Hilton

The Eibach progressive rates are 399-560 front, 285-372 rear.

Its my DD and the roads in Australia are terrible quality - I wanted something that wouldn't shake my teeth out of my head on the commute too

GlenL

The way it works is that when the spring is compressed the narrowly-wound coils come in contact. Those stop compressing and the remaining, active coils result in a stiffer spring. It's not magic, it's mechanics. There are springs that are wound to be gradually narrowing and have a continuously changing rate, but the Eibachs for the 928 aren't done like that.

When the springs are extending from the rest position then the rate stays at the lower level. Yes, the rear lifts up more because as the weight is transferred the coils extend more with a soft spring than with a stiff spring.

I'm curious as to how you think these springs work.

Thanks to Xlot for giving the right numbers.

RKD in OKC

as opposed to the stiffer springs pushing the car up at the stiffer rate as they are decompressed? How do your physics make a lower rate compression spring push a car up more than a higher rate non-progressive spring? The only way your progressive springs would lift the unweighted end more than non-progressive springs is if the non-progressive springs were softer than the low rating of the progressive springs. Are you sure you were comparing progressive springs with a higher low rate than the non-progressive springs like maybe a 300-500 progressive to a 200 non-progressive.

The progressive springs I've had on my cars with windings determining the progressive rate instead of wire size were still progressive even though they looked like you describe. You can't tell by looking at springs how they are actually functioning, you really have to put them on a spring dyno. Just ask any spring manufacturer.

The factory progressive springs I got from Porsche Motorsports for the front of my 951 were 180-410 and the wire the spring was wound from was tapered. The winds were a constant spacing. Porsche were no longer manufacturing the 180-410 springs when I wanted to upgrade my second 951 a few years later so we took the factory progressive springs (from the first 951 I did the upgrade on) to a local coil spring manufacturer and asked to have them duplicated. We got back a spring the same wire size the length of the spring and progressive windings. The spring copies looked just like you describe ie., the windings did not get narrower from one end to the other but looked like the were widely space at one end and narrowly spaced at the other. AND when we installed them on the car all the narrowly spaced windings closed up and were touching each other. We thought the same you did, didn't think they were really progressive and asked the spring manufacturer about it. They invited us to come watch both springs be dynoed on a spring dyno. Guess what, both the tapered wire spring with constant spaced windings and the constant wire spring with narrower and wider spaced windings DYNOED THE SAME WITH PROGRESSIVE CHANGING RATES AT THE SAME COMPRESSED LENGTHS!!! On the spring dyno they were exactly the same.

Also the factory tapered wire springs fell out of the top of the spring perches when you lifted the car and you had to be very careful when setting the car back down to make sure they were in the top hats correctly. It was kind of a hassle. We told the spring manufacture and they were able to change the spring windings and length so it would not fall out of the perches AND had the same rates at the same lengths as the original tapered spring.

Fabio421

I'm no expert on the subject, but I've got a chassis setup book that was written by a reace engineer who once worked for one of the big three U.S. automakers. He claims that an automotive coil spring does not lose it's spring rate although they do settle a little and take a set. I'll see if I can't get the exact quote and post it.

GlenL

I'll address this part of your comment as it's the central issue. When stopping the car will have weight transferred from the rear wheels to the front. Since the fronts have more weight the springs compress and the rears, supporting less weight, extend. As the rears are using the lower spring rate they will extend at that rate. If the rear had a standard spring at a higher rate it would extend less while supporting the lower weight.

Sounds like you've handled some truly progressive springs. The Eibachs aren't wound in that way. There are two sections where each section is the same tightness. This is why the Eibachs are not actually "progressive" but have two rates. I was going to mention tapered wire but the 928Intl Eibachs don't have that, either.

What would be great is to take a pile of 928 springs and have them tested. Then we'd have the hard numbers needed to make decisions. Are GTS springs that much stiffer the OB springs? What's a GT spring? Are the paint codes that important? I've done my own "tests" by standing on a coil while having someone eyeball the compression. Most look like 400lbs/in in front and 300 in the rear. Very Scientific!

ew928

So the rear springs, with lower rate, hence less 'push', will extend more.

So a lower rate spring will extend more than a higher rate spring with the same force pushing down in it.
This physiks stuff is so counter-intuitive.

RKD in OKC

Sorry Glen that rear raising less with a higher rate spring is just wrong. These springs in compression NOT tension. If the springs were in tension trying to hold back the extension I would agree with you. But since they are in compression they are pushing the car up. Less push equals less lift period. According to your scenario lighter springs the same length would have a higher ride height?

Again if you are comparing an Eibach progressive spring that is say 278 to 372 to a stock rear spring of say 178 then the progressive spring will push the rear up more under braking than the stock spring because the low end of the progressive spring is still stiffer than the non-progressive stock spring. BUT if you compare at 278 to 372 to a 372 non-progressive the progressive spring will lift the rear less under braking.

Maybe if I describe this as if we had lower weight helper springs like some racers run. Just for illustration lets say the main spring is 372lbs, it is shortened to make room for a helper spring on the end that is 278lbs. At rest the helper springs are fully compressed and the main spring maintains normal ride height. This is a setup some racers run for the same reason there are progressive springs to smooth out the ride or maintain tire contact on rougher surfaces. It is smoother because the car lifts less as the extended limit is reached keeping the tires in contact with the road surface, not because the car is riding on softer springs.

With a helper setup under heavy braking as the rear decompresses or lifts off the main springs until the main spring is extended to the point it can not longer push 372lbs. As the rear continues to lift the weight on the rear of the car is decreased until the helper springs which until now are fully compressed start to be decompressed or extend. So now the force from the springs to lift the car comes from the lighter helper spring of 278lbs not the main spring at 372lbs. The force from the springs to push the rear up is less the rear rises less. Does not matter whether the springs are progressive or not. Less spring causes less lift.

And AGAIN the progressive spring with the same wire diameter the length of the spring looks exactly as you describe, two distinct areas of distance between the winds. And yet on a spring dyno they are fully progressive to match the spring rates at the lengths of the tapered wire progressive springs.

If you do some research on progressive springs you will find that most racers use non-progressive springs for two reasons. Their suspension setup guys don't understand progressive springs as setups with progressive springs is a lot more difficult. It is not just a simple calculation to pick a spring like with non-progressive. And two, progressive springs are more expensive to change to maximize the suspension for a specific track because you can't just slap in a spring, you have to determine the rate, where that rate centers, and how it progresses to what lighter rate. That's a lot more than just slapping in stiffer of softer non-progressive springs. But the fact there are a lot of racers using helper springs shows there is a need for a lighter spring towards the end of travel.

ew928

A coil spring is a torsion bar wound into an ascending circle.

RKD in OKC

Ernest, yes you are correct, the difference is the ends of the torsion bar are captured so it can work in both directions compression AND tension, say 400-0-400 given enough travel. A coil spring with non-captured ends only work in compression. A torsion bar can also be made progressive by how you operate the lever rotating the bar. Not so with coil springs.

There are some 944 Turbo guys I know that install coil overs and leave their torsion bars in setting the torsion bars to static zero at their ride height. This means the torsion bars apply force to keep the body from lifting instead of holding the body up. Because they are resisting the coil overs pushing up force as the suspension decompresses or extends they kind of make the non-progressive coil overs into progressive springs.

GlenL

First off, springs are rated in force per unit displacement. There is not an intrinsic force. The force is due to being compressed, or extended, a certain amount. The force from the spring exactly matches the force applied to it.

Here's an example:

If you put a 1000 lbs on a 200 lbs/in spring the spring will compress 5 inches. If you remove 200 lbs the spring will come up 1 inch. If you repeat the exercise with a lighter, 100 lbs/in spring it'll compress 10 inches to start and some up 2 inches when the 200 pounds is taken off. (Two is more than one.) This is exactly what happens at the rear of the car when you apply the brakes. As weight is transferred the lighter, lower-rate spring extends more. The heavier spring won't push it up more because it's stronger, it pushes up less because that's what is needed to support the weight of the car.

There are dynamic effects where a stiffer spring creates a quicker movement. This is why stiffer springs get stiffer shocks. After the system responds to a sudden weight change the stiffer springs will settle to a position with smaller length change. Going way back, that's why a combination corner will behave oddly with progressive springs as the car quickly shifts weight between sides.

Racers use single rates for a variety of reasons. The #1 reason is that it's a waste of time, money and effort as tracks are smooth. Then there's that single-rate springs behave more predictably.

I'm talked out. Finish up if you'd like.

RKD in OKC

Just think about that.

Lizard928

Actually RKD, I am afraid that Glen is correct with his statements.

RKD in OKC

It's not how many pounds you take off the spring, it is how many lbs are setting on the spring.

dr bob

It's the difference in length between normal loaded lbs vs normal loaded minus transferred lbs. These loadings will always be positive numbers in the 928 when stopping in a straight line. The length of the shock/spring assy is limited to the extended stop length of the shock; the spring is always under compression even with the car supported on the frame with the wheels dangling.

To determine how much total travel there would be on any given weight change, one would need to know the lengths of travel in the spring covered by the two windings. ie: there are threee inches of travel at the lower rate before the higher rate. Remember also that the spring is in the middle of a lever (the fulcrum), so the actual load on the spring needs to consider the length and angles of all the sections of the lever (includes wheel offset and width numbers), and the angle of the shock/spring relative to the position of the lower control. arm. Is it ever simple?

S4ordie

There are no springs in a 928 suspension. There are coils however. Just like our cars are propelled by an internal combustion engine, not a motor. A motor is electric. Not sure how these misnomers entered into the automotive lexicon but htey have been around and erroneous for decades.