Progressive rate rocker arms determine how much the compressive force at the wheel will vary with suspension travel. The higher the rate, the more the suspension will stiffen as the wheel moves upward. This will reduce the likelihood of bottoming out when landing from a jump. Progressive rate 2 rocker arms are standard on 1/16 E-Revo, 1/16 E-Revo VXL, and 1/16 Slash. Adding progressive rate 2 rocker arms to 1/16 Summit will lower the ride height and stiffen the suspension.