Think of leaf springs as a stick. If you notice, each spring gets shorter towards the bottom of the pack. Now tell me, is it easier to bend a long stick or a short stick? The bottom, much shorter springs are rated higher to withstand a load. Hence the name overload springs. The top springs are the cushion springs, so essentially all leaf packs are the same except for the load they are rated at, minus the fact that they are different in size and slightly different in spring rate.
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SOLD- '13 JKR with lots of goodies
Current- '98 TJ Sahara: build coming in October