This is the Pro Molded Inner Foam Inserts for 24mm Tires from HPI.

FEATURES: Rubber tire inserts.
          Helps increase grip and stability of tires.
          Prevents tires from caving in during high speed & cornering.
          Yellow, and Rated SOFT.

INCLUDES: Two rubber inserts.

SPECS:    Width:            20mm (0.79")
          Outside Diameter: 60mm (2.36")
          Inside Diameter:  46mm (1.81")

COMMENTS: This is installed between the tire and wheel so they resist moving
            while the car is cornering.  For really high-grip tracks, use
            a firmer tire insert in the front with a softer insert in the rear.
                                                                   cmc 02/23/02
                                                                          ir/kh