These new 4S progressive and linear springs have been specifically designed to complement the SLP shocks, measuring up at 20mm long. The main advantage and benefit of progressive springs is that as the car initially enters the corner they work like soft springs to provide maximum traction. At the same time, when the chassis begins to roll in the middle of the corner, the spring rate progressively increases until they work like a hard springs, which allows the car to maintain maximum cornering speed. The 4S springs, like all the other Xray springs are manually measured, selected and matched into pairs to guarantee precisely equal length and damping characteristics.

#308264 Xray 4S Spring Set Progressive C=2.5-2.8 (2)
#308274 Xray 4S Spring Set C=2.3 (2)
#308275 Xray 4S Spring Set C=2.5 (2)
#308277 Xray 4S Spring Set C=2.9 (2)

Source: Xray