Suspension Advice
 

I need some suspension advice. I have been searching all kinds of different fourms, but can't decide what to do.
Current Set-up: Megan Racing Coilovers 8kg/mm(448lb/in) front, 6kg/mm(336lb/in) rear. ride height: about 13in, wheel hub to fender lip all around. Had this setup for about a year and a half now. I know its lower than I should be, but corners great and keeps up with Evo's and Sti's on the spagetti bowl.

Had King springs installed before the coilovers. I don't know what the spring rates were, but steering response wasn't the greatest.

Problem: I'm getting tired of the bounciness on small bumps, especially on long trips and on wash board snow conditions. I've tried adjusting the dampening from very hard to soft, but it doesn't help. (I actually learned about this in Systems Engineering, If the spring rates are to high, you can never get the small oscillations to go away over small frequencies, no matter how much damping there is.) Now I'm experiencing it first hand.

I don’t want to spend a lot of money, but am willing to if it comes to that, $2000

Here are my choices with questions:
1: I have been talking with the megan racing rep. Supposedly the dampeners were designed with a tolerance of plus or minus 2kg/mm. He said I could buy any 2.5 inch spring from Eibach, 180mm front free length and 200mm rear. I am thinking about buying a set of 6kg/mm for the front and 4-5kg/mm for the rear. I think this might help the bounciness and soften the ride a little, but I am worried about bottoming out the struts. If I do this, should I get a slightly longer free length spring and put a slight preload on it when it is installed? Do you think that spring rate combo would help?

2: Go with a softer spring rate coilover setup. I was looking at Tein SS-P, Whiteline G4’s (not made for 2004 wrx anymore), or something similar. Anyone know how the Tein SS-P’s ride?

3: Get a spring and strut combo with front and rear swaybar setup. I was looking at Eibach Sportline’s, Crucial racing, and Prodrives with KYB struts. Don’t know what swaybar to get. There isn’t a lot of info on the Sportlines, but they are progressive with drops of 1.7 f and 1.2 r. Anyone know about these?

4: Raise up my car now to recommended ride heights and see if that helps. If I do this, I will need to get an alignment. I don’t know if this will help with the high spring rates. I will raise my car up if I get the softer spring rates on the megans anyway.

Sorry if this is a long post, I have been thinking about this for a while and can’t decide on what to do. I know I want something softer that handles well and isn't bouncy on small bumps. It seems like there are so many choices. Suggestions will be greatly appreciated.

Thanks,
Dave

Eibach makes some great springs. Drop your rates to something like 380/290 and give it a shot. It'll cost you about $120, but as long as you're in the ballpark of what the strut can handle, you should help clean up the ride, though it will likely be "better" but not "all better".

I do like eibach springs. Does anyone know about the Sportline's? I am leaning towards getting softer spring rates on the Megans. If I do get the softer spring rates, should I get the same free length spring or a little longer and preload them? Anyone know how the tein SS-P's are?

Scott was actually suggesting that you get some 2.5"ID springs from Eibach to put on your Megans.

I think that is what I will do. The free length of the springs are 180mm f and 200mm r. Should I get slightly longer free length springs for the softer spring rate and preload them on the coilover or just get the same spring length? I will most likely get 6kg/mm front and 5kg/mm rear.

I doubt that the length would need to be adjusted. You can always raise the perch just a bit to counter the additional sag, unless of course you're already at the max height and the car is still too low...

max height on the megans makes a subie look like a 4wd off road buggy

Rereading your first post, I have a couple questions and thoughts before you go buying springs.

Megan V1, or V2?

What tires/wheels do you have at what pressure?

What is your alignment and was it done post coilover install? By whom?

What if any preload do you have on the springs?

At you ride height, you are in positive camber land which depending on the answers to the questions above may have a significant impact on suspension feel.

Have you found a damper curve for your coilovers? depending on that curve, softer springs may introduce other issues.

I do love my Crucial Springs. You might want to couple them with something better than KYB AGX if you do decide to go that route though. I have a theory that even on the highest setting, they aren't dampening enough which I only notice when I get 1st gear wheel spin coming out of a corner (AutoX issue only). Observers see my front inside tire hopping since I switched from the Eibach Prokit to Crucial (both on the AGX's). I did add a little power too at the same time though so who knows.


Other than that minor issue though, Crucial>Eibach.

I'm considering Koni inserts, Tociko D-Specs, or maybe even STi take offs for the future.

BTW, I have a set of Eibach Prokit springs for sale in the Classifieds section. :)

Please note, the Eibach springs I'm talking about have nothing to do with the Eibach springs Cody's comparing with the Crucial springs.

I'm talking about 2.5" ID Eibach springs that come in virtually any length and rate for use with coilovers. These guys: http://www.ground-control-store.com/...gory.php/CA=31

The same ones I used on my Koni's, and will probably end up tossing over my Tein's:

I'm not sure what version the coilovers are. I had them for a year and a half, the street series. I'm riding on 225 18" rota's at 35psi f and 32 psi r. My camber is at -.5 f and -1 rear since I do a lot of freeway driving, at least used to, zero toe as well. There is still a lot of adjustment with these, I can drop it to the wheel well or raise it "like a 4WD buggy" :) . I don't know what the damping curve looks like. I talked with the Megan rep a while ago and asked what the critically damped setting was. He didn't know what I was talking about until I explained my self. Critically damped is the setting where there is no overshoot after a bump. He didn't know what the optimal setting was. I know the 2.5in springs are different than normal springs you would install with regular stuts. I was looking on eibach's sight and you can get pretty much any spring rates and length's you want. lots of choices. I think I will try softer spring rates on the megans first and if it doesn't work, try a different option. Thanks for the feedback

Ah, 225 18s at relatively high pressures... What tires?

I don't see that you said if those alignment numbers were before or after you put on the coilovers and set them at that height? Those camber numbers are fairly positive even for a stock Impreza.

Do you still have your stockers? If so, put them on and tell us how it feels at OEM tire pressures on the same road.

By changing to 18s, you have added probably 100+ pounds of basically undamped spring rate to your suspension. (Austin, help me out here...)

That is tolerable on stock struts, but as almost everyone who has tried 18s on Imprezas has found, it is not fun, especially on coilovers with limited adjustability. 17s appear to be about as far as most go for the street.

Also, as I mentioned, you are in negative camber land at your ride height due to the Imprezas suspension geometry. Any amount of lateral load is going to quickly take your outside front to zero and then quickly positive. So, while not directly related to bounciness, If you want the car to handle remotely well, you will need to raise that ride height at some point

Depending on the road, you may even be running out of damper throw at that ride height. At rest, how much damper piston shaft is visible? Did you happen to see how much was visible at full compression of the damper before you installed it?

Those camber numbers are post coilover installation. The car still seems to corner well, even with the low ride height. I had my 18s on with king lowering springs and it was never bouncy with that setup. For lowering the coilover, you actually screw the whole coilover up or down, it does not affect the strut travel. You can adjust ride height by screwing the strut up, but I never did that and don't want to. So I should be getting full strut travel. When I get the softer spring rates, I will be raising it up. I will probably get a better alignment of -1 f and -1.5 to -2.0 r.

And the spring rates for those were??? And how much did they lower it? Not to 13" I bet. And they were probably designed for the stock dampers... So how is that a valid comparison.?

So how much Preload if any is on the current springs?

Your future numbers have the negative at the wrong end for better handling, and you may think it handles OK now, but unless you have the Strano front bar, your outside tire is is lifting the inside edge by quite bit in the corners at that ride height. That is just how the Impreza suspension geometry works.

Bring it out to autocross next weekend and we can probably take pictures of it if you don't believe us.

Oh, and what the heck is the "critical damping setting"?

In general, you adjust bump to damp the unsprung weight and rebound to allow for maximum contact for a given surface condition. Unfortunately, you have a single adjuster that either adjusts both or rebound only. In either case, unless the engineers really screwed up at Megan, it is unlikely you will find spring rates that match the Megan dampers better than the ones that came with them.

I wouldn't spend any money trying other springs on them.

I know my old lowering springs are not the greatest comparison and they did not lower it to 13 inchs. I agree that it would handle better if I raised it up, which is what I will do. I'm sure my tires go into positive camber when i'm cornering hard, no need for the attack, I never said I didn't believe you. I never auto-x'ed it, but if I did I would adjust the camber plates to get as much negative camber as possible. It should handle at least a little better than. Also, If I was riding with my front wheels at -2 degrees, the insides of my tires would not last. I would be going through tires quickly.
FYI,
Critically damped is when the spring settles to equilibrium position the fastest, without overshoot. Critically damped is inbetween under and overdamped, pretty much the best setting for a spring and damper(maybe not for comfort). For underdamped, zeta<1, crit zeta=1, overdamped zeta>1. Check this link out: http://en.wikipedia.org/wiki/Damping
also here: http://cat.sckans.edu/physics/shm.htm
You calculate zeta from k(spring rate), c(damping coef), and m(mass). If Megan designed the spring and damper, then they should have differential equations that solve for zeta at different damped setting. Therefore, they should know what the critically damped setting is. If I knew what that setting was, I would know were under and overdamped settings would be.
On most non performance cars, suspension is designed underdamped for comfort.

If I were to get softer springs, they would have more of a chance of being overdamped. The softest setting may work if I try this. It would cost $120 for the different springs and $80 for alignment. so the most I would lose is $200. Plus I would raise the car up for handleing purposes. The other choice is to get a completly different setup, so maybe this is worth a try.

FWIW, I have run -2.2 camber in the front all summer long for the last two summers, putting ~25K miles on my tires and wear is perfectly even. However, most of those miles are on the stock 16" rims and that may allow the tire to mold to the road a little better. Regardless, from my reading, you can usually do -1.5 to -2 and as long as you run zero toe, your tires will wear fine.

your arguing with the suspension genious...

I'm not attacking, you are the one who posted asking for input.

I have just been asking questions, sometimes without answers and offering options and explanations.

Hopefully this one will make sense.

Critical damping setting is not an automotive term, and this is not a simple system. Automotive dampers typically have at minimum different high and low speed damping for each direction of motion and may or may not be linear. Many automotive springs are also not simple as they are one or more way progressive and may or may not have helper springs that further complicate things. And we won't get into the overall vehicle dynamics related to weight transfer and such which also impact damper design.

At most, your coilovers have one knob that affects anywhere from one to all four of the above damping rates, but most likely 1-2, most likely high speed rebound, and maybe high speed bump.

Unless you have spent a fortune, your 18s and larger rubber have added upwards of 20-40# of unsprung weight and as I said, added I am guessing, 100# of basically undamped spring rate in the sidewalls of the tires.

Your softer springs and stock dampers effectively insulated you from those.

The stiffer springs and performance oriented dampers likely have significantly lower tolerance for low speed motion and thus on the road, they feel bouncy as the tire sidewall is transfering significantly more force than stock. Due to the low speed damping, changing springs is unlikely to have a significant impact on perceived ride quality.

We have had a bunch of people had and then dumped their 18s and others dumped their coilovers or bought a daily driver due to unacceptable street ride quality. The rest of the coilover owners understand that bouncy on the street is one of the prices you pay for a performance suspension. Harmonics from trucks and such are the real problem, and there just isn't much of a way around them and still have a setup that is anywhere near optimal under performance driving conditions.

This is why lowering springs are often the best option for a primarily street driven car where the objective is looks, not performance.

So what I'm looking for is to maintain performance, but not have as bouncy of a ride. This is why I didn't know, and still don't know, if I should buy softer spring rates for the coilovers, or just sell them and get a spring and strut combo. Megan said the struts were designed for a plus or minus 2kg/mm spring rate from the original spring rates. Do you think I should just get springs and struts or give the softer spring rates a try? I will be raising my car up if I make a change.

I do know the low pro tires add extra spring rates, if you model a car, you take into acount everything, like spring rate of the tire, springs, dampers, center of gravity, roll, spring rate of the seat(for personal comfort), arm angles, etc. It can be modeled by a systems equation, but is complicated.

I know a lot of people like the prodrives, which I am considering. I am interested in the sportlines, but don't know if the drops are to agressive for springs. They are progressive spring rates. Is there anything else you recommend? If I do the spring, strut combo, I may look into front and rear sway bars.

Thanks for your comments

Not to be OT but most of these threads end with:

"I bought some (insert coilover name) they are sweet."

and not with:

"I had a custom valved/damped/ride height adjusted $3k system that is perfect."

I think you'd like a good spring/strut combo and sways...way more daily drivable. That's what I have and love it. I take road trips a couple times a month comfortably, I can still mob through the snow, and I still beat better cars with coilovers and wider tires every once in a while. :)

If you do go this route, I suggest you choose struts first and then springs. There's only so many struts to choose from, but the selection of springs is endless.

A bunch of people on the board have stock struts and reasonable lowering springs and appear to like them on the street. That is probably your best bet if street comfort and lowered look are your objectives.

As we have likely scared most of them away, you might want to start a new thread and/or search as I believe it has been discussed before.

Thanks for the advice. A spring and strut combo would probably be my best bet. Softer spring rates on the Megans may be worth a try, i'll figure something out.