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Ended up purchasing a set of KW competition coilovers a while ago.
This setup was bought 2nd hand, looks a bit rough but was a decent price for such a high end coilover. I bought them with the intention of changing out the springs and swapping in custom rates.
The KWs came with the original springs for the fronts, but an MCA setup on the rear. Front springs are 61mm KW springs with unknown spring rate. Fronts have a helper spring which make annoying noises coming to a stop.
The rear springs are 200mm, 16KG 62mm MCA springs. These would be way too stiff for my liking.
I ended up going with HR ID springs
Front: 60mm internal diameter, 200mm length, 50N/mm
Rear 2.5" internal diameter, 10" length, 600lbs/in
Converted to kg = 5.1KG front, 10.7KG rear
These rates were chosen to be street friendly on an E82. This should control the body roll and prevent the rear wheels from unloading, resulting in a loss of grip on tight corners. On an E92 you can get away with higher spring rates as it is a heavier chassis with a longer wheelbase.
From my experience the rear springs and shocks dominate the ride quality. I have ridden in E82s with 12KG rear springs and felt it was too stiff for daily driving, not to mention its not the best for grip over rough mountain roads. I decided to go for a bit less stiff. A 10" length was chosen to provide adequate length to run a sensible ride height. This length will not be loose when the suspension is at full droop as well. This rear setup should allow me to run a similar ride height to what I'm running now, which is quite sensible.
For the fronts I chose 5kg springs. The front spring rate is important to prevent excessive body roll, which can unload the rear wheels in tight corners. Currently, the kw V2 front spring rates are 3.5kg, which are a bit too soft and allow the rear inside wheel to unload on sharp tight corners. With a torsen diff this would make it pretty useless and feel like an open diff when it unloads. I have driven quaife/mfactory equipped cars on similar spring rates and it feels very inconsistent. With the M3 diff it means the wheels spin up a bit before locking, which is annoying. With stiffer spring rates you can get a torsen diff to work, have driven a 135i with 6kg/12kg springs and felt like it made the Torsen diff work. I am hoping this setup will give me more traction with the M3 diff.
Another thing to consider is that the KW setup comes with spherical bearings for the front top hats. The spherical bearing is used to position the angle of the front strut, while the steering motion is handled by the shock itself rotating. This means the spring has to rotate on the spring perch. With no thrust bearing to reduce friction this can be the source of noise and clunks, and also potentially increase steering effort. Springs will also coilup when compressed, so resistance to that motion can also effect the spring rate.
To solve this, I decided to purchase an ABS sheet and got a coworker to laser cut them for me. This bit of plastic can dampen the metal-metal contact between the spring and perch and also reduce the friction and allow the spring to rotate easier. I also purchased some thrust bearings for the top side, which allows the top hat to rotate much easier. my only concern is the needle thrust bearings I've purchased are unsealed, but might be fine if I just replace them periodically before every alignment.
Front struts are ready to install, still have some bits to sort out for the rear but will hopefully install soon. I liked the thrust bearings so I will replicate the setup on the rear as well, hopefully this will allow the rear adjustment to be easier, as it will not fight the adjustment collar while it turns.
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