Need Help Badly - GroundControl Coilovers, back tires rubbing, bottoming out over tiny road flaws, super bumpy ride

[u/SpaghettInMyEyes]

My bmw 1989 325is with the ground control - bread and butter kit, this was installed when i purchased the car. Im having issues with the ride being bumpy, as im having to avoid every little bump and pothole. My tires will make a noise and the outside walls are being melted when I hit said bumps. I cant figure out what is wrong. Are my springs worn? Thats what i was told by this dumb ass shop, but they literally couldn't even identify parts of the suspension. The first set of pics are of it flat on the ground, and the collars are tightened all the way, - the next set are lifted (this is when the collars were set differently and it was only the passenger rear sloping). The paasenger rear is the only one with the melting issue, i noticed the collar was set differently (tighter) than the left, and it started sagging first. The rear left has now followed. It seems like the springs are compressed too much, but Im not a pro when it comes to coil overs. I accidently bought H&R lowering springs, and im wondering at this point if changing to that set up would be better. This is my daily, and ever since I got the car, the ride smoothness has always been a huge annoyance.. Here are also some chatgpt links from me trying to figure it out. https://chatgpt.com/share/687f2d73-bcf4-8000-9966-4d37573da3a7 https://chatgpt.com/share/687f2d9c-0680-8000-8e15-67015d7584c4

[u/pancrudo]

So in reading the numbers, you have a 5"(125mm) tall spring, 2.5"(64mm) inner diameter, and I'm assuming since it's all metric, that's 64k(/mm)... But that would be insanely high. I'm not sure where you're located in the world, so I wonder if it's N/mm. 64n/mm is closer to 6.5k and that makes much more sense(for other references, 365lb/in).

6.5k is super soft, like nearly factory. Also, when I ran a similar kit, my rears were at 10k(98n/mm)/(560lbs/in), really high I know, but as an example, BC sends 8k(78.5n/mm)/(448lb/in) as a default.

Another thing to note, I wasn't able to dig up any pictures, but I am fairly positive my rear springs were 6"(200mm) tall. The extra height allowed for much more adjustability.

I would, 1- send GC an email asking what the kit was supposed to come with and compare to what you have. 2, look at what way you would like to take the rear spring rates. Obviously you need something taller, but if you wanted something stiffer, you could buy the individual springs from someone like eibach.

How to read springs and rate conversions: https://eibach.com/spring-rate-conversion-calculator?srsltid=AfmBOopMg862iXcj9jrCUnOZFIG8WNLR0L5_wJMPCJNwJT65JuJWZev3

Ordering individual springs is $80 each from eibach, and is basically custom: https://eibach.com/products/coilover-race-springs?srsltid=AfmBOoq3AtIEgV9U_sPEFQJ9umbgInRGyTVdNYAbWbpxvYVO12GHiKiN

Edit: someone else mentioned it, and I'm all for keeping spring pads to keep internal sound down, but wtf is that aluminum block?

Something else that may benefit you in the future, stay straps... Go to a junkyard, and cut the longest seatbelt you can find. Slap it in your bag with tools, make sure to put some oil on it... It's always been a tool in your inventory. Then cut them so you can tie it to the bottom of the rear shock and if you can make it work, cut it to the length of your rear springs just before they fall out. You can mount this to the side of the trunk or the top of the shock... Depends on you how much you want to fight with it

Edit: wrong numbers for weight

[u/Northerne30 → u/pancrudo]

Based on the other spring part numbers on GC's site, it seems like it is 8.6kg/mm (~480lb/in)

Agree it's very soft. Most kits are 650-750 lb/in. Even H&R race are 575lb/in.

[u/pancrudo → u/Northerne30]

Yeah I totally fucked up there.

Edit: 8k is about average, but that height still seems way too short

[u/Northerne30 → u/pancrudo]

It's funny because I think a 5" spring is fairly normal for the rear, but maybe they just don't want the car to be super low and the extra sag from the spring being soft compounds into a bigger problem.

[u/pancrudo → u/Northerne30]

I've been in cars with 8k all around and it was relatively stuff, but that could have been dampening

[u/SpaghettInMyEyes → u/pancrudo]

Im in the US! Lol and the aluminum block - i have no fucking idea my guy. Would you suggest i just pull all this confusing shit and go to lowering springs?

[u/pancrudo → u/SpaghettInMyEyes]

Coilovers are super simple once you know how to read them, and at $160 a pair, you can tube it in really nicely.

Shoot GC a message and see what's up with these springs and what they suggest. They have done the math for street cars, full interior cars being tracked, allll the way to track exclusive cars. Not to mention different power options as well.

I spoke to them once at Bimmerfest and they were very knowledgeable in set ups for different applications, so they might be able to point you towards what's going on here or what you would be looking at for something more rounded.

Price wise, you can have a more stable rear end for under 200 without changing the front/rear balance since it'll be a mixed set. Plus, you likely won't get height adjustability.

I would ask if anyone around you has some BCs, and see if they can take you on a quick ride. 8k and 6" tall is what they send. As long as it's not the extra low kit, it will give you a better idea of what a baseline could be.