
Sidewall slap is also wound around a spool, separated by fabric and then dried, stacked in cases called ‘books,’ and stored for later use. If needed, it’s immediately transferred to the next step in the process.
Step 4: Carcass Creation
As we explain in How Bike Tires Work, the carcass (otherwise known as the casing) is perhaps the most critical section of a bike tire, since it acts as the underlying framework and determines its shape. It also defines how the tire conforms to surface irregularities, along with its rolling resistance.
Due to its importance, we’ll split this part of the process into two different stages.
Fabric Mesh Creation
First, a third-party manufacturer twists nylon textile yarn together, treats it with chemicals that promote bonding with rubber, and forms it into a grid pattern.
Upon arriving at the tire manufacturer, this yarn is stored, and its temperature and humidity levels are carefully monitored since both of these factors can impact its tensile levels.
Fabric Mesh Application
When the time comes, webbing is passed through very hot rollers at a 45-degree angle. This permanently bonds it with the tire’s casing rubber through a process known as calendering. This is what’s ultimately responsible for defining the tire’s shape and providing it with a great deal of support.
The density of this webbing yarn is measured in threads per inch (TPI), which references the number of threads contained in one square inch of the casing. Why does it matter?
Casing mesh with fewer TPI (< 80) typically features larger gauge threads and more rubber, resulting in tires that are heavier and more rigid. Higher TPI casing mesh (> 100), on the other hand, features finer threads and less rubber, resulting in greater strength, lighter weight, better flexibility, and improved suppleness. It also results in a higher price.
In addition to rubber compound and mixing process, Continental further explains that the yarn’s makeup can have an impact on a tire’s performance: “Very fine [casing] material makes the tire smoother and protects the carcass from punctures,” while “coarse fabric is more cut-resistant and makes the tire more robust,” they say.
Bottom line: Much like rubber formulation, “the choice of fabric,” they point out, “depends on the purpose of the tire.”
Cutting & Assembly
After calendering, casing rubber enters a cutting station where blades slice it into premeasured strips. In most instances, it will take multiple pieces to construct a single tire.
Similar to the tread section, after being assembled by hand, these casing strips are wound onto spools with fabric backing that prevent sticking. They can be used immediately or stored for later assembly.
If used immediately, raw tread rubber is added to the carcass by wrapping a layer around the drum and firmly sticking it down. Continental points out that tread thickness is also a significant contributor to a tire’s ride quality characteristics.
4B: Bead Creation & Insertion
While everything above is happening, a specialized wrapping machine takes steel or aramid (Kevlar) fibers, twists them together, and forms them into rings. These rings, formally known as the tire’s beads, are then covered with a thin layer of protective rubber.
With the casing already wound around a metal spool, Schwalbe tells us that a specially trained employee inserts beads on each side by hand. Once inflated, the strength they provide will help hold the tire onto the rim after it’s inflated with air.
Related: How to Buy a Bike Pump
Next, a mechanical apparatus simultaneously folds the casing’s rubber over both beads. If additional puncture protection is needed, a Kevlar strip is forced into the rubber as well.
Step 5: ‘Green’ Tires
The extruded rubber for the tire’s sidewalls is now glued into place, and the tread is applied to the center. With all of the layers set, the bike tire is removed from the building machine. At this point, it’s known as a ‘green’ tire.
From here, a specialist loads the tire into a branding station, where hot metal plates quickly bond labels to its sidewalls.
Green Tire Vulcanization
It’s then placed inside a clamshell-like mold over a tubular bladder. The shell is sealed, steam is pumped into the bladder like a balloon, and the interior heats up to 180°C for between three and six minutes. Overall, this accomplishes two goals:
- The pressure created by the steam-filled bladder presses the tire’s rubber against the sides of the mold, which contains a negative impression of the tread. (Schwalbe points out that you might think of this as a waffle iron.)
- The heat generated during the vulcanization process also fuses all of the tire’s layers at the molecular level, thereby hardening and elasticizing the rubber.
Similar to many other components involved during the manufacturing process, such as rubber formulation and threads-per-inch, time spent ‘cooking’ depends on a tire’s desired traits. For example, the general rule of thumb is that the longer it spends vulcanizing, the harder—and less elastic—a tire will be.
Step 6: Quality and Safety Testing
Finally, the tire is removed from the mold to cool. Eventually, it’s sent to a warehouse for distribution.
After cooling, each tire is inspected for flaws (e.g., bubbles, voids, etc.), whether by a specially trained employee or by bombarding it with x-rays and visualizing the results on a screen.
In most instances, one tire from each batch also undergoes in-depth quality control testing. This might involve one or more of the following:
- Simulating road wear by spinning it against a metal wheel until it goes bald (usually after thousands of miles). Sensors simultaneously report if the tire remains balanced and true.
- A different wheel that features a large bump to simulate hitting a curb several thousand times in a row.
- Applying a great deal of pressure onto one or more sharp metal studs, which checks a tire’s puncture resistance.
- Pressurizing the tire until it pops off of the rim. To pass, some manufacturers require that their tires survive at least 8 lbs of pressure per square inch before moving to the distribution phase.
The Bottom Line About What Bicycle Tires Are Made Of
It’s easy to overlook the substantial technology and manufacturing prowess involved in the development and production of every tire that graces your bicycle.
The next time you go for a spin, though, take a brief moment to marvel the many decades of progress associated with its rubber compound, additional ingredients, mixing process, mesh formation and calendering, and quality testing—all of which combine to make sure you have the smoothest, most enjoyable ride possible.
Keep rolling: Common Bike Tire-Related Words & Phrases