STF stands for Street Tech Formula. These skateboarding wheels are made by Bones specifically for street skateboarding. Most skateboard wheels are made from urethane, and have a hardness rating stamped on the wheel (95a, or 100a û always with an "a" after the number). STFs are made from BonesÆ own unique blend, and donÆt have a hardness listed. Instead, they are built specifically for street skateboarding in every way.Sold in sets of 4