This is Pro-Lines VTR Closed Cell 1:8 Truck Inserts. Choosing the right insert for the right condition can be the difference of winning, loosing, or just plain having bragging rights over a competitor on the track. Pro-Line now offers an aftermarket insert for those tracks that desire a firmer insert set-up. Closed Cell Technology Channel details capture the tire so carcass flex is kept to a minimum No air gaps in between tires unlike other brands Perfectly balanced 100% water safe Reusable Extremely durable Light weight For use on tracks that desire a firmer insert VTR Closed Cell Inserts (2 pcs)