Enigma I just checked into this and I'm sure my logic is correct. In tandem springs (side by side) they would each carry half the load. But when stacked together they each carry the same load of 448 lbs. There is no way around this. If I stack two books one atop the other and stand on the top book. The books are not sharing half my weight they are bearing all of my weight with the bottom book actually carrying a little more because it carries the weight of the top book as well. If I place them side by side and stand with one foot on each book then they share half my weight between them.

The formula for stacked springs would be something like this.

K combined=1/(1/k1 + 1/k2) where k1 is the spring rate of the first spring and k2 is the spring rate of the second spring.

So K = 1/(1/448 + 1/448) = 224 lbs/in

If you had two springs with differing rate say 500 lb/in and 225 lb/in you would get an actual assembly spring rate of:

K=1/(1/500 + 1/225) = 155 lbs/in

This would be the rate of the assembly until the 225 lbs/in spring bottoms out them the rate jumps to 500 lbs/in.

So you can see the Cosmo dual spring set-up shouldn't really have an actual 448 lbs/in spring rate but a more sensible 224 lbs/in spring rate. They also seem to offer 280 lbs/in rate which in a dual spring set-up would equal a 140 lbs/in spring.

Well fellows here is my 2 cents.

I must be the 1977 280z experiment here without knowing much I had bought these off EBAY.

Fortunatly enough 4 me they appear to be alright (thanks cosmo) with very light if any modification. At first glance you want to cut at the top of the old oem bearing support and slide that onto the aluminum top that Cosmo supplied. So as far as the installation being "as advertised"? 4 me it is going well for the type of job we are talking about. It is my first time tackling suspension.(gotta man up sometime)

FYI-The illustrations in the cosmo manual showed more of a shock or rear type application but I surfed the net and found couple examples.

However we will see AFTER I load it all up and test drive, which will be a bit later. cURRENTLY I am replacing the bushings,brake pad,boots ...all that.

.....................................................................................................

Thank god I am lucky lucky this was operated in Brea California who had an anal owner (like me now) for the last thirty years, I am enjoying going through this car with the factory manual.

Mods so far:

valve job

New 2 1/2 ehaust and high flow converter (tried to keep it looking stock with OEM manifold and large stock muffler for quietness) still it growls

all std. services done

injectors & replaced

clutch replaced

Radiator/thermostat

Thermistine switch,wtr cool switch

If you had two springs with differing rate say 500 lb/in and 225 lb/in you would get an actual assembly spring rate of:

K=1/(1/500 + 1/225) = 155 lbs/in

This would be the rate of the assembly until the 225 lbs/in spring bottoms out them the rate jumps to 500 lbs/in.

So you can see the Cosmo dual spring set-up shouldn't really have an actual 448 lbs/in spring rate but a more sensible 224 lbs/in spring rate. They also seem to offer 280 lbs/in rate which in a dual spring set-up would equal a 140 lbs/in spring.

This is wrong. I logically don't get how adding two stiff springs to the same suspension gets you a soft spring. The real solution is to add the rates together and divide by the number of springs. As stated by John Coffey in the Hybrid Z thread on this subject:

Actually, that's a common misconception about dual spring setups. Both springs are involved at all times so the spring rate you get with duals is not what either spring provides on its own. Here's a dual spring example:

1. Spring A: 250 lb. in.

2. Spring B: 500 lb. in.

Let's apply a 250 load to the dual spring combination. Most people would gues that spring A would compress 1" and spring B would compress 0". Not true. Remember, spring B will compress 1/2" with a 250lb load so it is definately involved. What you actually have is a combined spring rate of 375 lb. in. (the average of 750 / 2) so the dual spring combination above will compress 2/3" and this spring rate is a constant UNTIL one of the springs in the dual setup bottoms.

It all went good, but unless you want to grind down all the speed bumps in your community I say set the new spring perch supports and lock rings flush to the top of the threaded standoffs.

The old struts and coils looked good ;but worked bad these should be nice with the KYB struts and bushing kit that I installed.

Later I'll do the rear,resting now because the new clutch slave cylinder wont bleed even with a power bleeder- probably will be fine after a nap.

All kidding aside it was pretty labor intensive the first time around but saved alot of bucks and bragging rights (anyone can pay a mechanic to do it)

Dave

Man, I've tried to post in this thread for the past week and it keeps failing. Hopefully this one works...

My simple dual spring example above is wrong because I didn't take into account that one of the keys in calculating spring rate is number of free coils. Jeff's calcualtion is more accurate.

Here's the calc I'm using now to come up with dual spring rate numbers:

Ko = Overall rate

K1 = First spring

K2 = Second spring

Ko = (K1 * K2) / (K1 + K2)

This is an approximate calculation but its close enough for our use. It comes up with a spring rate of 166.67 lb. in. for the combined 500/250 spring in my example. That's assuming equal height springs. If one spring is shorter then the other, the calculation gets skewed in favor of the longer spring.

Well I guess I stand corrected in this case, sorry for passing along incorrect info.

Bringing up this very old and dead thread because I just bought my Cosmo coils and want to preach haha...

A big factor to keep in mind with this set-up is that the lower spring is very soft! its not your typical spring being that its coils are flat, like a slinky. The full stiffness of the high top spring rate doesn't take effect unless the bottom spring bottoms out(middle sleeve contacts bottom spring support), which is the intended function of its design. The bottom spring is actually easily compressible by hand, partially. I'm not sure what the actual rate would be for them, but this makes what Cosmo states very understandable. That being that, the lower spring is design for more response in corners. So from my analysis, if your driving straight, your not too stiff because the soft lower spring isn't compressed all the way and is limiting the top spring. But when applying the extra Gs in a turn, it bottoms and allows the full force of the top to take effect, thus limiting your body roll from there. Additionally, the lower spring's rebound reaction out of a corner, I assume, would generate better acceleration response.

I'm open to corrections and/or discussion if anyone still cares about this topic haha..

-Bryan

FYI... spring rate calculators:

http://www.swayaway.com/calculators/swayawayCalc.php

If it's flat it's probably a "helper spring" that prevents the main spring from flopping around when you lift the car with a jack. Not a necessary item to have and doesn't do anything at all when you're driving, because it's compressed all the time.

Here's a helper spring with a description:

http://www.paragon-products.com/product_p/cs100-2.5.htm

Here's Cosmo's webpage clarifying spring rates (280 in/lb and 336 in/lb) and also showing pics of the flat springs:

http://www.cosmoracing.com/productinfo.asp?cid=259&pid=1340

It certainly looks like a helper spring to me. It would be pretty easy to take a picture of the springs with the car on the ground and figure out if they're doing anything when you are actually driving.

Edited June 11, 201014 yr by jmortensen

Jon Mortensen, with the new style G.C. camber plates, do you still reccommend putting them on top of the strut tower vs. up underneath?

Or if you want to save some money start looking for the parts individually from a circle track racing shop, like Pegasus or Behrents or Coleman Racing or Pit Stop USA. You can buy the springs, hats, threaded collars, etc cheaper through those types of places, although my understanding is that the Ground Control and Modern Motorsports stuff is the highest quality.

Im just starting to play with my suspension on my Z but when I first lowered my LS I took the cheap short cut for a few weeks ( cut springs ) until a circle shop got me set up and I did just as explained in the post I quoted.

I got the springs, sleeves, and tops and welded the sleeves to a set of KYB's.

With cut springs on a 4k lb car I was riding on my bump stops even after cutting them down a lot. The ride was stiff as hell and I had no rebound in the front end at all. Once I got my stuff from the shop even 300 something lb springs was almost exactly like riding cut springs on the bump stop. It was so stiff it started rattling parts loose from my interior even after adjusting the rebound the best I could.

Now I have a question. Im probably going to do the same thing this time around ( might get the GC's,... dont know ). What would be a good rate to get for a 76 280z for street use? Im also going very low as well.

Spend some time reading...

http://forums.hybridz.org/index.php/topic/38589-strut-sectioning-faq/

http://forums.hybridz.org/index.php/topic/50944-the-strut-thread-koni-illumina-tokico-carrera-bilstein-ground-control/