H&R 36mm I.D. Race Springs are crafted from special lightweight (hf) 54SiCr6 spring steel with multiple lengths and rates for optimal tuning and setup. 36mm main and tender springs includes main springs ranging from 2.76" to 5.12" in length, and 72 lbs./inch to 686 lbs./inch spring rates. Tender Springs are available for optimal fine tuning and performance. TENDER SPRINGS: H&R 36mm I.D. Tender Springs are made of trapezoidal (flat rather than round) spring wire for a low block height and maximum travel. These springs are designed for demanding motorsport environments, and, like all H&R springs, have been pre-blocked at the factory so they will never set or sag.