[DaveJB]

I suspect Fortune tested their setup with a non M Z4 - I'm pretty sure the S54 engine weighs 100 lbs more. So, they messed up the specs of both front and rear springs. That is, I upgraded to Swift springs and apparently Swift doesn't sell springs in the same lengths as their in house stuff (6" IIRC) so they sent 1" shorter springs both front and rear. IIRC I got 5" Swift springs for the front and rear when these coilovers need 6" springs. Also, the rear Swift springs, while being too short were also just too weak. Over a couple years they compressed/sat shrunk massively. You wouldn't believe the pics I have of them when the rear started bottoming out. So, I've sold the car but here's what I figured out it needed.

Front: 6" Fortune Auto 10k front springs
Rear: I bought my own Eibach 6", 10.7k springs (part 0600.250.0600)

Yes, higher spring rates and the car didn't ride any worse.

Okay, why would it make a difference? The issue I first experienced up front was bottoming out. I figured out that while they claim the usable travel was about 4", the car's static sag was over 50% so as the car sat it had less than 2" of travel before bottoming out. This was all with the too short 5" Swift springs. However, you may be right. The difference might have been when adding the longer springs I also increased the preload which would cause the car and shock to compress a bit less when the car is set onto the ground. Basically, increasing static sag/ride height. So, from my best memory the available suspension travel was just less than 3". So, this with higher spring rates took care of the front end bottoming. Before I did this, it would bottom out on sharp bumps that weren't even 2" tall, and it sounded like a gun going off.

Anyway, I have an 85 email conversation thread with them over the many years getting this worked out. If you need any more help PM me. Attached is a pic of the car as it drove off after selling it, after all the coilover issues were solved.

EDIT: one more note about the rear end bottoming. I found that the spring perches were too long and the sound of the rear bottoming out was the two spring perches slamming together. I shorted them a little. Here's a picture of what they looked like with the overly weak 5" Swift Springs before I put the 6" Eibach springs in. See how close the spring perches are at rest (the black threaded things).

I hate to say it, but at least for the Z4 M I don't recommend these guys. Look, the support and lifetime service are great. So, maybe said another way, do NOT accept the fitment they spec out. Get at least 10k 6" front springs and at least 11k 6" rear springs. In hindsight, since it took so much effort to get it done right I wish I'd bought the Bilstein B12 Shock & Coil Spring Lowering Kit.

And in case anyone is wondering why I'd replace such truly bad ass looking car (IMO), it's because I decided to buy one of these. A 2013 427 Anniversary Edition Corvette. Over 500 hp, convertible, just an incredible car. I LOVE IT.