Roller skate wheels are defined by diameter (size) and durometer (hardness). But elasticity is the most underrated characteristic.
If you believed the manufacturers, roller skate wheels would only differ in size and hardness. But other characteristics, which are not commonly cited, can be more important for the performance of a wheel.
The hardness is measured by the Shore Durometer. It is the depth of an indentation in the material created by a given force on a standardized presser foot. Mostly Shore A scale is used. It measures the hardness of flexible mold rubbers and elastomers, that range in hardness from very soft and flexible, to medium and somewhat flexible, to hard with almost no flexibility at all. Some harder indoor wheels are specified in Shore D. The Shore D scale measures the hardness of hard elastomers, semi-rigid plastics and hard plastics. In practice there is an overlap on the two scales.
Most wheels are made from polyurethane (PU), an organic polymer and versatile elastic plastic material. Besides the hardness PU can have a range of other attributes:
- Load bearing capacity
- Abrasion & impact resistance
- Bonding capability to the core
Not all of these parameters are same important for usage in roller skating. However for outdoor skating over rough asphalt, the flexibility and resilience is most crucially. The wheels have to recover from distortion quickly, when you hit edges or stones, and unload the wheels by reducing the pressure. Otherwise the wheels are getting stuck. Only micro-movements, but the fast sequence of the distortion and unloading results in a perceptible difference.
Differences can be huge: I worked for some years in a skate shop. To show the customers the impact, we had two small balls of PU, each 78A. One ball bounced like crazy when dropped to the floor, one was literally dead, bounced quarter of inch and stayed on the ground.
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That’s the elasticity of good outdoor wheels. Rebound lets you bounce over edges and pebbles and makes sure that the wheels don’t get stuck. If you hit an edge, a soft outdoor wheel will be disorted. When your foot lower the pressure to get over it, it is important that the wheel rebounds very quickly into the original shape. To avoid to get stuck and to regain the power. Elasticity is more relevant than diameter when skating on sidewalks and in streets. #rollerskating #berlin #streetskating
The elasticity is also more important for smooth outdoor skating than the diameter. The advantage of larger diameter is mainly the “better roll”, so longer distance with one stride = faster. The angle between the surface and the tangent line of the wheels, which determines, if the wheel will strike the obstacle in acute/obtuse angle, of 65 mm diameter wheels is not such lower, compared to 62 mm, or even 57 mm diameter. This cannot outweigh lack of elasticity of the wheels. A high-rebound 57 mm diameter wheel will perform better than a less elastic 62 mm or 65 mm wheel. Similar a resilient 85A wheel will be perform better than a less elastic 78A wheel.
Radar Energy wheels are popular and widely recommended for outdoor skating , because they have a high flexibility and rebound, more than any other wheels I have seen in the last 20 years. Their higher elasticity compared to other wheels allows a quicker “take off” of the wheels when they stick at an edge. I’m using the 57 mm Radar Energy. Although the 57 mm could be little bumpy, and 62 mm Energy should be the better selection for most skaters, and my recommendation.