Stroker Performance Review

I deal with a lot of springs. The thing I know for sure is that the rate is very highly dependent on the wire diameter and the total length of wire in the spring. A variance of .001 changes things to a significant degree. This is why PAC springs are awesome - they control everything down to the gnats ***.

That said, the discrepancy between the catalog numbers and the numbers in reality probably comes from manufacturing tolerances. Almost every material made will come at the very min diameter, min thickness, min everything because material manufacturers are cheap bastards. The engineering is all done to 'nominal' sizes though. If a .100 shim only adds 30 lbs, and not 39 then the rate is closer to 300 than it is to 391 - or the installed load is lower than advertised. This can happen for a lot of reasons too. Rate is often measured on a 'new' spring, but once a spring is compressed beyond the intended working height, it will take a 'set' and everything changes (rate, pitch, preload, etc). There's also spring 'creep' which happens to almost any spring left in any amount of compression for long periods of time.

All this to say: never go strictly by the numbers, they're only there for doing the initial work but MUST be verified by measurements otherwise you simply cannot know. Just like compression ratio, installed centerline, etc.