The definitive Ground Control coilover suspension discussion thread - step on in!

[Redshift]

I. History and Background

Most of you know I have just installed a Ground Control coilover kit optimized for mostly street but also some track days. The biggest benefits to me are a more "sure" feeling and much more even tire wear thanks to the front adjustable camber plates.

There are a few other threads here showing what my car looked like after installing the setup, and today I was able to fix the ride height (well, almost, see below) and get the car aligned for both street and track. Here they are:

Fun with coilovers (AKA "Slammaroni") - [PICS] - posted 10 June 2004
M5 Ground Control Kit (Slammaroni Part II) - posted 13 June 2004

II. Technical Details of the Kit

The GC setup consists of 350/315 Eibach springs, coilover sleeves with custom GC perches, Koni adjustable shocks (difficult for the rear, but easy for the front), GC custom rear shock mounts, and GC custom front camber plates. The plates allow you to get anywhere from -2.2 to -2.8 degrees of camber depending on several things, including factory tolerances and ride height. You can discuss your requirements with the GC crew and they may tweak the spring rates for you.

III. Details on My Installation


A. Ride Height Discussion

My final ride height, as measured from the center of the plastic "roundel" on each wheel to the bottom edge of the fender lip, is 13.5" on the front wheels and 13.0" on the rear wheels. This retains the stock "rake" of 0.5" higher in front. This was also recommended by GC. For reference, the stock ride height settings according to the alignment shop's information, as measured from the lower edge of the wheel lip to the bottom edge of the fender lip is 23.8" front and 22.8" rear.

I specified my GC setup to be stock ride height, but when I went to adjust the rear ride height, I was only able to get 13.0" as shown above. Jay at GC recommended that I check something out to see if they made an error of if they assembled my setup wrong as there should be a lot more adjustment left at this ride height, but on my car there is not - the sleeve is as far down as it will go and there are no more threads showing above the perch collar.

My concern lies mainly with track driving - with this setup I have I asked Jay at GC about possibly contacting the bump stops too frequently at the track and he stated that this ride height was perfectly fine and that while I may hit the bump stops in a few places at the track, it should be very few and not frequently. He says GC added more suspension travel than the stock suspension has, and that even the stock suspension will hit the bump stops (he says it was designed to do that from BMW) occasionally. As an aside, this ride height necessitates some preload on the springs even at full droop, but I was also told that this is normal.

Based on his recommendation I am going to leave the ride height as it is right now and take the car to the road course this weekend and drive it in the instructor group and see how it feels.

B. Alignment

The "before" numbers don't matter so much, because we just installed an entirely new suspension. With the current ride heights mentioned above, the current settings are as follows (we put weights equivalent to the average weight between me and my wife and one other passenger in the passenger seat up front and equivalent to 3/4 tank of gas in the rear):

FRONT, STREET
Camber, L: -1.5 deg
Camber, R: -1.6 deg
Caster, L: 6.2 deg
Caster, R: 6.4 deg
Toe, total: 0.14in

REAR, STREET
Camber, L: -1.7 deg
Camber, R: -1.9 deg
Toe, total: 0.07in

---------------------

FRONT, TRACK
Camber, L: -2.2 deg
Camber, R: -2.2 deg
Caster: not recorded
Toe, total: -0.10in

REAR, TRACK
* Same as street settings *

NOTE: The settings for front toe are made on the tie rods which happen to be behind the axle, meaning that when you add additional negative camber by tilting the tops of the struts inward using the camber plates, you add even more positive toe out. To remedy this, I need to mark each tie rod and give it one full turn to pull the toe back to slightly negative for track use, then back again for the street.

C. Tire and Wheel Clearance

So far I have tested three different tire and wheel combinations on the front and here are the results of each:

1) BMW 8.5x18 M5 wheel with 245/35/18 Michelin Pilot Sport tire: plenty of tire to shock clearance. No fitment problems.
2) BMW 9.5x18 M5 wheel with 275/35/18 Toyo T1-S tire + 3mm spacer: 1-2mm of tire to shock clearance. Slight rubbing during some turning (seen on the shock but never felt or heard in the car). Main problem is that the T1-S has a thick protruding rim protection bead sticking out of the side, causing slight clearance issues.
3) BMW 9.5x18 M5 wheel with 275/35/18 Toyo RA-1 tire + 3mm spacer: 3-4mm of tire to shock clearance. Although the section width of this tire is wider than either a Michelin Pilot Sport or Toyo T1-S street tire, it has no rim protection bead, and thus ends up with more clearance than the street tire. I plan to check for rubbing soon, but did not feel or hear any during the 20+ mile drive home including some spirited driving.

So far I have tested two different tire and wheel combinations on the rear and here are the results of both:

1) BMW 9.5x18 M5 wheel with 275/35/18 Michelin Pilot Sport tire: plenty of tire to shock clearance. No fitment problems.
2) BMW 9.5x18 M5 wheel with 275/35/18 Toyo RA-1 tire: plenty of tire to shock clearance. No fitment problems.

IV. Driving Impressions

A. Street Driving Impressions

So far, I can say that there is much less body roll. The car feels more sure-footed on all types of pavement. Despite the loss of about 0.75-1.00" of ride height, the ride quality suffers very little. The trade-off of a bit of ride quality is well worth the improved feel and handling. The additional front camber brings the neutral feeling back to the car. The rebound over harsh road imperfections is stellar. There is no cyclical bouncing up and down after going over large bumps - the car just rebounds and "sticks" there. I feel even more confident pushing it into, through, and out of corners than before.

B. Track Driving Impressions

Coming 21 June 2004!

V. Pictures

Click each picture for a 1600x1200 view. Larger ones are available if you e-mail me. Pictures taken with track wheels/tires mounted (BMW 18x9.5 wheels all around with Toyo RA-1 in 275/35/18 size all around and +3mm spacers up front).

Side view, normal level:


Side view, ground level:


Rear quarter view, normal level:


Rear quarter view, ground level:


Driver's side "along the car" view, taken from rear:


Driver's side "along the car" view, taken from front:

[Off]

Brian,

Thanks for the write-up! It answers a lot of questions I had with the GC setup. Your success only makes me wait with more anticipation!
I'm glad it'll be worth it, GC is shipped my coilovers today (placed order three weeks ago )

Quick question, with the camber plates delivering a neutral ride now, how do the 275's feel up front? Is the difference still as dramatic as with the stock suspension?

Thanks,

Syd

[chunpng]

Brian,

Excellent write up, and thanks for your technical info. Since you have the Dinan rear sways, you should have better performance than my set up with the stock sway.

I'm slightly surprised that the factory ride height shows a slight rake backwards (front higher than rear). When I set up my RUF with PSS9s, all shops recommended a 1 degree forward rake (front lower than rear). My M5 have a forward rake now (front is 1/4" lower than rear). Can you educate me on the rake issue?

Thanks.

CP

[Lscman]

Sounds to me like Jay is surprised with your low "max" rear ride height and trying to come up with reasons to justify it. I can think of no reason for the low max ride height...except for: spring is too soft. If your rear sits lower than OEM with less than 1" of spring preload, then the GC spring is, in fact, softer than OEM. This extrapolation is accurate because the factory springs have almost no preload (too). You can install the factory perch with your hands because the spring is compressed less than 1" at full droop. These are the facts.

Do you by chance have a stereo upgrade or some heavy options? We need to find out why your car is lower in the back than chunpng's.

Are the bumpstops different than OEM? You have not yet commented on this. The suspension does not have more travel than OEM unless GC changed the bumpstops. The bumpstops in the pics appear to be BMW, but perhaps smaller....from a 3 series or something.

If the collars are set for max lift and you're still at 13", then I think a stiffer rear spring (of the same uncompressed length) is appropriate. A spring that's about 20% stiffer of the same length would still give you preload but allow ride height range to be raised about an inch....to a max value of at least 14".

This is because the 315 spring is compressed about 5" under suspension loading. With a 20% stiffer spring, it'll compress about 20% less with the same chassis load. That'll give you about 1" more height which should allow you to adjust between 14" and 13" instead of 13" and 12".

[Gus]

Hi Brian

Looks really good - how would it handle all those speed bumps and cobblestones we drove over in London?

[CSBM5]

Originally Posted by chunpng

I'm slightly surprised that the factory ride height shows a slight rake backwards (front higher than rear). When I set up my RUF with PSS9s, all shops recommended a 1 degree forward rake (front lower than rear). My M5 have a forward rake now (front is 1/4" lower than rear). Can you educate me on the rake issue?

I think there is a chance for confusion here since what is being measured is the distance between the wheel center and the bottom of the fender lip. Even though this shows a greater height up front, that does not mean the car is not raked down toward the front. The front fender is much higher to allow for clearance with the wheels turned, so trying to determine absolute body rake from the fender measurements won't work (unless you know the difference between the fender lip heights relative to a common plane on the body). If you measure something like the difference between the body height from just behind the front wheel to that just in front of the rear wheel, you will see a forward down rake. Hope this helps explain it!

[CSBM5]

Originally Posted by Lscman

I can think of no reason for the low max ride height...except for: spring is too soft. If your rear sits lower than OEM with less than 1" of spring preload, then the GC spring is, in fact, softer than OEM.

Rick, I'm not sure that is the case here, but I'm just thinking from my gut here having seen the deflection with load on the rear (i.e. Brian standing in the trunk ). I think Brian and I can easily find out by placing the same static load in our two cars and measuring the deflection.

[stanm5]

Brian,
Thanks so much for the in depth write up. I've been considering a coilover setup and your findings are making my choice much easier.
One question, is the GC set up superior to the FK or KW setup? If so, why?

Cheers,

[Teutonaddict]

Originally Posted by stanm5

One question, is the GC set up superior to the FK or KW setup? If so, why?

I have no idea what the GC coilover set costs (anyone willing to provide insight? ) but one advantage that I see vs. the KWs --that I own-- would be linear rate springs in the rear on the GCs. That, and you can specify the spring rates that you desire on the GCs, whereas they're pre-determined on the KWs.

If I were making the choice today, assuming that $$ were comparable, I'd likely go the GC route....

-Dave