This is the optional Purple 6, 4 rate shock springs for the Traxxas Revo monster truck, Traxxas offers a wide variety of spring rates for the GTR shocks to precisely compensate for lighter or heavier truck configurations and different rocker installations, The springs use high-quality spring steel with durable powder-coated finishes, Spring rate is indicated by colored dots on the end of the spring, Traxxas Revo, SKU: RCHOB1183