Inline skate wheels are made of polyurethane, a material that revolutionized various forms of skating, including skating on inline skates.
Polyurethane
Modern inline wheels are made of polyurethane, a material that revolutionized skating. Before the widespread adoption polyurethane, roller skates and skateboards used composition wheels made from clay or rubber mixed with sawdust and other fibers. These composition wheels were prone to swelling from moisture and could shatter on impact, contributing to a decline in skateboarding by the late 1960s.
In the early 1970s, roller skaters experimented with polyurethane wheels, but they proved too grippy, and slow for roller rinks with wooden floors, where composition wheels performed better. However, by the mid-1970s, skateboarders repurposed polyurethane wheels from roller skates, shaving them down to fit skateboards. These wheels excelled on streets, and urban terrain, offering greater durability, and impact resistance compared to their predecessors. Their elasticity provided a smoother ride over rough surfaces, helping to spark a resurgence in skateboarding after its late-1960s slump.
In the early 1980s, Scott Olson similarly repurposed polyurethane wheels from roller skates, shaving down tens of thousands to fit the skates sold by his company, Ole’s Innovative Sports – later known as Rollerblade. The same qualities that made polyurethane wheels ideal for skateboarding, such as durability, impact resistance, and a smooth ride, also benefited inline skating. This innovation helped bring inline skating out of obscurity and transform it into a popular outdoor sport during the 1980s.
What truly set polyurethane (or urethane) apart from previous wheel materials was its versatility. As an elastomer, it could be molded into any shape, and formulated to achieve a wide range of properties including color, hardness, grippiness and rebound. This adaptability allowed for the development of specialized wheels for different inline skating disciplines, from large, hard wheels with pointed bullet profiles for outdoor racing to small, soft wheels with rounded profiles for indoor hockey. Quote: "when discussing industrial applications, urethane and polyurethane are essentially the same material. The terms are used interchangeably, and there is no functional difference between them."
Another remarkable aspect of a polyurethane compound is that it can be formulated to achieve high elasticity, regardless of the desired values for other properties such as hardness. In material science, this elasticity is often measured with the Bayshore Resilience test. In the inline skating industry, it is referred to as wheel rebound. A wheel with high rebound efficiently converts energy from a skater's stride into acceleration, while a wheel with low rebound dissipates much of that energy, leaving little to propel the skater forward. High-rebound wheels are beneficial across all inline skating disciplines, enhancing performance and efficiency.
Structure
From the 1910s to the 1970s, many wheeled skates served as precursors to modern inline skates. Most of these early skates featured small wheels made of rubber or rubber reinforced with fibers, often with relatively simple constructions. In many cases, the wheel was merely a rubber ring shaped like a donut, with a bearing cylinder press-fit into its hollow center – an example of which can be seen in the 1966 Chicago Roller-Blade. Other designs incorporated two metal discs to enclose the bearing cylinder more securely within the rubber wheel, as seen in the 1975 Super Sport Skate.
These early rubber wheels were designed to flex under load, providing grip on road surfaces. Nevertheless, the same elasticity that enhanced their performance also made them increasingly unsuitable for securely housing precision ball bearings, which inline skates began adopting from roller skates in the 1980s. A misaligned bearing could cause the wheel to tilt, resulting in constant dragging against the frame. This friction not only impaired performance but also generated heat, which could soften the wheel and further compound the misalignment issue.
To address this in roller skates, metal bushings were introduced to hold two ball bearings and a spacer in precise alignment at a 90° angle to the wheel axle. While effective, these bushings added weight and introduced new issues such as metal grinding noises. As plastic became more common, metal bushings were replaced by injection-molded plastic hubs. These single-piece hubs not only securely encased the bearing assembly but also interlocked with the elastic wheel without additional hardware. Furthermore, plastic hubs reduced overall wheel weight, unlike metal bushings, which increased it.
In the 1980s, Rollerblade collaborated with Kryptonics to develop polyurethane wheels for inline skates. The inline industry adapted hub innovations from roller skates, integrating them into inline wheels. These wheels were manufactured by placing a plastic hub at the center of a mold and then pouring molten polyurethane around it. The liquid polyurethane flowed into cavities and holes in the hub before settling. As it cooled and solidified, it formed a secure interlock with the hub.
Modern inline wheels consist of two main components: an outer polyurethane tire, shaped like a donut, and an inner plastic hub, also known as a core. The hub features a hollow center designed to accommodate a spacer and two ISO 608 ball bearings. Made of hard plastic, the hub securely holds the bearings in place through a friction fit - something the softer polyurethane tire cannot achieve. On the other hand, the soft polyurethane tire is able to deform upon ground contact, cushioning the landing and gripping the ground.
Hub
The outer rim of a wheel hub, known as the interlock, is buried within the polyurethane tire. Unless the tire is made from clear urethane, this rim remains hidden from view. Its size, shape, and design vary depending on the wheel’s outer diameter and the demands of different skating disciplines. Beyond mechanically securing the tire to the hub, the interlock rim also enhances the wheel’s overall structural rigidity.
Some manufacturers apply a bonding agent to the hub before pouring the polyurethane, creating a chemical bond that reduces reliance on mechanical interlocking. Others use polyurethane-blended plastic hubs, such as "Estaloc," developed by Robert J. Labeda of Labeda Wheels in collaboration with B.F. Goodrich, which chemically bonds with the polyurethane tire during the molding process.
The visible portion of the hub varies in size depending on the wheel’s overall dimensions and the requirements of specific skating disciplines. In some small wheels used for aggressive skating, the visible hub may appear as a thin ring between the bearings and the tire, just large enough to serve as a bearing housing. In contrast, larger racing wheels from the 1990s, typically around in diameter, often featured hubs occupying nearly 50% of the wheel’s diameter. By the 2020s, even larger wheels from to feature hubs that take up most of the wheel’s diameter.
A wheel's volume increases in proportion to the square of its diameter given fixed width. Thus a solid polyurethane wheel of substantial size would be prohibitively heavy and excessively flexible, making it impractical for inline skating. Even if the wheel consists of largely a rigid hub, it would still be prohibitively heavy. To address this, large inline wheels adapted design principles from polyurethane stroller wheels, incorporating lightweight yet rigid spoked hubs with an interlock rim to secure the polyurethane tire. These spoked hubs, also known as open cores, feature an outer rim connected to the bearing housing by distinct spokes.
In contrast, a full hub, also referred to as a closed core, consists of a solid disc with no clear separation between the outer rim and the bearing housing. Between these two designs is the semi-open core, which features a solid disc but with small hollows drilled between the outer rim and bearing housing to reduce weight.
Spoked cores are generally lighter than full cores but may offer reduced rigidity and structural integrity. As a result, aggressive skaters typically favor small wheels with robust full cores to ensure durability against the high impact of jumps and landings. Conversely, speed and marathon skaters prefer large wheels with spoked hubs, as the open core design allows air to circulate around the hub, helping dissipate heat generated by the bearings during prolonged high-speed use. Without adequate heat dissipation, the polyurethane tire may soften, increasing deformation during rolling and potentially leading to separation from the hub.
Diameter and profile
Inline skate wheels in the 2020s range from as small as to as large as in diameter. Also reference the companion video shown in the article. However, the wheel width remains industry-standard at , regardless of size. Wheel frames are designed for specific setups, accommodating a set number of wheels of particular diameters. Despite these variations, all frames assume a uniform hub width and polyurethane tires no wider than .
Wheel profile refers to the shape of the wheel’s contact surface when viewed head-on, that is, the portion of the wheel that touches the ground. While any wheel diameter can, in theory, be paired with any profile, certain combinations prove optimal for different skating disciplines. Also reference the companion video shown in the article.
Some inline wheels feature a flat contact profile, resembling car tires with a rounded rectangular shape. This shape provides a stable upright rolling experience with a strong grip on the ground, due to the larger surface area. Flat-profile wheels work well for disciplines that keep skates upright while rolling, such as aggressive skating. Typical aggressive wheels are relatively small, ranging from to in diameter. Grindwheels, installed as middle wheels, are even smaller, typically under . Made from hard polyurethane or plastic, they have reduced traction to minimize wheel bite during a grind. However, where grip is needed, such as during stair bashing, the flat profile’s large contact surface helps compensate for the grindwheels' inherently low traction. In the 1990s, aggressive wheels and grindwheels trended toward completely flat profiles. However, in the 2020s and beyond, aggressive skaters began favoring wheels with slightly rounder profiles, offering a balance between stability and maneuverability.
Most inline wheels have a round profile rather than a flat one. This is because skaters don't always roll with their skates perfectly upright. Much like ice skating with blades, many inline skating techniques involve edging - that is, skating on the side of the wheels. This requires the skate to tilt at various angles, depending on the move being performed. A round profile maintains a consistent, usable contact surface across different edging angles, even if that surface is smaller than what a flat-profile wheel provides when fully upright. Disciplines like inline hockey and freestyle slalom typically favor round-profile wheels of smallish to medium size, with diameters no larger than .
At the opposite end of the spectrum from the flat profile is the bullet profile, also known as the pointy, thin, narrow, or elliptical profile. This shape is usually found in larger wheels ranging from to in diameter. The combination of a bullet profile and large diameter serves the unique demands of speed skating, where racers aim to satisfy two seemingly conflicting goals.