I guess I'm confused why you want stiffer springs in rear, when they don't have as much weight to carry.
don't forget about motion ratio......
Please do elaborate
<- newb just trying to learn
motion ratio= ratio of "free" spring rate of the spring sitting on your coffee table
versus actual wheel rate dictated by that spring installed and articulated by the suspension geometry.
a spring has its "free rate".....the one that a spring rate tester would reveal. A car has its front and rear "wheel rates".....this is the real "working" spring rate because of lever-arm action within a given suspension geometry not to mention the additional rate from anti-roll bars.
Easy translation.....on a Focus the front suspension's motion ratio may be closer to 1:1, that is the given "free" spring rate translates to similar rate at the wheel, while on the rear, because of the geometry it takes more "free" spring rate to equal the wheel rate of the front.
Therefore a disparity of F/R spring rates on "paper", do not have as much of a disparity
as applied in the Focus suspension.
Pulling #s out of my arse for discussion purposes--
A front "free" spring rate of 325 may translate to into an effective wheel rate of 300 because of the given geometry, while a rear "free" rate of 375 may translate into an effective wheel rate of 275 for example.
Begs the question.....is there a "better" or "best" motion ratio? Well not really. One could say that a ratio that would allow the highest rate at the wheel would allow the lightest spring to be used. Always a desireable trait. However, most final motion ratios are just the end product of suspension packaging goals, or other suspension characteristic goals - NVH, camber curve, toe change/control etc.
I'll leave it to geeks far superior to me to share what the Focus' exact motion ratios are front and rear, as well as even provide examples of other cars' motion ratios....Mustangs etc.
I'm just a surfer for fawk's sake