The 2-Layer ( Dual Layer) design with this ?undersized? X6 core is one of our UNIQUE designs and inventions that we started end last century !!! The advantage of the Dual Layer is that the wheels will get MORE rebound, MORE grip and MORE smoothness in riding. The first layer color will tell the Hardness of the Urethane: SOFT: PURPLE / MEDIUM: RED /HARD: YELLOW Our new Radix Line that will set the new standard again and will change the scooter sport for ever; like we did before... The 30mm wide wheels will replace the 24mm inline skate size wheels as it will give the user more grip, more angle, more control and more air time! The Radix Line wheels are available in 3 different hardness: SOFT, MEDIUM and HARD ! SOFT: just below the ?standard? hardness of the ?regular? scooter wheels for most smooth riding and super high grip. MEDIUM: Equal to the ?standard? hardness HARD: Just above the ?standard? hardness ideal for warmer areas and smooth surfaces like the skate park. Fits the Eagle Supply, District and Addict forks and decks. More brands will follow. Eagle Supply will supply you with all spacers needed for any brand on request. EACH MODEL HAS DIFFERENT HARDNESSES FOR INDIVIDUAL CHOICES.