Description Reviews Spring Rubbers are used to add progressive rate to your linear race spring. Four different Spring Rubber durometers (50D - 90D) available for desired rate tuning. Fits springs with 2.5" to 3.0" inside diameters. Adds approximately 1.5" of solid height to your spring. Made in the U.S.A. and sold individually. Spring Rubbers are used to add progressive rate to a linear race spring. Durometers ranging from 60-90D. Sold Individually.