The core is the hard center of a longboard wheel. Stiff and durable, it exists to hold the bearings in place, maintain the circular integrity of the wheel, and support the urethane cast around it.
A wide core will offer more support to the urethane which means that the thane will deform less, making it roll faster and easier to slide. A narrow core will allow the urethane to deform more, creating a softer but slower ride that will also enable the wheel to better conform to the riding surface. Wider cores are better suited to high speeds and sliding, while narrower cores are more capable when riding over rough surfaces and gripping corners.
Cores are traditionally positioned in the wheel in one of three ways: centerset, sideset, or offset.
A centerset core will have an equal amount of urethane on either side of the core. Centersets offer the advantage of allowing you to flip the wheel in the case that you should start to cone your wheels when sliding, or just want to adjust the ride characteristics. With your weight distributed more evenly across the width of the wheel, centersets will also wear more slowly than many wheels and offer more traction than other core positions. Wheels that use this core position: Mini Ozone, Ozone
Wheels with offset cores are the most common style available today. In an offset wheel the core is positioned between the center and the inside lip of the wheel, which makes them well-balanced for different types of riding. Wheels that use this core position: Freeride, Mini Slide, Slide
Sideset wheels have cores that are placed towards the very inner lip of the wheel. These wheels are extremely effective for easy sliding and make for great freeride wheels.