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Dump your spring rate and suspension advice here

10K views 58 replies 15 participants last post by  thebigjimsho 
#1 ·
Gents,

Hoping to get some feedback on how to get back to some more neutral handling after I overhauled my suspension.

Currently running Penske 7500 shocks on 4 corners, Sprint springs (10kg/~570lbs front (10 inch length), 12kg/640lbs rear (10 inch length). Have all other suspension mods inc. beefy swaybars, trailing arms, beefy rear tie rods. Obviously with the S/C up front we have a bit more extra weight to deal with.

Given that I slapped these on without much thought car's handling has changed quite drastically. From being somewhat neutral to understeer happy, it's now very tail happy. Not necessarily a bad thing for the fun factor but really a nuisance when you're trying to get around a track quicker than the debris collection truck.

Need to get a bit more neutrality back in, maybe soften up the spring rates also and get the rear under control. Right now car is a bit unsettled (front and rear).

I'm thinking of breaking the pattern of rear to front spring rate bias and going with 500lbs shorter springs in the rear and 550lbs in front (move up to 12 or longer springs to take up some shock acreage that's going to waste right now).

Unfortunately can't quite ditch the rear hotchkis sway bar (threw away the factory one) so need to work with it in place.


I'll get the car properly aligned and corner balanced.
 
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#3 ·
Unfortunately, it's been forty years since I took vehicle dynamics (ME 498) while an undergraduate in engineering, but IIRC it should be able to be corrected by anti-sway bar (torsional) rates and coil spring rates alone (assuming no changes in tires, bushings, ride height, etc.). I know, I know ... I'm not a bit of help.
 
#4 ·
Cheers rand... :) Send me a book or two to read.

I'll play around with my adjustable endlinks a bit as I've not checked them out thoroughly. Swaybar is supposed to be neutral with no preload with car down on flat ground, right? Maybe they're preloaded as I've not adjusted them since I started this whole ride height circus.
 
#5 · (Edited)
... Swaybar is supposed to be neutral with no preload with car down on flat ground, right? ...
Anti-sway bar and preload are mutually exclusive on flat ground. Only during body roll does one side pull down while the opposite pushes up to resist body roll.

Am I right to think that while turning, you want the rear axle to move in a manner that effectively steers the rear toward the outside of the turn thereby causing the car to rotate about it's CG better thereby (again) lessening understeer (i.e. going toward neutral steer). Too much would obviously cause oversteer, a condition that can work better than caffeine to wake you up at anything approaching triple digits (metric OR Imperial units).
 
#6 · (Edited)
I am not an expert on the subject but what I gather from things I've read you're on the right track with reducing the rear spring rates. I always thought the rears were purposely lower than the fronts in most set ups. That along with adjusting the rebound on the dampers and playing with tire pressure might also be helpful. I heard some internet chatter (take it for what it's worth) that you should try to get away with the lowest spring rates in the rear you can. Maybe finding out what the actual wheel rates are from front to rear may help as well. Not sure how to do that, but it is a better correlation of the front in relation to the rear than spring rates. Heres an interesting article from GRMS on tire lateral load transfer distribution and wheel rates. The front seems to be a higher percentage than the rears in most cases. 50/50 is a loose set up. Just remember that spring rates can act like a sway bar. It does cover over and under-steer. The last two paragraphs kind of sum it up. Good luck, I'm sure you'll sort it out.

http://grassrootsmotorsports.com/articles/lean-less/

PS zzila's thread says he started with 750 in front and 450 in back. That's in sharp contrast to your set up.
 
#7 ·
PS zzila's thread says he started with 750 in front and 450 in back. That's in sharp contrast to your set up.
For what it may be worth - I just ordered a set of Penke 7500s from Anze. I will be running at stock ride height and the set up is more street than track. The spring rates are to be 700 front and 400 rear. I questioned the rear srping rate, because without the nivomat effect the spring alone will need to accommodate the occasional passenger or luggage weight. Anze's answer was that the coilover 400 lb was closer to a 500 lb spring in the stock location.
 
#10 · (Edited)
Where are your springs in the rear? Are you running the rear coil-over setup that zzilla had ANZE put together, so the rear springs are on the rear dampers? Or are your rear springs in the stock location, like my KWs and (I think) like the Ground Control setups? As crankedupforit and darkman already mentioned, the difference in spring locations will make a significant difference in the rates you should be running. If you're running coil-overs in the rear then it's no surprise that the 12kg/mm springs are making it tail-happy - that's a lot of rear rate!

FWIW, 12kg/mm is actually ~670#/in, and 10kg/mm is about 560#/in. In other words, your front/rear stagger is even bigger than your post indicates.

I'm running the "stock" KW V3 springs up front (10kg/mm) and 10kg/mm springs in the rear (stock location with adapters to fit the 60mm springs). The "stock" KW rear springs were 9kg/mm, but I figured that GM chose to put matched front/rear springs (400#/in) on the car, so I would do the same. I'm running a stock front swaybar and a Hotchkis rear swaybar, which feels quite a bit more nimble than having both the front and rear Hotchkis swaybars installed. The car is (IMO) still prone to understeer in slower corners, but not horribly so.

My understanding is that pretty much all of the coil-over springs are good to go. SSS seems to be the only exception - I've read more that people have more issues with spring sag, rates not matching from one spring to the next, etc. Hyperco, Swift, H&R, Eibach all are very good quality.
 
#13 ·
Where are your springs in the rear? Are you running the rear coil-over setup that zzilla had ANZE put together, so the rear springs are on the rear dampers? Or are your rear springs in the stock location, like my KWs and (I think) like the Ground Control setups? As crankedupforit and darkman already mentioned, the difference in spring locations will make a significant difference in the rates you should be running. If you're running coil-overs in the rear then it's no surprise that the 12kg/mm springs are making it tail-happy - that's a lot of rear rate!

FWIW, 12kg/mm is actually ~670#/in, and 10kg/mm is about 560#/in. In other words, your front/rear stagger is even bigger than your post indicates.

I'm running the "stock" KW V3 springs up front (10kg/mm) and 10kg/mm springs in the rear (stock location with adapters to fit the 60mm springs). The "stock" KW rear springs were 9kg/mm, but I figured that GM chose to put matched front/rear springs (400#/in) on the car, so I would do the same. I'm running a stock front swaybar and a Hotchkis rear swaybar, which feels quite a bit more nimble than having both the front and rear Hotchkis swaybars installed. The car is (IMO) still prone to understeer in slower corners, but not horribly so.

My understanding is that pretty much all of the coil-over springs are good to go. SSS seems to be the only exception - I've read more that people have more issues with spring sag, rates not matching from one spring to the next, etc. Hyperco, Sprint, H&R, Eibach all are very good quality.
My rear shocks/springs are not coilovers (I've not done the conversion yet) so everything is in stock locations. The only addition is the ground control perch at the rear.

The only other observation is that the front 10inch long spring I was running seems a tad short installed on the Penske's (given it's a full proper coilover so spring is fully contained within shock) so I'm thinking of sticking with the same rate but getting something longer. I don't think the shock is bottoming out (though I get a nice clunk on heavy comp/rebound, still unsure where that's coming from).
 
#14 ·
Are the 7500's non adjust /single adjust/ double adjust? as far as spring rates the rear are going to be lower due to most of the weight of these cars are in the front 60% front 40% rear(these numbers are from Cross weighting the car before any improvments ie stock) ." Hyperco "springs are excellent springs and are run on many different race series. I dont' know where you bought the shocks from if they were from "ANZE ENGINERING" he can and will help with any type of set up you need. Or you can purchase the Hyperco springs from him and he will still help you set your car up. Angelo at Anze have set up many cts v race cars and knows how to get it right( 516) 671-3960 as far as my set up 750 front 450 rear spring rate is perfect so far, but I have not had the chance to use them much due to the fact the car is in the fabrication shop for a widebody kit. So when I get to the track things might change(weight /wing/etc..)
 
#15 ·
Thanks sir. I'll give them a call. Out of interest, how long are your front springs (length wise).
 
#16 ·
I keep saying Sprint. They're actually Swift metric coilover springs...
 
#17 ·
I'm also on Penskes with stock locations and use 750# front and 550# rear.This combo is ugly on street but great on track.I don't know spring length but think I'm 1 inch lower than stock.IMHO your rear is too stiff and too tall.I'm no expert but I tried lowering the front only by 3/4 inch last time at track and car gained oversteer.After 1 session I changed back and once again became fairly neutral.Again,I'm no expert at set-up but think relative height from front to rear is pretty important.
 
#18 ·
Thanks for the feedback. Sounds pretty aggressive. I'm guessing you're still running shock next to spring config in the rear. I was thinking of going an inch shorter on the rears and use that GC perch for street.
 
#20 · (Edited)
I, too, am in need of some advice from you suspension gurus.

I'm currently running the GC Kit (lowered about 1" front and 1/2" rear) with 750 lb / 800 lb Eibach springs. I like the setup, I don't have much understeer (or excessive oversteer) at all, and the car feels nice and planted and balanced (neutral).

I bought ichpen's used QA1s, and plan to swap my current FG2s with them. I really am excited about the adjustability of the QA1s.

My question is this: I think I am going to need to bite the bullet and order some shorter springs for the rear because for some reason, I can't get the rear down as low as I would like it to be (for the street). I need the extra 1" of drop that shorter rear springs would allow me to have, and I could still raise the ride height up plenty if I needed to.

I am thinking about keeping the front Eibach 750 lb/in springs, and going with some Swift or Hyperco 14 Kg/mm (784 lb/in), 2.50 in diameter coils, in the 9" length (I'm guessing right now, I think the Eibachs are 10" tall). I have had great luck with running 50 lb/in stiffer springs in the rear compared to the front, but with all this talk about going lower in the rear, I am wondering if I should drop down to a less stiff spring in the rear. I know that the guys changing over to the true rear coilover will need the much softer rear springs because of the mouning location being moved in-board, but how about use guys who still use the stock suspension setup (geometry). Any help would be MUCH appreciated.
 
#21 ·
I don't think you can go to a shorter spring on the rear w/ the GC setup. Remember the kid in Sacramento who cut his springs...and they fell out. The 10" springs stay in the perch, with some movement, when the suspension is fully unloaded. Maybe a 9" spring will stay, but I'd be very worried. And I have NO idea how it would affect the V's ride and handling...
 
#22 ·
Yeah, that is definitely a concern, but without completely changing my suspension, I don't really have any other options but using a shorter spring to get the ride height that I am looking for. The funny thing is that when I was running the 650 lb/in springs in the rear, my ride height was closer to where I wanted it because those springs would compress slightly under the cars own weight, but the heavy 800 lb spring don't compress at all. Anyway, I am pretty much ready to drop the $200 on two (2) rear springs and have the shop put them in at the same time as they do the QA1 install, but I guess I am more concerned about the spring rate. If I can make the car handle even better with a different spring rate, then I would like to do that at the same time for no extra expense.
 
#23 ·
I'm no expert on the GC setups, but do you have the MightyMouse spacer mod in the rear? My understanding is that without the MM mod the stock rear struts will pump themselves back up to (near) stock ride height. I've never had to deal with that, so my understanding could be incorrect. If my understanding is correct, and if you don't have the MM spacers, then the ride height issue will be resolved by going to the QA-1s, since they aren't Nivomat dampers and thus don't pump themselves up.

$200 for 2 springs is a bit much. Make sure you check the various circle track racing vendors - HRPworld, Coleman Racing, Pegasus Racing, Racer Parts Wholesale, etc - to get a good price. Should be closer to $150 shipped.

As for rates, if 750/800 is working for you now, I wouldn't change it. That's just based on the "don't change too many things at once" theory - the more stuff you change, the harder it is to figure out what changes were good and what were bad. I would get the QA-1s installed and play with those a bit, then see if you want/need to change the spring rates. Swapping out the rear springs is really pretty easy if you're keeping the springs in the stock location with adapters.
 
#24 · (Edited by Moderator)
I'm no expert on the GC setups, but do you have the MightyMouse spacer mod in the rear? My understanding is that without the MM mod the stock rear struts will pump themselves back up to (near) stock ride height. I've never had to deal with that, so my understanding could be incorrect. If my understanding is correct, and if you don't have the MM spacers, then the ride height issue will be resolved by going to the QA-1s, since they aren't Nivomat dampers and thus don't pump themselves up.
The GC Kit came with a spacer to fool the FG2 Nivomats from pumping up, so that isn't the issue, but thanks for taking a stab at it anyway. :)

$200 for 2 springs is a bit much. Make sure you check the various circle track racing vendors - HRPworld, Coleman Racing, Pegasus Racing, Racer Parts Wholesale, etc - to get a good price. Should be closer to $150 shipped.
That is what I was thinking too, I would feel a lot better only having to spend $150. It just really sucks that I can't get that extra 3/4"-1" of drop out of the rear with my current setup! So then I spend another $150 for that perfect stance so the car looks good at car shows.

As for rates, if 750/800 is working for you now, I wouldn't change it. That's just based on the "don't change too many things at once" theory - the more stuff you change, the harder it is to figure out what changes were good and what were bad. I would get the QA-1s installed and play with those a bit, then see if you want/need to change the spring rates. Swapping out the rear springs is really pretty easy if you're keeping the springs in the stock location with adapters.
Great advice (and TBJS too)! I was leary about wanting to change too much, I just don't like wrenching on the car more than I absolutely have to, so I tend to overdo it when I am having work done on the car anyway. But, I think your advice is sound, I will probably just order (2) 784 lb/in (2.5 dia, 9" length --> can anyone confirm this is the correct measurements I would need?) Swift springs and go from there.

BTW, what is the best and easiest way to just pull out the rear springs and change them to new ones? I had envisioned trying to do it without even unbolting anything (maybe using a small spring compressor to squish the spring enough to pop it out from under the GC spring perch), but that might not be possible...
 
#29 ·
Yeah, that sucks. I looked on Hyperco's website and apparently that is the only size that doesn't come in a 2.5" diameter. They have all different lengths around it, but no dice on the 9"...ridiculous. I guess I'm stuck with one of the other brands.
 
#28 ·
Best advice is to change one thing at a time.
If your tire pressures are correct, you have no preload in the rear sway bar and you are aligned correctly then reducing rear roll stiffness will in theory reduce oversteer.
You can reduce your rear roll stiffness by doing a few things. Changing the effective lever arm on the sway bar will change the torsional stiffness of the sway bar. Changing your spring rates with the current spring location will all so change the roll stiffness. If you change both spring location AND spring rate, talk to someone who has done it so you know where to start or then you will start changing more than one thing.

If you do get an adjustable sway bar then this is the quickest and easiest way to tune the car while at the track.

Read all of Carroll Smith's books, Prepare, Tune, Engineer and Drive to Win. Chassis Engineering by Herb Adams is a great one too.
 
#31 ·
Great info, thanks a bunch! I need to pick up those books for some light reading ;)
 
#32 ·
For whatever reason, 9" is some sort of bastard length. I've never understood why, but everyone makes 6, 7, 8, 10, 12, etc, but not 9". Interesting that Swift makes 9" in their metric (65mm) springs, but not in their 2.5" springs. :confused: I would just wrap some electrical tape or something like that around the lip of the spring perches to fill in the slight gap when using the 65mm springs with 2.5" perches.
 
#33 ·
I've been reading through this thread, pondering spring rates, and wondered where you guys wound up with your Penske and KW Variant 3 setups. Are you still rocking your present handling balance, and if not, what kind of experimentation have you done in the meantime?

I had a very similar setup to Tweeter (Ground Control with 650F/750R), which I might have been using to compensate for the poor compression and rebound dampening of the FG2s. As they say, the best thing to do when your suspension sucks it to prevent it from doing anything.

Now that I have KW Variant 3's, I'm debating the following spring combinations, based on the Swift catalog:

Option 1 (Soft 46/54 ratio)
- 2x Stock KW V3 front springs (8" length, 70mm ID, 100 N-m / 574 in-lbs)
- 2x Z60-228-120 rear springs (9" length, 60mm ID, 120 N-m / 672 in-lbs)

Option 2 (Medium 50/50 ratio)
- 2x Z70-203-120 front springs (8" length, 70mm ID, 120 N-m / 672 in-lbs)
- 2x Z60-228-120 rear springs (9" length, 60mm ID, 120 N-m / 672 in-lbs)

Option 3 (Stiff 46/54 ratio)
- 2x Z70-203-120 front springs (8" length, 70mm ID, 120 N-m / 672 in-lbs)
- 2x Z60-228-140 rear springs (9" length, 60mm ID, 140 N-m / 784 in-lbs)

I'm lowered pretty far, so the only two things that I'm truly certain of right now are a) I need more rate in the rear to prevent my 285s from rubbing on hard, sustained corners, and b) I want less understeer (which I may be able to derive by tuning the shocks). Presently, my KW Variant 3's are set to stock settings, which is:

Front Axle
- 0.75 turns from full hard for compression
- 0.75 turns from full hard for rebound

Rear Axle
- 1.0 turns from full hard for compression
- 1.0 turns from full hard for rebound

Another important question is, has anyone with a KW Variant 3 found out how much spring is too much for the KW Variant 3's rebound dampener valving?
 
#34 ·
Another important question is, has anyone with a KW Variant 3 found out how much spring is too much for the KW Variant 3's rebound dampener valving?
I was told by KW that about 200 lbs is the max increase in rate that the stock valving can handle. They also told me that the warranty is voided if you change the stock spring rate on the KW V3s. Not sure how they would know that you changed them. The issue then becomes figuring out some new settings to start with. With this they have not been too authoritative with the response. I suppose you can quantify the increase of the spring rate as a percentage and then apply that to the stock settings but that's yet to be proved out. I'm debating as to whether I can make the V3s more track worthy or would just going to Penskes be the solution.
 
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#35 ·
Great info. Frankly, the stock springs and damper settings on the KW Variant 3's are super comfortable compared to my Ground Control setup. But the Ground Control setup felt much snappier when pushing the car hard.

By the way, do we have a live axle, and if not, what on Earth is that thing in the back? Most of the calculators that I've seen are only good for A-arm setups. The Eibach calculator includes the equations for a live axle, which seems closer to what we have, but not quite correct.

Ultimately, I plan on putting together an Excel spreadsheet, with measurements for everyone to see (including sprung and unsprung weights, as best as I can get). Maybe even something like the Excel Web App this guy did (see the right sidebar).
 
#41 ·
Great info. Frankly, the stock springs and damper settings on the KW Variant 3's are super comfortable compared to my Ground Control setup. But the Ground Control setup felt much snappier when pushing the car hard.

By the way, do we have a live axle, and if not, what on Earth is that thing in the back? Most of the calculators that I've seen are only good for A-arm setups. The Eibach calculator includes the equations for a live axle, which seems closer to what we have, but not quite correct.

Ultimately, I plan on putting together an Excel spreadsheet, with measurements for everyone to see (including sprung and unsprung weights, as best as I can get). Maybe even something like the Excel Web App this guy did (see the right sidebar).
This is the best excel spring rate calculator i have found to date. http://www.pirate4x4.com/tech/billavista/coilovers/Part_1/BV%20Spring%20Rate%20Calculator%20V6.4.xls
 
#36 ·
I'm still running on the same setup I've had for years now and really like it. My heavy springs seem to play well with the FG2s, but I don't put a ton of miles on the car either and I've only had this setup for about 7000 miles.

Btw, I've said this before in other threads, but I have more rubbing on hard turns in the rear with my 285/35r19s (currently installed) than I had with the 295/30r19s. That extra sidewall kills you.
 
#38 ·
That is literally the perfect setup for the first gen V as far as suspension goes. :)
 
#47 ·
As darkman said, all of the CTS-Vs were Nivomat, whether they had the FE4 or optional FG2 dampers, so the FE4 springs are the Nivomat springs. The FE1 dampers were paired with lower spring rates on the V6 cars.

I'm guessing that whoever installed the FE1 dampers just left the stock springs, in which case they're most likely quite under-damped. Having seen the FE1 dampers on a V6 CTS in the junkyard, I have to say they're tiny compared to the FE4/FG2s, and that lack of volume probably compounds the effect of them being under-damped. Putting some FE4 dampers back there would make the car ride and handle a heckuva lot better, methinks, albeit at pretty high cost (nearly $200 each on rockauto and amazon).
 
#46 ·
The CTS-V is a Nivomat car. The CTS (FE1) uses rear springs (25754802) that differ from the springs on the CTS-V (25752977). Obviously, FE1 shocks differ from the FE4/FG2 shocks. Although, there have numerous debates on the subject on this forum, I believe the rear height of the cars depends in part on the spring rate and height and in part on the leveling capacity of the shock.
 
#48 ·
Thanks guys! I just found this Sachs info also. All I had up until yesterday was the gm FE4 part number 25752922. I did not know they were nivomats until I received them in the mail yesterday. Two for 170 on ebay. They look beautiful. I will measure my height before and after so we can see the difference between the FE1 and FE4. It will be nice to finally have the correct parts back there.

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