After extensive research (special thanks to
fe1rx for his epic Ohlins write up and to
vtl and
PeterY for the extensive conversations we've have had) I decided to do the koni coilover conversion.
This basically replaces the swift spec Rs with linear springs where I am able to specify the rate and length of the springs. Which then leaves the question of how to select the "correct" springs....
In order to determine this it is important to set requirements:
1) Approximately 20mm increase in ride height front and rear compared to the current swift Spec Rs
2) Springs must be preloaded such that they are not loose at full droop.
3) The setup should limit spring travel to the springs usable stroke and prevent any coil bind.
4) Allow for 235 tyres to be fit up front in the future.
Rear Springs
The biggest issue for the rear was finding a suitable lower spring mount for flat bottom springs - I wasn't comfortable leaving the rear spring unsecured. Due to the shape of the camber arm it was difficult to find and off the shelf part. Fortunately I discovered that ohlins will sell the part from their kit so i decided to go with that. (wasn't cheap).
Rear spring rate and length was pretty straightforward to determine - Look at what others have done and ensure that there is good adjustment range (i.e. suitable position on the upper perches). I settled on a 10.6kg (600lb/in) 10" rear spring this puts the adjuster 35mm from the body for my desired ride height, perfect ! A 9" spring would have also worked just would have been mounted 1" lower on the perch and by virtue of a shorter spring would have had less usable travel.
As for brand i decided to go with Hypercoil - Mainly because i could bundle the spring with other stuff i was ordering from summit racing. Ride height was spot on where I wanted it to be based on the calculated perch location suggesting the advertised spring rate is accurate
Initial impressions with just the rear springs
-Huge improvement in ride quality - which i assume is attributed to the increased suspension travel. No longer does the rear crash over bumps (the crashing from the front is now much more apparent).
-Significant increase in rear end grip especially over rough surfaces - Can feel the dampers working hard to ensure that the wheels are constantly in contact with the ground.
-Car balance - Unfortunately with the increase in rear grip the car has a tendency to understeer far more... The car also tends to snap quite hard if the front is loaded up and you go near the throttle. I anticipate this will be cured with when the front springs go in.
Front Springs (have not been fitted yet)
On the e82 front springs are a little tricky to get right and ultimately depend on your goals for the car. As my intention is to fit 235 tyres to the front in the future, spring length is a huge consideration. This steers you towards a 7" spring which due to its shorter length has less usable travel than an equivalent longer spring.
First step was to take some approximate shock measurements (only suitable for shocks which mimic stock shocks exactly):
Top spring perch to top of knuckle attachment - 385mm (extended length)
Top of knuckle attachment to top of shock body - 240mm
From body to bottom of rim (extended length i.e. full droop) - 640mm
Spring data Z65-178-060 178mm length 6kg/mm
Usable stroke 107mm
Maximum Stroke 117mm
Shock travel = 385-240 = 145mm
Clearly this is more than the springs maximum stroke however this doesn't account for the space occupied by the bump stop. The beauty of the Konis is they have an external bumpstop giving me the freedom to choose any length I like and also alter the point where it engages.
I have decided to go with the bumpstop that comes with the externally adjustable konis on the rear (to keep things consistent). This is a 55mm bumpstop with a soft linear progressive characteristic, that when fully compressed is approximately 22mm in length. Removing this length from the shock travel results in 123mm of total shock travel still more then the maximum available from the swift springs. In order to ensure the spring is never coil bound I will be using a 12mm slice of HDPE to engage the bumpstop 12mm earlier, this ensures that the spring can never be coil bound, with the last few mm past the usable spring stroke having significant bumpstop resistance.
The last point, ride height! the 6kg springs will compress approximately 60-62mm from their free length due to the weight of the body. Full droop length minus this compression results in an approximate ride height of approximately (640mm - 60mm) 580mm satisfying criteria 1 (assuming minimal preload).