Rear suspension repair and Swift coilover spring upgrade:
For this I selected Swift 10" 65mm ID rear coilover springs with a 672 lbs/in or 12 kg/mm spring rate. Why 12k with a 7k front spring? My previous setup used TCKs own VVS coil springs at 350/600 lbs/in. This accounts for a 58.33% front biased spring rate. As I liked the F/R suspension balance, I wanted to preserve that ratio. With these rear springs, the ratio at least with my (possibly incorrect) math is preserved.
I also opted to add Swift thrust sheets top and bottom of the springs, and after discussion with our local suspension expert, Fe1rx was kind enough to supply me his spare set of custom rear lower spring isolators. More on these below. Not pictured are new lower shock mounts. One of the mounts was stuck on the old TCK damper, and the other had 30k miles on it, so I opted to replace them as a set with new OEM Febi Bilstein mounts.
Despite my love of the TCK coilover setup, I've had some issues with the rear dampers. My left rear damper bottom mount or stud nut seized on the right damper, and the left damper nut couldn't be removed at all. Perhaps they got overtorqued? I'm not sure.
I need to cut the nut off the left damper so I can see the threads. I had to remove the lower mount torx bolts (E12), lube the mount itself up really well, and pull the whole thing out of the lower camber arm. Almost didn't come out.
With the damper out I was able to spin up the rear TCK spring height adjuster and remove the spring without dropping the camber link. This is one of the huge benefits of coilovers, the bolt to drop the camber link is torqued to 122 ft lbs. Without an impact gun, it's a workout to get that loose and then to tighten again after.
With the TCK VVS rear spring out, I dusted out the rear camber link. I've been wrenching on my own cars for years now and every time I do suspension work I am still blown away by how much dirt/sand/gravel gets stuck in there.
On to the rear lower spring isolators. With standard coilover setups, the spring sits in the recess of the arm. BMW secures the stock rear springs with lower and upper rubber isolators/spring pads. After switching to TCK SA coils, I could occasionally hear the rear spring popping/knocking in the lower arm. If I had to wonder, probably getting stuck and then popping loose upon rebound. I had noticed that Fe1rx made a set of isolators when he did his Ohlins coil install. I reached out and he graciously sold me his spare set. Shipping from Canada is always pricy, but these are worth it.
As you can see, the back side of the isolator is machined out to match the curvature of the recess in the well of the camber link. This gives the spring something FLAT to sit on. Adding thrust sheets to this and the height adjuster above allows the spring to spin even when compressed.
With that, the isolator and thrust sheets were installed, and new 10" Swift springs monkeyed back into place.
Here's a closeup of the spring sitting on the isolator. It fits so well the thrust sheets stay put under the spring, which is a good thing. Provides a smooth surface to allow the spring to rotate and further reduce the chance of binding.
So with that done, I opted to put my old Bilstein B6 dampers back in on the rear. They had been abused heavily in their tenure on the car a few years back, but they're still in great shape. New rear mounts went in, the 12mm upper shock mount sleeves for the Dinan RSM were swapped back for the necessary 10mm sleeve since the Bilstein dampers are not externally adjustable.
Then, nothing left to do but repeat on the other side! This was easier as the TCK stud on this side was only damaged slightly and the nut came off with some force. As compared to being totally FUBAR on the left. Thankfully I had a matched set of quality rear dampers sitting around right?
And another closeup..
Impressions and final thoughts
First thing, I weighed the TCK VVS springs, the Swift coilover springs, and my old Swift Spec-R springs.
REAR:- TCK VVS springs (600 lbs/in): 5 lbs 1 oz.
- Swift coilover springs (672 lbs/in): 4 lbs 14 oz.
- Swift Spec-R springs (503 lbs/in): 5 lbs 15 oz.
FRONT:- TCK VVS springs (350 lbs/in): 2 lbs 1 oz.
- Swift coilover springs (392 lbs/in): 2 lbs.
- Swift Spec-R springs (201 lbs/in): 4 lbs 7 oz.
This is good information to have. Despite having an almost 100 lbs/in increase in spring rate, the swift coilover springs are 3 ounces lighter. Each. Coupled with the front springs, I have removed 8 ounces from the suspension. Really insignificant overall, but I like removing unsprung weight where I can.
What's really shocking to me is the difference between the Swift coilover springs and the Spec-R springs. 4 lbs 14 oz on the front axle, and 2 lbs 2 oz on the rear axle weight savings. I didn't weigh the front dampers, so real weight savings switching from OE BMW style strut to coilover is unknown, but here we are.
The differences are pretty minor overall. I don't know if it's the new springs, or the thrust sheets, or both, but the car is much more compliant on bumps and sudden elevation changes. Bump steer is reduced, and harshness is reduced as well, despite increasing spring rate. Changing to the bilstein rear dampers seems to have increased rigidity / tightened things up in the rear. I wonder how the valving compares between the B6 and TCK Koni external. I know the B6 is a much higher pressure gas tube.
I had some lateral movement under WOT in dry conditions which is now resolved. I suspect it's a combination of removing a sagged spring, and possibly the high pressure gas tube dampers. I need to drive more and start putting the car through its paces to compare the straight TCK SA setup to the new TCK/Swift front and B6/Swift rear setup.
With the increased rear spring rate I feel the car now needs a limited slip differential. I am getting some more tire spin in the corners, though despite keeping the ratio the same as before, the car feels more neutral. I probably haven't pushed it hard enough yet.
The next (and probably last) steps for the rear end are going to ultimately be the HPA M3 type rear lower camber link, TCK Dual (or maybe single) adjustable M3/1M rear dampers, and I think a Wavetrac LSD.
As always: any questions, fire away! 