Bushings

Bushings are typical of rubber and are attached to the truck. These small parts help keep the turns stable and accurate. The better quality and material used, the better the performance they will give you on your turns.
PU rubber is a really good material for bushings and will usually be what you can find here in this category.
Since wheels can have a certain hardness measured using durometer, bushings can also be the same. Heres the higher the value of the tougher bushings.