Quote:
Originally Posted by ghost.M
You're right - there is less data on that thread about front rates than rear rates, and they don't mention the other factors you've highlight above. Seeing as no-one else (at least not yet found on this forum) has done the work to calculate, the rate estimates on that thread are about as good as you'll find anywhere though. Also, the OEM F80/F82 front spring rates aren't very stiff either.
Reading the below thread you'll find the F80/F82 crowd as well as the engineers at Swift Springs would disagree with you about the handling characteristics of high spring rate variances between front and rear on F-Series bmws. In addition, the AFE Control Coilovers for F30s (couldn't find any reviews on these) have an even higher front rear variance than the Swift Springs! Not trying to argue here, just suggesting the data seems to go against your intuition on this and maybe the front rate estimates on babybmw aren't too far off.
https://f80.bimmerpost.com/forums/sh....php?t=1054041
185 lb/in Stock M3/4 Front Springs
575 lb/in Stock M3/4 Rear Springs
268 lb/in M3 Swift Spec-R Front Springs
715 lb/in M3 Swift Spec-R Rear Springs
279 lb/in M4 Swift Spec-R Front Springs
726 lb/in M4 Swift Spec-R Rear Springs
https://afepower.com/afe-power-430-5...oilover-system
275 lbs/in Front, 1100 lbs/in rear |
There is a lot of information missing in effective spring rates, as the engineered bump stops BMW uses are in use over much of the suspension compression, so the steel spring rates are only applicable over the initial compression and all of the extension. Without looking at all the components, not a lot will be learnt about the effective suspension stiffness. The low front steel spring rates are quite similar to my fun car that weighs 1/3 as much as the M3/M4. One of the conclusions likely is that changing the front steel spring rate will have little effect when not addressing the bump stops and front anti-roll (sway) bar too.
The compound spring rate under hard cornering is going to be a lot higher than the quoted rate. The polymer bump stops also have a progressive spring rate that increases with compression, compared with a steel spring being either linear, or progressive when coil-bound with variable pitch coils. The F20 OEM front struts only have about 10mm of compression at normal ride height until the polymer bump-stop adds to the spring rate.
Another issue is going to be the effect of the rear anti-roll (sway) bar with stiff rear springs relative to the front, ensuring that it isn’t too stiff to give balanced handling and good traction under acceleration, too stiff and the inside rear wheel will easily lift while the front outside wheel is too heavily loaded.
If the front to rear rates are this disparate, then I wouldn’t change the strut / coil-over springs without understanding the total effective rate and adjusting the other components too.