How Bike Tires Work: Your Quick-Reference Visual Guide
We’ll help you understand how bike tires work by peeking beneath the surface, exploring its three core parts, and learning how each impacts ride quality and performance.
As we detail in How Bike Tires Are Made, the process involves multiple high-tech manufacturing processes. But, at their most basic, these rolling marvels involve just three core elements: the carcass, beads, and tread.
Here, we’ll zoom in, explore these components in detail, and see how they come together to make bicycle tires work.
All About A Bike Tire’s Bead
Imagine picking up a bike tire and slicing through it with a pair of scissors, from one side to the other. If you looked at it head-on afterward, it would resemble an upside-down ‘U.’
- Steel wire, at lower price points
- Synthetic materials at high higher price points, such as aramid fibers (Kevlar is one common example)
Why Beads Work As a Tire’s ‘Backbone’
These strands—which Sheldon Brown refers to as “the “backbones” of a tire,” since they act as its main support structure and also determine it’s size, or diameter—are covered by a hard rubber compound for protection.
Then, they’re folded into the casing’s rubber by a tire building machine to form a wedge, which is what grips the rim and keeps the tire secured in place once the inner tube is filled with air.
The Importance of the Valve
How does air make its way into the inner tube?
Through a relatively simple device known as a valve stem, which uses a removable core and an elongated body to maintain proper pressure. Presta valves are longer and thinner than the much more common Schrader valve, and also feature a lock nut at the end to keep air inside, versus the latter’s internal spring.
Related: Bike Pump Buying Guide
Flexible vs. Rigid Bead
Although both steel wire and aramid fibers provide more than enough backbone support for most applications, there is a structural difference between the two materials: specifically, steel is rigid, whereas Kevlar fibers are flexible.
As a result, tires built with flexible beads are usually shipped folded, which has aptly given them a designation as ‘folding’ tires.
In addition to their flexibility, Johnny Sprockets points out that the flexible bead found in folding tires also typically makes them lighter, easier to install, and less-complicated to remove than their non-folding counterparts.
A Bike Tire’s Carcass (aka Casing)
A bike tire’s carcass references a network of cloth fabric—often made from nylon or other polyamides, although cotton and silk are used in some tire models—woven between each of its two beads.
Compared to regular cloth, which utilizes crossed threads, Sheldon Brown explains that a bike tire’s threads are arranged in layers of parallel threads, with each layer running perpendicular to the next at a 45-degree angle. Perhaps the easiest way to think of it is as a grid.
Why the Carcass is a Bike Tire’s ‘Heart’
When calendered to the tread’s rubber, it’s this layering and 45-degree angle application that’s responsible for a tire’s overall stability. Schwalbe adds that the density of the materials used to create casing mesh can also have a significant impact on a tire’s personality.
Together, while Mr. Brown refers to a tire’s beads as its backbone, he calls the carcass “the heart,” since it determines the shape and acts as the underlying framework.
The Importance of Threads Per Inch
The density of this mesh is measure in ‘ends per inch’ (EPI), also commonly referred to as ‘threads per inch’ (TPI), which references the number of threads contained in one square inch of casing.
In general, bike tires with lower TPI counts feature larger gauge threading and greater amounts of rubber in their casing mesh, resulting in heavier weights. Higher EPI tires, on the other hand, feature finer thread and a much closer mesh weave. What does this mean in the real world?
How TPI Affects Performance
According to Arlington Heights, IL-based Village Cycle Sport, “the higher the thread count, the more flexible and supple a tire will feel, which improves ride quality, handling, and control.” However, this will “also increase cost,” they emphasize.
Not All TPI Measurements Are Equal
As a general rule of thumb, mountain bike tires start at 60 TPI, while road tires typically begin somewhere around 120. REI tells us, however, that TPI count can go all the way up to 320.
If you encounter an EPI/TPI greater than 150, though, Schwalbe recommends exercising caution, since it’s often the case “the number of strands of all carcass layers is added together.”
For example, they explain, if a manufacturer advertises that one of their tires boasts 200 TPI, in reality, this could mean it’s constructed “from 3 layers of 67 EPI, each.”
Getting to Know a Bike Tire’s Tread
The third principal component is the tread, which refers to the exterior part of the tire that comes into contact with the ground.
Tread Exterior Design
Because it’s subject to the most significant amount of wear, tread rubber is extruded through a die during manufacturing so that it’s thicker in the center and thinner on the sidewalls (the area between the tread and each bead). The center section is also often grooved with different patterns to improve grip.
Tread Rubber Compound
Depending on the surface it’s designed for (pavement on the road, dirt on a trail, washed out gravel, smooth wood at a velodrome, etc.) the rubber compound manufacturers use to create a tire’s tread can have a significant impact on handling and performance characteristics. This is obviously in addition to attributes like casing mesh density and bead material.
As just a couple of examples, harder rubber formulations provide greater resistance to wear, but can also reduce traction. Softer compounds, on the other hand, maximize traction (e.g., when cornering) but are prone to wear faster.
REI explains that dual-compound tires attempt to offer the best of both worlds since they “feature a softer rubber on the outside that contacts the ground, and a harder rubber between the tread and the casing.” This results in “better grip and better cornering in almost any terrain.”
In addition to the tread’s rubber compound, its groove pattern and depth—which is determined by a specially designed mold during vulcanization—can play significant roles in overall performance, especially when it comes to traction.
At one end of the spectrum, there are slicks, which feature little-to-no perceptible grooving. These tires are often used for indoor racing, where riders focus on minimizing rolling resistance and maximizing speed.
At the other end of the spectrum are mountain and ‘fat’ bike tires, which feature much deeper grooving that’s spaced farther apart to maximize traction in loose, quickly-changing off-road conditions. These ‘knobbies’ greatly increase rolling resistance on smoother surfaces, though, and could actually slow down a rider.
A Bike Tire’s Sub-Tread
Whether on-road or off, some bike tires also feature an additional ‘sub-tread;’ an extra layer of Kevlar or nylon that’s inserted underneath the primary tread to help boost puncture resistance. However, the tradeoff is that this sub-layer also adds weight and increases rolling resistance.
Given these details, Johnny Sprockets admits that when it comes to finding a tread compound and pattern combo that meets your needs and preferences, it’s often “a tradeoff whether you require top ride quality or durability.”
Bike Tires: The Sum is Greater Than Its Parts
Based on everything we’ve written about here, along with our in-depth illustrations, we can see that while it might look like a single piece of rubber from the outside, a bike tire consists of three core parts: the beads, the casing, and the tread.
Perhaps more importantly, we can now understand how each of the materials and assembly processes used (e.g., beads made from steel wire vs. Kevlar fibers, threads-per-inch (TPI) measurement for casing mesh, and tread rubber compound) cumulatively contribute to a tire’s unique characteristics.
Keep rolling: Bike Tire Glossary