Raising and softening the rear

[filtercoffee]

My car is on Fortune Auto MSC-1 coilovers, 8kg/mm front, 10kg/mm rear linear springs. 235/40/18 front, 265/35/18 rear. The rake was pretty neutral, with the rear roughly .5 inches above the front measured at the side skirts.

Today I swapped the rear 10k springs with the stock Z4M progressive springs. I kept the coilover's shock in place. I did this because I wanted to raise the rear ride height for aesthetic reasons, and didn't feel like adjusting the rear spring perch (lazy, also the design sucks, also wanted to see if this change would improve ride quality). Now the rear sits about an inch higher at static resting position. The ride quality is _greatly_ improved. Front to back weight transfer is slower so the car doesn't push you around as much when you shift from 1st to 2nd. Much more composed on on the street.

Also - it could be a placebo affect, but I feel the car is more willing to turn in. and in the middle of a turn, it feels more balanced, less prone to understeer. Can someone tell me why this is? Is it more likely the height change or the softer spring?

[filtercoffee]

I think some of what I'm feeling is just the increased body roll.
Drove it on the highway, it's definitely a bit floaty/bouncy now. Feels like a luxury car compared to before (the good and the bad that comes with that).
On the lookout for some Eibach or H&R lowering springs for the rear to test as a middle ground.

[maupineda]

I found your posts very interesting as that is exactly what I have been thinking about doing. My car rides on H&R Coilovers, and it is overly stiff and I have been looking at TCK springs, just trying to figure out what rate would be best for my needs. they have linear springs only, rating from 400 to 700 in increments of 100's.

Your 10k springs are ~550, which is a nice in-between from what I have read. the most feedback I found was about Z3M users (Z3Ms are lighter, but have similar suspension design and 50/50 weight distribution), and in general, they said a 400 and even 500 spring would be soft, and that 600 is the sweet spot, with those cars being lighter I would think for our platform we may need a slightly higher rate, but maybe this logic is plain wrong. A guy who claimed to be a suspension designer said that for the Z3M a nice rate for street and track with appropriate dampers is 350/500

Anyhow, you seem to have a rate I would consider a happy mean, if you just want to raise and soften it, you could go with TCK 500 and take the hit and mess with the adjusters to set it where you want even though they are a PITA.

I wish I knew the rates H&R use, I just know they are rock hard, and progressive coils are consumed with just the weight of the car leaving just the stiff coils as active, which makes the ride very crappy. I read somewhere the stiffer section of the spring can go from 700-1000 lb/in, and that is too much. in my case, I am almost ready to buy linear 600 for my setup and try it out.

lastly, I have found that from the factory the rear hight is set 13mm lower than the front (referencing the wheel and fender arch), I assume this is needed to achieve the 50/50 weight; if the rake is inverse, then you are shifting some more weight to the front, I don't think it is a big issue for normal driving, but this will affect how much weight the rear carries (less) which adds a bit to the crapy ride if that makes sense.

[maupineda]

Originally Posted by filtercoffee

I think some of what I'm feeling is just the increased body roll.
Drove it on the highway, it's definitely a bit floaty/bouncy now. Feels like a luxury car compared to before (the good and the bad that comes with that).
On the lookout for some Eibach or H&R lowering springs for the rear to test as a middle ground.

Interestingly now the OE springs feel "plush" as even the stock setup is said to be stiff. the issue I see is that now you have a stiff front and a soft rear, which adds traction on the positive side of the things. if you like how it rides, you can compensate roll with a stiffer antiroll bar at the rear. but roll is not a bad thing.

[filtercoffee]

Try it out! swapping out the rear springs is relatively easy and what a huge difference it makes. My luxury car statement is not totally fair, it's not that bouncy like a car without shocks, but it does move a bit more than it used to on the highway. it's a good change. 10k really was too stiff for the streets. Maybe OK for track only. Maybe our seating position in the z4 close to the rear has something to do with it.

I can send you my rear spring setup from the fortune auto kit for you to try if you're interested.