Progressive-rate 1 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. Rocker arms are available in 3 progressive rates with 1 having softest rate and 3 being the stiffest. Progressive-1 rockers provide the plushest ride for absorbing bumps and surface irregularities. Progressive 1 rockers require new push rods (sold separately). Use push rods 5318 or 5319X.