Here are the specifications of that spring. I see in other posts, some efforts at mathematics, swing arm length, ratios, and the like. I disregarded them, relying instead of individual specifications such as length, wire diameter, total coils, and ends. I also ordered two additional springs of very near specifications, but they're just too light. One of them would work with an additional inner spring. I do not like two springs, as they rub, weakening both springs.
Part #: 4115 O.D: 1.531" LENGTH: 4.25" ID: .907" RATE:1003 MAX. DEFLECTION: 0.48" MAX. LOAD: 478 LBS
WIRE DIA: 0.312" TOTAL COILS: 9.5 MATERIAL: Spring Steel ENDS: Closed Ground FINISH: Zinc Cost: $20.09