Spring rate "stacking"

If a 200 lb / in spring and a 100 lb /in spring are working "together" is the overall rate 300? One spring would be inside the other. It can't be this easy.... can it?

Yes it is that simple. parallel springs total rate = sum of individual rates.

But the terminology sure gets fun!

For instance, the word "stacking" in your subject suggests an image of putting in the springs in series (one on top of the other), which would have a wackier calculation:

((200 * 100) / (200 + 100)) = 66.666...

Which isn't what you asked, that having been answered by AC. I just couldn't stop myself. Plus it has the dubious usefulness of pointing out that you are correct in thinking spring calculations can get kinda weird, it's just that the one you're after is simple.

Yeah, they won't be 300lb unless they are mounted concentrically, like double or triple valvesprings. Sticking one on the other will reduce the rate, just like cutting a spring (in effect, making two springs out of one) will have a higher rate for each one.

Now, I've seen how they have developed digressive springs. Rate goes down. It uses two springs, and the lighter rate spring is installed heavily preloaded, so it only moves until there's a certain amount of load on the top spring. (The spacer between the two springs has a limiter on its up-travel)

I use a setup like that on the Targa Miata, actually. I can lock out the soft spring after a certain amount of travel. Come to think of it, I could probably flip the springs and lock out the stiff one

Really, you set them up so the soft spring is just locked out at static ride height.

Keith wrote: Really, you set them up so the soft spring is just locked out at static ride height.

Or not. If we're talking stacked here instead of parallel you can have it so that the soft spring takes up the first X" of travel so that the car doesn't skitter across rough roads and then the stiffer spring comes into play.

If done right you'll never notice the transition.

This is also what a lot of modern cars do with long soft bump stops. Half of your suspension travel is on the bumps. The transition is smooth and you never feel it.

Gearheadotaku wrote: If a 200 lb / in spring and a 100 lb /in spring are working "together" is the overall rate 300? One spring would be inside the other. It can't be this easy.... can it?

No, it goes to the softer spring. If the softer spring bottoms out then it becomes 200. This is the principal in a progressive spring.

In reply to iceracer:

Though the conversation has turned to stacked springs, the OP was asking about a pair of springs mounted concentrically, not stacked one on top of the other. In his original question, 1" of travel at the perches would compress the 100 lb/in spring 1" AND compress the 200 lb/in spring 1".

And just for clarity, I'm not sure what you meant by "it goes to the softer spring"... If you mean that the rate for stacked springs is the softer rate until coil bind then becomes the firmer rate, that's also incorrect. See Hooke's law on springs in series (summarized in my earlier post). You are correct that once one of the springs hits coil bind then the rate becomes that of the remaining unbound spring, but that's probably obvious...

I also see that while I was having fun, I only muddied the waters surrounding the OP's question by carrying on about springs in series...

Define " together". If stacked the spring rate would be that of the softer spring I don't see anything under Hookes Law that relates to springs in series.

Of course I am not an engineer so maybe I don't understand.the language.

I doesn't actually work like that. For example, take a 12 inch long 100 lbs/inch spring and count the coils. Not cut it in half and count the coils again. You have the same wire diameter for the springs, but half the coil count, so the rate is doubled to 200 lbs/inch. That's the principle behind cutting springs to increase rate. Now take those two and stack them back up but with a spacer in between them and magically you have taken two 6 inch 200 lbs/in springs and stacked them to make a 12 inch 100 lbs/inch spring. Now that I've convinced you that the combined rate of the springs is lower than the springs individually (unless they're collapsed), you just extend that and add a bit of math to come up with the general ideal that the combined rate of two springs (non collapsed) is always lower than the rate of the lower spring. Once that collapses, the rate jumps to the rate of the higher spring. Lots of cool custom rates that you can come up with by stacking springs. Neat stuff.
But as the original poster asked, if you stack them one inside another, they're simply additive and the world is simple and happy.

Stacked springs don't "work off of the light spring until it bottoms out". The load works on both of them, so both of them compress.

If you have a 50-lb tender spring and a 400-lb main spring, putting 50lb of load on the assembly will compress the tender spring one inch and the main spring 1/8 inch. If the tender spring bottoms out before the suspension does, the spring rate will jump to that of the main spring when that happens. It's possible, depending on the rates and spring lengths used, to have a stacked spring arrangement that runs out of suspension travel before the lightest spring bottoms out.

I still like the digressive spring, it's one of those problems like AWD systems that is fun to try to wrap one's head around. Say you have a 200lb/400lb spring combo, and you want the spring rate to drop after 2" of compression. So you figure that it takes 800lb of force to move the suspension two inches, so you have the 200lb spring in a holder that keeps it compressed by four inches. (The lighter spring would have to be the lower one, and you need to rig a limiter for it that can still slide down) Until there's 800lb of load on the suspension, the rate is 400lb. Past 800lb, the rate drops to something like 300lb. It's supposed to be awesome for things like touring cars where they want the suspension to be stiff, but still handle things like bouncing over curbing without upsetting the chassis.

Also supposed to be awesome for high speed off-road work.

iceracer wrote: I don't see anything under Hookes Law that relates to springs in series.

http://en.wikipedia.org/wiki/Hooke's_law#Multiple_springs (there's a section to be expanded titled "Equivalent Spring Constant (Series)".)

carguy123 wrote:

Keith wrote: Really, you set them up so the soft spring is just locked out at static ride height.

Or not. If we're talking stacked here instead of parallel you can have it so that the soft spring takes up the first X" of travel so that the car doesn't skitter across rough roads and then the stiffer spring comes into play. If done right you'll never notice the transition.

I've tried that. It didn't work well - trying to get the damping right was impossible, and I ended up with a considerable amount of initial roll. It worked better to just have the right damping for the primary spring rate.

Having the secondary only working on droop improved things considerably. The guys at AFCO (who have a fair bit of experience with setting up for rough surfaces!) concurred.

I've been running it for years. Ground Control got it spot on the first time.

Gives me compliance on the rough parts, which saves my back, and keeps the car planted and allows me to run a firmer spring than I would have otherwise for the street.

My what a discussion I've caused. Glad the answer I needed was a simple one!