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I own an 08 in need of new shocks. I’ve been considering moving to the 2012 suspension and I wanted to know how much different the spring rates were. The results indicate very little difference in spring stiffness. It would be nice to include 2011 and 13-14 springs into this shootout. I can complete testing in about an hour if someone is game.
METHOD:
*I used an electro mechanical Universal Testing Machine. I captured the lower perch with an epoxy cast of the bottom of the spring with 100lbs seating load. The top perch is directly on the machine.
*I seated the springs using 250 lb then unloaded to 100 lb. At that point testing was displacement control up to about 1200lb.
*Displacement and load were recorded 100 times a second and post-processed in excel.
*All 8 springs were tested (2 front, 2 back for each year). The pairs of springs were then averaged to produce the results. Measured stiffness within the pair was similar averaging 0.12% difference between matching springs. The largest difference was 0.9%.
*Stiffness was measured from 1.5” of stroke to the end of the test usually around 2.25” thus capturing only the operational range and not the unloaded(droop) portion.
*Limitations: I don’t have the height of the springs in the shock so I can’t say what pre-load is. I also don’t have measurements to relate spring travel to wheel travel.
RESULTS:
From 2008 to 2012 the front was made 3.5% softer and the rear was made 1.7% stiffer.
Here is the load curve for only the front springs. This is actually 4 curves, but the matching pairs each line up on top of each other.
Here the curves for the rear. There’s less agreement primarily due to seating differences. The measured stiffness have a better match within the pair.
DISCUSSION:
This lends a lot of credence to the finding that replacing the 2008 ISF shocks with Bilstein produces excellent results. It appears the factory didn’t really change the rates noticeably during the change over to new shocks. More testing should be done on other model years and possibly on other springs to confirm that I’m not measuring some statistical fluke.
I did measure the dimensions of the springs and got reasonable answers. The measurements of the springs suggest the biggest difference between the front springs are 0.25 more turns on the 2012 which provides the approximately 20lb/in softer stiffness.
My machine won’t move fast enough to measure shock response, but that would provide a clearer picture of behavior on the car. I have a feeling the 2008 setup was over damped and that’s why they were able to get a softer ride without significantly changing stiffness.
At this point in the life of my 08 it’s under damped (old shocks) and I would hate to stiffen the response. This is more evidence I should be looking at 18” wheels.
Good lord you're amazing, thank you for taking the time and energy to record these results for us!
Back in 2018, I couldn't find any information relative to our factory spring rates. Thank you
I’m working on some suspension stuff for those of us who have comfort as a priority... I may want to send you some RCF springs if you’re game to try those as well?
06-13-19, 01:52 PM
On ISF Factory Spring Rates
