AKA is the pioneer in closed cell inserts. This insert was designed to decrease the lateral deflection of the tire while at the same time increasing the vertical impact absorption therefore providing better handling in the bumps. Using softer foam or hollowing out the inner circumference like other companies do still leaves the car feeling squirmy with a less accurate feeling through the bumps. Not to mention the loss of durability. Gil Losi and knew this compromise was not the solution. The solution they found was the addition of lateral grooves to the inner diameter. The lateral grooves allow the insert to flex vertically to absorb the large impacts while still maintaining lateral stability and overall durability AKA inserts are known for no compromises. The result is a more predictable handling car without giving up durability. The Black (Medium) insert is ideal for smooth, high-grip tracks and very warm ambient temperatures (90F/32C+).