Technical Rear coil springs

Need some help, I'm building a 40 chev coupe, BBC 427, 6-71, all steel body. I have a straight axle under the front and want to use coils in the rear with regular shocks. I found a kit that will use a coil spring, 2.5 ID, sets on top of the housing and the top bracket mounts to my 2x4 box frame. I was thinking 12 in spring with a 200 lb rating. Do you think that would be a good starting point? I wish I had scales but I don't.

 

Here goes; weights, spring rate etc

 

Thanks, I can get close.

 

For a street car I would shy away from 2.5 ID coils [unless you want to "tie" the springs to prevent them from jumping out ]
Example:
A Ford Mustang 6 cyl only has 85lb/in leaf springs, and the Boss mustang has 152lb/in springs
[Road Racer Mustangs only 170lb/in]

So unless you intend to lay the coilovers down to alter the motion ratio ,it will be over sprung.

Don't just think about stiffness [lbs/in] but also "load rating". Load Rating is how much the spring compresses to meet a point of equilibrium with the weight upon it.

Usually this is about 5-7 inches!
So a nice soft 2.5" Id coil would probably coil bind on compression [or jump out on "Droop"] This is why manufacturers us larger ID coils.

Assuming the springs are vertical and mounted on the rear end [1:1 motion ratio]
Lets do some "GUESSING" maths.
If your car weighs 3000lbs and has 52/48% front rear bias ,the rear would weigh 1440lbs.
Deduct approx. 200lbs for the rear end and wheels = 1240lbs or 620lbs sprung weight each side.

now if you divide the 620lbs by 5" which is a reasonably comfortable load rate ,it would = 124lbs/in [ you would jack the car 5" before the wheels leave the ground.]
Understand a lot of the load rating can be compressed with a spring compressor so a shorter travel shock is required [this is very normal]

Your suggested 200lb/in springs with the above estimated loads = 3.1" , That is stiff enough for Road Racing cars.
Note: the best way to weight un-sprung weight [instead of guessing 200lbs] is to jack the car up onto stands and remove the springs and use scales under the wheels. [bathroom scales work OK]

Edit: If your rear coil-overs were laid over 30 degrees the motion ratio would give a wheel rate of approx. 125lb/in if you were using 167lb/in springs [which would be fine for the above parameters]

 

kit include way to stabilize rearend? post pics

 

Car has a 4 link and a panhard bar. I was wanting to do coils, straight up and down with a adjustable shock. The easiest would be coil over shocks, but I would rather have something more old school looking but work.

 

Kit has plates that tie the top and bottom of the spring.

 

I'm thinking that 2.5 coils are for coil over setups. Or are you running a coil over setup?

 

If you're using a 9" Ford rear end, plan on 300 pounds unsprung weight.
These are some of the best coil shocks made, in my opinion:
http://vi-king.com/

 

You will want at least a 4.5 inch spring if you want to look old school.
2.5 inch springs are usually for coil overs. Are you talking about that kit from Jegs that puts a shock in the middle of the coil? Speedway sells all lengths ,rates and diameter coil springs.
Making your own mounting plates is an easy fabrication job and you can tell your car show buddies that I made them and didn't just buy a kit.

 

I may go on and make them. I looked at the kit again, it's $70 for one, plus shipping. I got more time than money. I'm going to use a 8.8 explorer rear end. Going to remove 3 in on the long side, that will give me a 56 1/2 total width and a centered pinion. They have 31 spline axles also. I'll check out Summit and see what they got. I have a new set of QA1 coil overs, new, but they have a 8 in spring, don't know the specs on the spring. They are a street rod, single adjustable, they are to short to really do anything with.

 

If you are looking at the coil spring kit from speedway, where the shock goes up through the middle of the coil, I used it 30 years ago in my '34 Chev 2 door. The kits is good, but the supplied springs were too soft. So, count on using something with a higher rate- the first set only lasted for about 20 years of holeshots! Yes, still running it.

 

I found a pair at Speedway, they are 4.5 OD and 175 lb. They are in the garage sale so the price is good. If I have to change them out when I get done, I won't loose much. They also had 5 in spring pads in there for $5 a piece.