Help Me Find Springs for My Buick

The time has come to give the Personal Luxury Coupe a little bit of love, and one thing it needs is new rear springs. 

Why? Because in order to lift the rear of the car, I simply inflated the air shocks. And while that did lift the car a few inches and provide some extra spring rate to absorb the weight of four people and tools...

They leak. So. Badly. After some, err, ALL the abuse, they leak down to flat after a few hours, and the shocks are blown, anyway.

I'd like to make this car faster off-road, which means getting the static spring rate closer in the rear (the front is actually quite good with the torsion bars cranked up all the way), then fine-tuning with a fresh set of air shocks.

Does anybody have recommendations for a set of cheap/junkyard springs I could substitute in place of the stock units? I'm not sure how to do the math, but it was riding pretty nice with 100 psi in the rear shocks. 

Here're specs for the stock rear springs:

https://www.rockauto.com/en/moreinfo.php?pk=200770&cc=1021891&jsn=440

And before anybody recommends it... I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. I will not convert this car to use 2.5" race springs even though I have a lathe and a pile of extra springs. 

Oh, and here's what the rear suspension looks like:

Wanna take a stab at the size of the piston that 100 psi was acting on? Or are the air shocks really more or less... gas charged shocks, where the only real acting area is that of the shaft? In which case, what's the shaft diameter? I mean, from there, the "helper" rate probably isn't hard to guess (though if it does have an actual chamber acted on by a full size piston, then it probably has a strong rising rate, while the volume of the shaft probably doesn't change the volume enough to change the rate a bunch)

PV=nRT and all.

See, that's where my math gets fuzzy. Without cutting one of these up, I really have no idea what sort of chamber I was filling.

This might be worth weighing the car and carefully measuring ride height as I inflate them to figure out their rate. 

Ohhhhhhh... Guessing we can probably call the OD of the shock lower the piston area, or thereabouts? Got a diameter? And was it sitting on its own wheels when you pumped it up to 100psi?

We'd know more about the range of rates if you, for instance, set it on the ground at 100psi, then jacked it up and found out A) how much droop that was and B) what the pressure at full droop is. I suspect there's a bunch of variation in pressure (and attendant spring rate) across travel with that setup.

e.g. if that's about 2" in diameter, then we've got about Pi square inches, let's call it 3, making it 300lbs of force at whatever height you measured it. But we need another point to guess at rate, so we can see pressure change per travel and turn that into a guess at chamber size and thus the range of rates you're getting... Even jacking it up 1" and checking pressure would tell you rate right around static ride height if you think that 3 sqare inch guess is ballpark...

This is a fun one; I'll try to come back it it later. BUT...

I think it'll be hard to find out much by inflating and measuring. Might be able to work it out using the change based on the known existing springs, but...

For a mental puzzle: Imagine it had no rear steel springs, just the air shocks. Until the suspension tops out, different ride heights shouldn't actually see different pressures. The car still weighs the same, so needs the same pressure on the pistons to hold it up.

Back to that Pressure x Volume = Number of Moles of Gas x Ideal Gas Constant x Temperature (and we can pretty much ignore the last two for our purposes)

So P x V = N. And P holds up the car just fine. But V gets bigger as the car gets taller (until top-out).

So if you add to N (more gas molecules), you just raise V until the car tops out and the shocks can't grow, at which point you start adding P.

Of course, since you do have metal springs, they'll be helping hold the car up less and less, 252 lbs per inch of height. Perhaps we can figure out the helpers with this info.

Measure ride height with shocks inflated to 100psi as your target number. Deflate shocks, check ride height again. Take a SWAG at how worn the stock coils are and play with dimensional info in this MOOG interactive chart.

https://www.moog-suspension-parts.com/universal_coil_springs.asp

Free height of the stock springs is quite tall...If you can drop down to a 4.07" ID in the square end bucket there are a couple options >16" tall, but they give up some load rate.

*edit. If you go pigtail/pigtail and cut one end off, CC80841 is 17.3" tall, higher rate and higher load.

Would 5" stock car springs work?

https://www.speedwaymotors.com/search?query=afco+springs&gclid=EAIaIQobChMI7bXF8JGN7AIVnOzjBx2f4A8PEAAYASAAEgKMzvD_BwE

Moog has a chart of all of their springs with all the dimensions on it.

My phone is being wonky right now, so I can't post it up.

I think you need a spring about 13.5-14" free length and 200 LB/in assuming you're content with the current unladen ride height.. That's just based on what appears to be the preload in the spring data and upping the rate a little over 50%. The 50% is a guess.

In reply to DeadSkunk (Warren) :

That would be my estimate as well.

Any idea what the small (pigtail) end diameter is? I assume the 4.5" measurement is for the main body of the spring.

This would be my best guess looking at the catalog

There are many spring calculators that are pretty darn good at getting you within about 20 lbs of spring rate.  You'll need OD, wire diameter, free space between coils, pigtail, blah blah.  Easy stuff to measure while it's out of the car.

From there, if you like the current rate of your springs (ride, oscillation, etc) you can choose a spring with the same rate, but X inches taller than what you currently have.  The nice thing about trailing arm suspensions is that the spring is 1:1 with the axle.  If you were to try the same thing with a double-A front where the spring is mounted between the fulcrum and the load, it gets hairier.

Pigtail springs are a choice that doesn't need to be kept.  Straight springs make more room for things anyway.

If you just need 2" or so, you could also get some spring spacers.  They just sit in the lower cup and space the spring up.  I have done that before.  I wouldn't go more than 2" or so.  I have also built some steel spacers that used some 3" steel pipe and a collar welded on to it at the right height.  Then I welded the steel pipe to the axle's spring cups.  Kinda like this:

I hope your spring looks better than mine.

If you have access to wheel scales you should be able to get a good estimate of what the air shocks are doing. 

1) Measure corner weights with the shocks at zero psi.
2) Air the shocks up to 100psi, block under the frame, and release the pressure in the air shocks. Try to block as close to the axle centerline as possible, or even behind it.
3) Measure rear corner weights again.

The difference in the corner weights between #1 and #3 should be approx what the air shocks are supporting. 

A less scientific method would be to look to see what the wagon version of the chassis has listed for springs. I would expect a higher rate and preload to provide a higher ride height. 

In reply to Jesse Ransom :

I heartily agree with your application of physics. +1

Hey, Tom have you considered converting this car to use 2.5" race springs since you have a lathe and a pile of extra springs?

Thanks for all the help, everybody! Time to do some more homework–I'll update this thread soon.