These wheel inserts (code S04-SI1) are designed to fit inside Slot.it 15.8 standard wheels (2x) and 4WD front wheels (2x). They're designed after the Speedline Racing wheels that equipped Audi Sport Quattro S1 rally cars. Read the Shapeways Scalextric FAQ for a suggested list of parts to be used with Scalextric models. This product is not a toy and it's only suitable for model makers of at least 14 years of age.