Ever wonder what the specifics of a progressive rate spring are? I did. They're sold as "180", but they're "progressive" so that can't be enough to go by.

The springs I tested are Weltmeister progressive 180s, and have a pretty clear wide coil spacing section, then a narrow one. They look to be pretty constant within each section. One of the springs is a touch longer than the other, which shows up in the graph.

When lightly loaded they are about a 100 pound rate, and when heavily loaded, they are about a 160 pound rate. The transition happens at about 340 lbs of force and about 3.75" of compression.

I'm using these for a V8 conversion, and since it'll be a street car, these will probably work quite nicely. Here's a graph:

Cool info!

Good to have some actual facts...

I thought progressives were only made in 160 though..

I have wonder about that as well, you would think its basic and should be in the description. Now looking at just the description of 180 whats with that?
Light load = 100 heavy load = 160 = 180 ?
I am running 165 on my V8 (Street) car and love them.

Well I would call these 160, despite the SP180 that's printed on them. Close enough for me though.

I'm guessing the 180 is supposed to be the heavy load rating. If I were a spring manufacturer I would include the light load rate, the heavy load rate, and the transition point. Maybe they think that is too confusing for the consumer, or maybe they just want to keep it a secret?

These are not really progressive. The larger spaced coils should be 180 lbs/inch. The more tightly spaced coils are 'dead coils'. These dead coils bottom out on each other and become solid under normal conditions. Then, only the remaining coils are 'active', giving it a useable spring rate of 180 lbs/inch.

The reason they do this is because they have to make the springs fit the shocks. If the spring was entirely 180 lbs/in spring coils, then your ride height would look like a 4x4, and there would be no way to lower it enough. So they put in these dead coils to make up the extra travel, while still allowing for full suspension travel. It's really a trade-off since like you have noticed, when the spring starts to unload, the spring rate decreases. Of course, that decrease is generally a good thing.

Demick

How's the fit in the lower spring perch? The SP180 I've seen look like they have a slightly larger diameter that doesn't exactly fit into the perch cup.

Thanks,