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      10-16-2019, 09:24 PM   #10
FaRKle!
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Drives: 328d Wagon, M2 Comp, i4 eD35
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Quote:
Originally Posted by ktw View Post
Even higher than 900 lb/in might be a good idea like you mentioned to get a good frequency ratio. I haven't seen any application of main+helper spring in a non coil-over-damper setup, but I also don't see any reason it wouldn't work.
The helper spring is to keep the main spring loaded during droop events. With a really stiff spring the compression isn't all that much, so on droop, it can become completely unloaded and loose on its perches. The helper spring prevents that.

Your existing springs have a similar function. Bilstein uses progressive springs with two spring rates. If you look at your spring at static height you'll see all the initial rate coils already bound! So in normal driving you're only using the final rate coils. If the suspension becomes unloaded, that's when the softer rate comes into play to keep the overall spring loaded and in place.


Quote:
Originally Posted by ktw View Post
My original thinking was to do 900 lb/in rear and then later decrease the front spring rate slightly. For that I need to find an upper spring perch to adapt the stock roller bearing to a standard spring size (perhaps the perch supplied with Ohlins R&T coilovers). That's a project for another time though, and changing rear springs is so cheap/easy, maybe I should start with a higher rate and decide later on decreasing both front and rear. Back to the spreadsheets and catalogues to do some optimizing...
I thought about upper adapters, but the ones I saw were all for 60mm springs. Bilstein uses 70mm ID springs in the front IIRC. Machining an adapter seemed expensive. It's definitely more straight forward to go stiffer in the rear.


Quote:
Originally Posted by ktw View Post
You make some good points about why manufacturers might choose the spring rates they do for coilovers. I don't have much experience with various cars/suspension setups, but I do think the B16s make some tradeoffs for "feeling fast" over actually maximizing grip, hence I'm here thinking about more modifications! Also the 428 lb/in rear springs are not very happy when there's a bike rack and two mountain bikes hanging off the back--sometimes there are practical considerations too.
428lbs/in is softer than the OE F31 spring (485lbs/in). Heck it's softer than F30 M-sport OE rear springs. Terrible choice by Bilstein IMO.
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