A 4.5mm internal shock spring, to be placed on the shock shaft, below the piston inside the shock body. The smaller diameter rests on the piston shim. These springs help the car drive more flat and stable, especially good on high grip tracks. Adjust your shock ball end to set the point that the spring engages. Undo shock end for later engagement, tighten shock end for earlier engagement. Works with most Touring Cars on the market.