The quality of the urethane used in a wheel is measured by its rebound. When in use a wheel loses energy creating heat and noise. The higher the rebound, the less energy is lost to friction in noise and heat, and will therefore be easier to propel. It may sound an odd concept, but a soft wheel is better than a hard wheel. A soft wheel is capable of absorbing vibrations from the ground, resulting in a smoother ride for the skater.
Softer wheels adapt more easily to the contours of uneven ground.
The relative softness of a wheel is referred to as its durometer. As a guide, the smaller the durometer, the softer wheel and the smoother the ride. The bearings used in an inline skate wheel determine how freely the wheel turns on its axel. A high ABEC grading indicates the most superior standard, with ABEC-9 the highest possible grade. Two kinds of lubricant are commonly used with bearings - oil and grease.