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Originally Posted by ktw
FaRKle! Great work as always! Been lurking on-and-off, and your analysis/modifications are always well thought out.
How are the 900 lb/in springs working in practice? I'm considering making the same change for my F31 (petrol instead of diesel).
Background is I installed Bilstein B16 Komforts a while ago--wish I had seen some of your analysis of these earlier! Bilstein say the f/r main spring rates are 289/428 lb/in, which by my calculations puts the f/r natural frequencies at ~1.8/1.3 Hz. (Interestingly the B16 spring rates give much lower rear frequency than front for all applicable F3x models--still scratching my head over that.) A bounce test by hand seems to confirm the front at 1.8~1.9 Hz, and the rear at ~1.5 Hz, so I'm still refining the motion ratios and masses on my spreadsheet, but it seems clear a higher rear spring rate will be beneficial.
I'm curious if the 900 lb/in springs worked as expected to provide flat ride paired with the higher rate front springs you already have. I'm also wondering if there's a way to know whether the B16 dampers will work ok with that much higher spring rate. A custom damper tune may come in the future, but not right now.
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I'm getting my rear dampers tuned for the 900lbs/in springs in Nov, however even running underdamped in the rear (they're tuned for 650lbs/in now) the ride is very smooth! There's a lot of construction and wavy/broken pavement around my office and I was actually surprised at how much smoother those features became. My rear frequency ratio is 1.25 (or 25%) of the front. The low speed underdamping makes the rear feel a little floaty, or take a bit longer to settle on turning/acceleration, but I can't complain about the ride smoothness.
Most coilovers I've seen (with exception of Ohlins R&T on 6cyl models) produce a higher front frequency. I believe they do this for a couple of reasons. The first is they want the user to have a "sporty" feel, meaning they want you to make you "feel connected to the road." If you've ever been in a really smooth riding car and looked down at the speedo and said "wow I didn't know I was going that fast!" (because it was so smooth), well that's what they don't want you to experience... They'd rather make slow "feel fast." The second reason I think they run really high front rates is to try and support the front of the car better due to the Macpherson struts. As the struts compress you lose camber, and thus cornering grip. Since they can't count on end users upgrading sway bars, they'd rather handle it themselves by stiffening the front springs to reduce roll. Lastly, higher front frequencies make the front more responsive due to shorter transients.
My final spring rates once I get the dampers retuned will be 245lbs/in front and 900lbs/in, in the rear.
If the B16 dampers' dampening range encompasses the regular B6's behavior, then there should be a setting to accommodate the 900lbs/in springs. Heck, you may even want to go higher than 900lbs/in (with a shorter spring and helper spring) if that's the case. I can look through the data I've built up to plug the numbers for ride frequency and damping ratios with the B6, and check PNs to get a better idea if that's true, but I usually charge a small consulting fee for that.