What Are Bicycle Tires Made Of?

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.

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.

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.

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.