Technical Help with leaf to coil conversion ??????

I recently ditched the worn out leaf springs on the back of my 53 Belair. I built a 3 link for it and am running an old Fenner hydraulic setup on the back. It's a very simple old setup that runs off the battery under the hood. I set it up using some slightly cut down 60 Pontiac rear coils, because I already had them and they were the size I needed(about 12'' free height and 4'' ID). Now that I have it on the road, they are way too stiff. I figured the 60 Pontiac is only 6-700 pounds heavier than the belair, so they should have been alright, just slightly firm. I was wrong. I figured the thing to do would be to find some coils that are as close to the spring rate of the original leafs and I should be good. From doing a little research I have found that my factory leafs only had a 100lb spring rate and the Pontiac coils have a 275lb rate, so that explains why they are too stiff. I searched through a Moog coil spring chart and found two coils that seem like they would work. The first being front coils from a 80-83 Honda Civic, which has a linear spring rate of 112 and rear coils from a 98-2005 Cavalier with a variable rate of 114. What I'm not sure of, is that though both coils have a slightly higher spring rate that the original leafs, both the Civic and Cavalier weigh 1500-1800 lbs and the Belair weighs 3300 lbs. Is the load from the Belair going to be too much for either coil and make them collapse, even though the rate is higher? Am I missing something or over thinking it all? Thanks in advance for any advice.

 

When you cut down a spring it gets stiffer for the same amount of deflection before being cut.

 

I understand that and I'm not sure how much the Pontiac coils that are on it now are cut. They were already cut when I pulled them off a friends car to bag it. That's why I'm looking at coils that are already only 12-14'' tall, so I can run them just like they are and I'll know that the spring rate is correct.

 

It's the same theory as a M2 setup, light duty coil springs have been used in almost every application imaginable. I would think the cavalier would be you best bet given the availability over the Honda units. #brazosbomb

 

I am not 100% sure but I think you are overthinking it because the spring rate is in # so a car weight was not in the picture when the spring was made just the # rate of the spring if that makes any sense.

 

Are you mounting the springs in front or behind the axle? Behind makes them at stiffer and ahead softer. The Pontiac had them ahead.

 

They sit on top of the link bars, right in front of the rearend.

 

Have you considered running accumulators somewhere in you hydraulic line? You can add or take out a little air to dial in the ride you like.
I'm not really familiar with hydraulics but know that is done sometimes to soften out the suspension

 

What ?

Let's say a non progressive spring has 10 coils,
Each coil has 10% of the rate and 10% of the load capacity.
Start there. (Explain how it got stiffer)
Each coil also has 10% of the free height and 10% of installed height.
When you alter free height in order to change the installed height, the calculated and preload on that spring is gone.

So you decide the installed height it too tall, then start chopping coils out.
Remove 1 coil and you drop 1/10th installed height, 1/10th of the rate & 1/10th of the load. Chopping stock spring for ride height adjustment always makes them weaker in their stock application.
In order to get close to the same rate and load with lower installed height you've gotta start with a stiffer spring and cut for installed height or the ride quality is gone.

 

The moog universal spring chart is pretty good reference, if you have a known starting point spring. Cavalier weighs 2900 lbs not 1800. That may be part of your confusion.

 

The spring rate will increase(stiffer ride) as you cut the spring shorter. This may not be that big of an issue if you're just chopping a turn or 2 but if you are planning to use a spring cut in half I would definitely take that into account.

I think adding accumulators on a tee somewhere are the solution to what you are doing though. It will be like adding an airbag.

 

Thanks guys. 31Vicky, yeah my first google search gave me the wrong info on the cavalier. 54fierro, I am familiar with accumulators and eventually I will probably add some to the system for better fine tuning, once I add more dumps and hook up the front. My Cavalier coils will be here on Monday, so we'll see if they do the trick.