Progressive springs: BAD idea????

[Guest Anonymous]

I had heard nothing but good about the Eibach pro springs. I was hoping to cut costs and avoid the coil over ordeal. But now, I hear from 2 different guys (and one a supposed expert...which I do not doubt) that they are not good. I guess different strokes for different folks, but read this...

 

"The entire theory is flawed. You want the car to react in a predictable manner. You want a linear reaction to bump (compression of the suspension). Otherwise you can not reliably predict how the car will behave. A progressive spring alters the spring rate based on its compressed length, the more you compress the springs, the stiffer the springs get. That means your spring rate in a corner will depend on your speed, which is a bad idea. It's like having a different car each time you enter a corner because the body roll, roll center, chassis flex, weight transfer, and more, all change as the spring rate changes. Also as the suspension moves the spring rate balance side to side and front to rear will change (in a hard braking situation the front springs will get stiffer and the backs will get softer, in a body roll situation the outside springs will get stiffer and the inside springs will get softer). The balance of spring rates side to side is an expected constant in chassis design, and the front to rear balance is something the engineers work hard to adjust to get optimum ride and performance. A car should not be altering these parameters as it goes. Also, the strut damping rate that works for the lightest spring rate will not work well for the heaviest, and vice versa, so you have totally different damping effects depending on speed, body roll, etc.. This has a big effect on rapid maneuvers and on corner entry/exit behavior. Progressive rate springs are just a bad idea from a handling perspective. For good handling you want a predictable linear rate spring that will compress X inches for every 1 lb load applied."

 

Any of you guys running these progressives that can give me your experiences and thoughts? Thanks!

[SleeperZ]

I am by no means a professional or even a good road course driver, but I love my Eibach progressive springs. The ride height is slightly lower than stock, and slightly stiffer. I run 60 series tires on the street and it's not harsh at all. My Z rated 50 series tire I use on the track is another story on the street.

 

That said, I love the car on the street, and I have no complaints about the handling on the track - it holds the corners and seems very predictable. Is it optimal? - most probably not, but I like it. The rest of my car's suspension is close to stock - just urethane bushings on the steering rack and swaybars (stock), the strut spacers in front, and an aluminum TC rod kit are the only mods, besides Tokiko heavy duty strut cartridges. I think as long as you are smooth in your steering inputs and make sure to transfer the weight in a controlled manner, the progressives will respond like a normal spring (by the way, only the rear spring is progressive with the Eibach - some other kits I hear are the opposite??)

 

My 2c.

[Guest Anonymous]

Odd. Your experts bring a good point to the table, the changing of handling characteristics as far as load on spring changes spring rate.

 

In this case they (Eibach) are combining an improved DTD ride that would have a softer spring with a race car eat the corners style that would have a stiffer spring. So you have benefits of both, ideal for the street racer.

 

But you can't get something for nothing. So i guess you trade this "street racer handling" for unpredictabilty in the turns. The thing to know would be how unpredictible it would actually be. Because if you could learn how the springs would perform under different conditions, then i think you could obtain maximum cornering efficiency. It would probablly just take some experimentation.

 

I don't think that Eibach is marketing those springs as ideal for race or track or even hard street use. They are a compromise. Yet, a damn good one in my opinion. There is no question what is going on my corners.

[danc]

FordZ8:

Think it really comes down to your driving preference, and expected ride quality, what problem you are trying to solve, and how large of a tire/rim combination you want to run. I'm using the Eibach Pro-kit with Tokico struts (Illumina rear) and am very satisified with the result. My converted SBC ZX weighs 78 Lbs. more than it did stock. My use is primarly straight line, and I wanted to eleminate the bottoming-out problem in the rear end under accelleration. The combination took care of all my needs without the proverbial log-wagon ride. There are also guys on the forum who will tell you they are Dogshit. Perhaps they are for their application, but figure out your needs, what you are willing to live with including cost, and you will be moving in the right direction.

Dan

[Guest Anonymous]

Well, maybe I will go for it still. I mean, if it gets stiffer and therefore harder to guess if you are getting into a curve aggressive, wouldn't it be better the car holds BETTER than expected, rather than worse?

[Mudge]

For a compromise progressive/dual/tripple rate springs are ok, for racing then I agree that straight rate is BETTER but I have known people happy with these, and have ridden in a dual rate Eibach Pro Kit spring Trans Am and the ride is just fine and the guy is a talented driver (races a third gen RX7).

[johnc]

I guess I'll drag my reply from ZCAR.COM over here too:

 

Formula 1, CART, IRL, ALMS LMP 900 and 675, and even Winston Cup (when they are cheating) cars almost all use some sort of rising rate (or progressive) suspension system. When aerodynamics become the major factor in how a vehicle handles then rising rate suspensions are critical.

 

You do need very sophisticated software to design these types of suspensions and you have to test them on the track and in the wind tunnel. So, are progressive springs and suspensions bad? Absolutely not - if you have the smarts and the money to design and test them.

 

BTW... tapered bump stops are a form of rising rate suspension and are used very effectively in Showroom Stock and Touring classes.

[David K]

That means your spring rate in a corner will depend on your speed, which is a bad idea. It's like having a different car each time you enter a corner because the body roll, roll center, chassis flex, weight transfer, and more, all change as the spring rate changes.

can i ask you where you got this? I believe this is BS because first of all, its NOT like a different car each time. your car will drive different every time you enter a corner at various speeds anyway. This is the same that a stock out of the box car would do. Dont listen to anything you quoted, because it only what a car does anyway. Adding good springs is a good idea.