Arguments Against Lowering the Car

[nando]

Since you have a convertible the geometry is slightly different from a normal hard top.

no it isn't. the suspension is exactly the same -the only difference is cabby's get slightly stiffer springs because they are heavier. there is no geometry change. there are only standard E30s, M3's, and ix's.

[Cabriolet]

Since you have a convertible the geometry is slightly different from a normal hard top. If you use normal e30 lowering springs it’s going to you about 2-3 degrees of more camber in the rear than a hardtop. So to do it right you don't want to go too low and you want your shocks to match. I’m sitting on Dinan springs and Koni single adjustable shocks and I have a camber correction in the back at the moment and GC camber correction for the fronts waiting in the box at home.

Also remember you chassis is SOFT, there is a ton of flex at the doors. So you can only do so much before that softness neglects any other improvements. Welding in a cage at the point is the only other option.

Optimal suspension is:
HR-50407 H&R sport springs for convertible so it sits higher in the back.(no butt sag)
koni yellows (my pref) or billie sports.
Rear weld in camber correction
New front control arms + full tie-rod replacement.
25mm/22mm sway bars.

At this point there is enough play in the suspension for you not to get wheel hop or slam your oil pan, but not too soft to get body roll or sloppy handling. And your geometry should be spot on with -3 degrees in back and -.5 up front.

[nando]

they aren't special, they're just a little bit stiffer for the extra weight.

[TimR]

I can't get to work or home with lowered sus. speedbumps, train tracks, other BS on the road. Tape some cardboard on your crossmember or oil pan 2-3 " long simulating where your car would be clearance wise if lowered, if that scrapes on your usual roads then your options would be lower but but big tires and wheels for clearance from the ground. Remember Verts. have special springs stock. Somewhere there's a vert spring thread.

[Brandon12V]

lol so true. Back when I had one, there was only one company that had coilover made to better the suspension. I don't remember their name but the setup raised a mkIV .08"-1.2". The only option to fix the suspension geometry, when lowering it, was go with different spindles. I can't remember but I think it was 20squared tuning or USRT that offered this option.

Edit: it's H2 sport spindles.

http://www.h2sport.com/products.php?productid=203

[davem]

I'm doing H&R Sport + billies shortly, I don't give a fuck about my "stance" but hopefully I will like it more than what I have (stock)...

[HarryPotter]

This. on an optimal set-up your control arms should be parallel with the ground, THROUGH a turn, with the center gravity a stock set-up has. when I first considered lowering my mkIV jetta, I found that it made the control arms sit like this \ /. Thus, creating a suspension that handles like shit. I later found out that the only way to make one handle better was to actually raise the vehicle slightly. Most mkIV guys never gave a shit about that, though. I know we're talking e30's here so no one needs to be a smart ass. Just giving an example. Low is not a bad thing, but precauations need to be taken if you still want the car to handle as good or better than stock. That said, you can easily get away with a cup kit. Slamming an e30 does things you don't want to happen unless you address the issues.... but it can be done right.

HAHA I love mark 4 guys. These 300 dollar raceland coils are superb!!! Sorry but anything that cheap for that much is garbage.

I mean seriously, strut housings, struts, and coil springs both front and rear for 300 dollars? That shit must be made out of fucking plastic.

[Brandon12V]

Parallel control arms aren't necessarily optimal. On an e30, unless you lower the center of gravity significantly by lowering the engine or other unrealistic things, you want the control arms to be slightly like this / \ for the front control arms at least, the rear semi-trailing arms may be a different story as I haven't done any kinematics calculations with them yet. One would have to do their own CG and roll center calculations to determine what ride height allows the CG to be the lowest while keeping the roll center as close as possible to the CG.

This. on an optimal set-up your control arms should be parallel with the ground, THROUGH a turn, with the center gravity a stock set-up has. when I first considered lowering my mkIV jetta, I found that it made the control arms sit like this \ /. Thus, creating a suspension that handles like shit. I later found out that the only way to make one handle better was to actually raise the vehicle slightly. Most mkIV guys never gave a shit about that, though. I know we're talking e30's here so no one needs to be a smart ass. Just giving an example. Low is not a bad thing, but precauations need to be taken if you still want the car to handle as good or better than stock. That said, you can easily get away with a cup kit. Slamming an e30 does things you don't want to happen unless you address the issues.... but it can be done right.

[HarryPotter]

Pretty much what I had figured. My control arms are damn near obtuse to the ground.

[nando]

A tubular front subframe wouldn't solve any bump steer problems, there isn't any space to raise the front subframe so there isn't anywhere to go. I want people to make RCA kits and bump steer kits for our cars :(

I think a better way would be custom struts with modified suspension pickup points. $$$, of course.

You mean a noticeable amount of bump steer :). You might if you autox heavily and have cut springs or something. Bump steer really manifests on harsh transitions or high suspension movements. 95% of the people on these boards wouldn't notice it if it happened or don't track their car. In which case, bump steer will almost never be an issue.

yes, true. A lot of times though it's just because people don't realign their cars after changing springs, the increase in toe out up front will feel really wierd and can cause "bump steer". a mild drop like 1.5" isn't going to show a whole lot though. Done lots of track/autox myself, as long as the alignment is good it's not an issue :)

as far as control arms pointing up/down vs camber curves, you have to compress the struts quite far to actually lose camber. on a lowered car with stiff springs, it's not going to compress a whole lot. I don't know about the effects on other things though.

[Morrison]

Downsides:

1: More extreme CV joint angles - results in slightly less efficiency and faster wear

2: Harsher ride - I don't wanna hear about how 600 lb rate GC's actually ride like stock.

3: Worse tire wear - if camber, etc. is not corrected.

4: Snow clearance - may not matter to some

5: Scraping when pulling into steep driveways - hazardous to front air dam

[slammin.e28]

The engineers at BMW knew how to design the suspension so it would perform optimally under all conditions the car was produced to face.

Now, two questions:

(1) Are you German?
(2) Are you an engineer?

If you answer "no" to either of these questions, you should consult someone who is and get their professional opinion.

(I have a friend who was born in Germany and knows engineering, so I'm set.)

[shiboujin]

was going for rule of thumb but thank you good sirs.

[Sagaris]

Parallel control arms aren't necessarily optimal. On an e30, unless you lower the center of gravity significantly by lowering the engine or other unrealistic things, you want the control arms to be slightly like this / \ for the front control arms at least, the rear semi-trailing arms may be a different story as I haven't done any kinematics calculations with them yet. One would have to do their own CG and roll center calculations to determine what ride height allows the CG to be the lowest while keeping the roll center as close as possible to the CG.

[LJ851]

suspension components (even your trailing arms) are designed to be parallel to the ground when the car is on level ground and with the full weight of the car on the wheels.

So yes, your control arms, trailing arms, and tie rods shouldn't angle up or down. The same physics that creates bump steer applies to control arms.

Not true. A trailing arm that is flat will have substantial squat issues under power. A lower control arm that is flat will start losing camber as soon as the suspension starts to compress which is no good for handling. There are very good reasons BMW designed the suspension with front and rear arms angled down. And dont give me the "just put on stiffer springs" argument, that does not correct the poor geometry.