Concept
Modern skateboard wheels are made of urethane, and come in many different sizes and shapes to suit different types of skating. Larger sizes (65mm and above) roll faster on asphalt and can handle road imperfections, but may weigh more and consequently have a slower initial acceleration. Smaller sizes keep the board closer to the ground, result in a lighter board, result in a quicker initial acceleration, produce a lower center of gravity, but also make for a slower top speed. What makes a good wheel is a fiercely debated topic within the skateboarding community (and even within the specific disciplines).Modern skateboard wheels each use at least two sealed bearing units per wheel.
Hardness
Wheels also are available in a variety of hardnesses usually measured on the durometer 'A' scale. Wheels range from the very soft (about 75a) to the very hard (about 99a). As the scale stops at 100a, any wheels labeled 101a may be harder, but do not use the appropriate durometer scale.Sizes
Longboarding
Large wheels are used in most longboarding and downhill skateboarding disciplines. Sizes range from 65mm right up to 100mm and beyond. These wheels almost always have cores of hard plastic that can be made thinner, lighter and more rigid than a solid urethane wheel. The durometer of choice is a function of the discipline -- e.g., carving is often satisfied with a softer wheel and sliding sessions preferring a harder wheel.Slalom Skating
Slalom skating requires mid-sized wheels (60-75mm) to sustain the highest speeds possible, yet provide control. The desired durometer depends on the event, rider, etc., but tend toward the low 80's in the durometer "A" scale. Slalom wheels tend to feature profiles that are designed to increase traction usually by way of a square or thin outside edge.Vert Skating
Vert skating requires wheels in the 55-65mm range as vert skating involves high speeds that smaller wheels are unable to sustain, but the weight and acceleration characteristics of larger wheels would prove problematic. Vert wheels are usually hard, so they can roll faster on the smooth surfaces encountered.Street Skating
Street skaters prefer smaller wheels (usually 48-55mm), as small wheels make tricks like kickflips and ollies easier. Street wheels also tend to be hard, e.g, 99a.Durometer
Durometer (or duro) is one of several ways to
indicate the hardness of a material, defined as the material's
resistance to permanent indentation. The term durometer is often
used to refer to the measurement, as well as the instrument itself.
Durometer is typically used as a measure of hardness in polymers,
elastomers and rubbers.
Durometer Scales
There are several scales of durometer, used for materials with different properties. The two most common scales, using slightly different measurement systems, are the A and D scales. The A scale is for softer plastics and is therefor the scale used for meausuring the hardness of skateboard wheels and bushings.Method of measurement
Durometer, like many other hardness tests, measures the depth of an indentation in the material created by a given force on a standardized presser foot. This depth is dependent on the hardness of the material, its viscoelastic properties, the shape of the presser foot, and the duration of the test. The durometer test allows for a measurement of the initial indentation, or the indentation after a given period of time. The basic test requires applying the force in a consistent manner, without shock, for 15 seconds, and measuring the depth of the indentation. If instantaneous depth is desired, force is applied for only 1 second. The material under test should be approximately 6.4 mm (.25 inch) thick.The final value of the hardness depends on the depth of the indenter's penetration. If the indenter penetrates 2.5mm or more into the material, the durometer is 0 for that scale. If it does not penetrate at all, then the durometer is 100 for that scale. It is for this reason that multiple scales exist. Durometer is a dimensionless quantity, and there is no simple relationship between a material's durometer in one scale, and its durometer in any other scale, or by any other hardness test.
Durometer of skateboard wheels as compared to various common materials
| Material | Durometer | Scale | |
|---|---|---|---|
| Ebonite Rubber | 100 | A | |
| Hard skateboard wheel | 98 | A | |
| Soft skateboard wheel | 75 | A | |
| Automotive tire tread | 70 | A | |
| Door seal | 55 | A | |
| Rubber band | 25 | A |
Durometer rating and relevance to Skateboard Wheels
While the durometer rating is a valuable measurement to determine skateboard wheel suitability, it will not give a complete picture of a wheel's performance characteristics. It is most valuable for comparing the same model of wheel that is available in different durometers. If one is using it to compare different wheels, other factors such as wheel profile, urethane thickness, urethane properties, etc. may affect wheel performance.Additionally, the durometer ratings for wheels are not performed by independent labs, so the ratings may be inaccurate or even misleading -- there are some wheels available with a durometer of 101a, which are an impossible rating on the A scale.
Broadly speaking, the durometer ratings of skateboard wheels is generally between 75a and 98a.
Skateboard wheels come in a variety of colors, sizes and degrees of hardness. Skateboard wheels have two stats -
- Diameter, which means how tall the wheel is. This is measured in millimeters (mm) for skateboard wheels.
- Durometer, which means how hard the wheels is. Most skateboard wheels use what's called the "a-scale". You don't need to understand all the details of how the a-scale is measured, just that the higher the number, the harder the wheel. For example, you'll see the hardness of the wheels written "a95" for a an average street skateboarding wheel. Softer wheels can be all the way down to a70, or even lower.
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