Video Overview and Installation Guide
DIY HAS Writeup
So, I've wondered for a while if one could DIY a rear HAS setup at a reasonable cost. If possible, this would solve common issues like not getting the rear height/wheel gap/rake you want, not being able to choose your rear spring rate and flat ride ratio, and having to buy expensive "BMW fit" springs.
Spring Mounting Hardware
I noticed that aftermarket coilover systems typically had flat ground springs (like standard coilover springs) that seated against their ride height adjusters, and wondered if it was possible to obtain just the rear ride height hardware. As it turns out, both
BC Racing (PN I-29-HLK) and
Solowerks offer their rear perch/adjusters. I ended up buying a set of the BC Racing perches/adjusters for $100 shipped (you can select any F2x/F3x/F8x on the linked site's drop-down menus and it'll bring up the right PN).
One thing I bought that was completely unnecessary was Swift's Thrust Sheets. These are supposed to act like a radial bearing to allow the spring to spin so it doesn't bind during compression/decompression. You really don't need these.
Next, in order to use a standard coilover spring I'd need a lower spring perch/pad to rest it on. Since that side is also ground flat the perch/pad needed to have a flat base. I looked through
Energy Suspension's Catalog and on page 14 found universal polyurethane spring pads.
PN 9.6116 and 9.6121 looked like they'd fit over the protrusion/mount in the rear camber arm, so I bought the cheaper of the two (9.6116).
This part's flange/seat is too large to allow it to sit flush in the lower camber arm, but that was easily remedied by cutting it to the appropriate profile.
Lower spring pad not sitting flush
Modified lower spring pad
Picking Springs
The last piece of the puzzle was choosing which spring to use. Both the BC Racing and Solowerks perches can use 60mm ID springs, although you'll probably need to remove the plastic perch on the BC unit. With the perch you can use 2.5" (63.5mm) ID springs. 65mm ID springs also probably fit fine, and you can choose to use a
60mm to 65mm adapter if you like.
To pick my spring I looked through
Hyperco, Eibach, and Swift's catalogs. For my particular application I'm targeting a rear ride freq around 1.8Hz, so a 900lb/in spring is what I was after. When looking through the catalogs and searching the PNs for the spring rates I wanted I was surprised at how cheap standard coilover springs are. I saw them ranging from $50 to about $80 each, and the ones I wanted (Hyperco 2.5" ID, 900lbs/in, 10" long), ended up only being $57 each shipped.
A 9" coilover spring is probably optimal for having the most adjustment range, however 8" and 10" springs are far more common. If you decide to go with a spring rate as high/higher than mine (or you want to drop your rear height really low), you'll probably want an 8" spring instead. If you're keeping things around OE rates or up to M3/4 (~650lbs/in) then a 10" spring will be fine. One thing that's important to note is that you want it so that when the camber arm is up and bolted to the rear knuckle the spring has some load on it so it can't pop out. This is a concern if you go with too short of a spring, and is why helper springs and couplers exist.
Rear HAS in Practice
Rear HAS installed
I ended up having to adjust my rear perch all the way to the top (minimum height) to achieve the 14" hub to fender measurement I was targeting. I might drop the rear a bit more, but will have to remove the locking ring to allow the height adjustment ring to get closer to the top. Many coilovers don't use a locking ring on the rear, like Bilstein, and KW, so I'm confident I don't need them.
So far I've been driving around on this setup about five days now and have been pleased with it. I'm also quite pleased this setup came in at <$250, which is less than it'd cost you to get a pair of M-Performance rear springs.
But what about the front?
I'm thinking about the front too, but haven't found anything yet that would be easy and cost effective...