As any skater would know, skateboard wheels are attached as a pair at the bottom of skateboards, held and mounted in place by specialized axles which are also known as trucks.
The longboard wheels or even those used by skateboards are normally made from polyurethane, which is a very hard type of rubber. These wheels also range in size between 48 to 100 millimeters. Other than size, the wheels are also available in different hardness and is measured by a durometer A scale.
The softest wheels used are at about 75a while the hardest one are at 99a. There were instances that I’ve seen some wheels having ratings of 101a and above but this would be inappropriate since the durometer A scale maximum measurement is up to 100.
Last but not least, longboard wheels and skateboard wheels are available depending on the skaters styled and preference of skating. For example, street style skaters would use the smallest wheel sizes of between 48 to 55 millimeters. Vert skaters who prefer using their jumping on ramps typically use sizes between 55 to 65 millimeters.
Slalom skaters would prefer using bigger wheels than vert wheels at 60 to 75 millimeters, which allows for constant maximum speeds at a longer period of time. Longboard wheels meanwhile are made typically for transport and cruising. The common sizes used for longboarding range from 65 to 100 millimeters and they provide better stability and comfortable rides.
For beginners just starting to skate, the best choice of wheels would be the softest you can get your hands on to provide grip and from there you can gradually increase the hardness as you advance in skill level.