Ever wonder what those numbers mean on your wheels? The numbers, followed by an "A", tell how hard the wheels are. Most skateboard wheels use the A scale for durometer, which is good - it helps make everything easier.
On the A scale, the lower the number, the softer the wheel. So, a wheel is 90A printed on it would be a relatively soft wheel, where 100A is pretty hard. In fact, there aren't many reasons to have wheels harder than 101A, though I've used wheels that are a lot softer than 90A. Softer wheels grip the ground better, but get torn up or develop flatspots fast. Harder wheels last longer, but don't grip. Longboards often have very soft wheels, where street skaters usually want harder wheels.
If you are shopping for wheels, check out the Best Skateboard Wheels list. For more help, read the wheels page of Choosing Your Skateboard to find out more!
Also Known As: a (written in lower case)
Examples: "Josh's wheels on his longboard are 70mm 80A Kryptonics, while the wheels on his regular skateboard are 52mm 100A Bones Onehundreds."