Hi Marquis Rex,
My university engineering learning days are over now, so although I get the gist of what you are mentioning, real world knowledge and experience based on solid engineering principles for this application I don't have.
In the days prior to coilovers and when there was a lot less choice of products including suspension packages for BMW's, I used to use a local company to me (long since gone now) who'd make me custom springs depending on what I wanted. In those days, you either went 'expensive' for Koni or 'entry level/cheap' for Jamex lowered suspension kits (I had a number of E30's through all of the 1990's). The Jamex kits I used to go for and then have the custom springs made. Many of West London's other BMW guys used to ask me how I'd get my ride height along with which wheels I was running - to which I never used to tell them ! My 'mistake' back then was to use the Jamex springs and/or the used springs from the car to have analysed/tested.
So my question to you is did you get brand new or used springs? I remember that there's such a thing as metal fatigue so a used spring will have some of that and thus, wouldn't be an '100%' benchmark. I'd suggest in you getting brand new OEM M5 springs to be tested, or, maybe write into BMW to ask them if they'd provide the specification from the actual engineering design.
I've really loved the H&R Springs I had on my previous E34 525i Sport using the oem MTEC shocks, and that car rode fantastically well for 30mm lowering, no one could tell or knew the car was lowered, ride quality was fantastic. I put H&R springs on my E39 Touring in 2006 and they're still on today! on OEM standard shocks, I Was told by numerous suspension speciaiist manufacturers that 30mm was as low I could go without using Sport/Shortened shock absorbers.
I'm thinking to go coilovers but part of me still wants to get al new Sport shocks and use D Faulkner Springs to make me some custom springs. Drop them an email or give them a call to discuss - Suspension springs for the Motorsport Industry | D Faulkner Springs LLP
There's another place I spoke to and for the life of me I Can't remember their name, expect they're just a few miles south of Birmingham (or at least they used to be a couple years back). I'd also say to speak to KW suspension as well as direct with Dinan in the USA so that you get a good spread of knowledge/expertise and real world testing/knowledge to augment your increased knowledge.
I guess brand new E39 M5 suspension would be nice, but having been in plenty, including fairly low miled examples (25-50K) I thought the ride was far too bouncy/wallowy and allowed a lot of body roll. If you aren't or won't be driving your M5 when you get it, enthusiastically then seems your project has more potential for you. If you will be driving more enthusiastically, I'd say alter you benchmark for a better handling/rated spring with respect to the characteristics you want.
2003 Ex-Police E39 530 Diesel Touring. Alpine White, 260,000+ Miles & climbing!
19" Hartge Nova 5 - with custom painted 'Flash Chrome' centres & Polished Lips (set of 5)
19" Hartge Nova 5 - Awaiting custom re-finish (set of 4)
19" Hartge Classic - with custom powder coated centres in shadow chrome & diamond cut/no lacquer lips (set of 5)
20" Hartge Classic - Awaiting custom re-finish
19" 3Pc Forged wheels - ID retained until re-built & finished (2 sets)
20" 3Pc Forged wheels -ID retained until re-built & finished
19" Centres - ID retained until rebuilt & finished
Full Custom Audio install - Arc Audio KS Mini ampliifers. DLS 5.25" 2 way front components & 4" 2 way co-axials. JL Audio 8W3-V3 subwoofer in stealth enclosure. Parrot Asteroid Smart head unit, custom door builds for DLS front stage & Earthquake SWS6 flatline subwoofers, custom rear pillar speaker pods. LumiDock iPod dock,
H&R Springs, ExoticsTuning front lip spoiler, Sport F/R bumpers upgrade, Carbon Fibre effect printed Vinyl roof wrap, High gloss black vinyl window trim wrap, Hartge bonnet/hood & tailgate badges, rear rooftip spoiler, matt Black kidney grille, projector foglamps.
Hartge auto gearlever, Hartge Handbrake handle, Hartge door sill strips, E46 M3 CSL Alcantara/Tri-stitch SMG steering wheel, full Black Leather Sport seats,
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Plenty more modifications to come!