Comparative Durometers

What’s with all the different ratings?

According to the Merriam-Webster’s Collegiate Dictionary Tenth Edition, the definition of durometer is an instrument for measuring hardness. The Rex Gauge Web site (durometer.com) states that a durometer is the international standard for the hardness measurement of rubber, plastic, and other nonmetallic materials. There are several types of durometers, each designed to measure a particular range of materials. Skateboard-wheel urethane is one such material, most commonly measured on the A scale.

There are actually about nine different scales for measuring durometer, but currently only four¿A, B, C, and D¿apply to the hardness of skateboard wheels. According to Instron Corporation (instron.com), another company specializing in durometers, each scale ranges from zero to 100, excluding readings below ten and above 90¿these measurements are not considered reliable, and the next appropriate scale would then be used.

The A scale is applicable to soft rubber, neoprene, wax, and other materials that normally yield under fingernail pressure. The B scale is useful for materials moderately harder, like a typewriter roller. And the D scale is utilized for hard rubber, Plexiglas, and laminates like Formica. More recently, wheel manufacturers have begun using the D scale on their dual-durometer, and some mono-durometer wheels.

The A scale has been used to measure the hardness of the skateboard wheel since its inception. Wheels manufactured throughout the 70s and 80s were much softer than the wheels ridden today, so the A scale was appropriate. As wheels hardened into the upper 90s on the A scale, wheel companies used a modified A-scale durometer to measure hardness, or just guessed. But when skateboarders required higher-performance mono- and dual-durometer wheels, a scale with greater accuracy was needed. Thus skateboarders were introduced to the D scale.

Even though it’s very common to see a hardness rating of 95, 97, 98, 99, 100, or even 101A on a wheel, literature from durometer-scale manufacturers states that these numbers can’t be accurate. Skateboard-wheel companies continue to use the A scale because it’s more familiar to skaters, and while a 101A wheel makes little sense to a chemist, it’s perfectly understandable to a skater. Darkstar has totally adopted the D scale to measure the durometer of their wheels. Their mono-urethane wheel registers at 52D, which would read about 102 or 103A if the A scale was accurate at that particular hardness. Australia’s Coretech dual-durometer wheel has an inner-core hardness of 75D, which is well above the reading capabilities of the A scale and a stated riding-surface rating of 98A.

Not to confuse the matter even more, but Ricta has begun offering its line of wheels with durometers using the B scale. Ricta’s wheel hardness is rated at 82B, which is equivalent to about a 99A or a 45D. The B scale is well suited for measurements between 92A and 55D.

The C scale, which is in the numerical range to measure the hardness of skateboard wheels, hasn’t been utilized by wheel manufacturers.

As long as skateboarders continue to push performance standards for wheels, manufacturers will invest in wheel-formulating technology to meet the required characteristics. As in most advances in technology, new and more defined measuring instruments are needed to better determine the product’s properties. The fact that we have three scales to measure hardness is just one example of how skateboarders are challenging skateboard technology to reach even higher levels of performance.

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