This is an optional Mugen Big Bore Rear Damper Spring Set.
These springs are made with 1.5mm wire making them slightly lighter than the previous springs and feature an improved coil design which will make both your buggy and truggy much more stable through bumps and jumps.
With the addition of the Soft rear springs to the Mugen "1.5 wire" spring selection, setting up the shocks on your buggy or truggy has now become much easier.