Need Help With Springs
 

New member in need of advice.Im right in the middle of a frame on restoration for my 1978 cutlass(touring car with daily street action) and once again ive hit a brick wall.Im having a helluva time guessing what shocks and springs to shove under this thing.Ive got a sbc w/ iron heads and an aluminum intake.The Stock weights are as follows;gvwr:4560 gawr F:2270 gawr R:2290 iI just bought a set of 245/35R20 iroc wheels.The wheels add an extra 248#'s of unsprung weight to the car(62#/wheel--124#/side).Ive upgraded to the big sway bars front and rear, bushings front and rear and bolted all of the braces i could find to it to help stiffen the car up a little.My intensions are to maintain the stock ride height within 1/2" + or - .I read a moog spring chart that pointed me in the right direction and Ive got an idea of what front springs look good on paper (moog:5662 =706#/in) (stock:5606 =420#/in).Im hoping to hear from someone with a similar setup or any feed back from anyone running these springs and Im also interested in hearing what shocks you are using.As for rear springs and front and rear shocks,I dont have a clue what to use.Im looking to stiffen up the ride but I dont want to over do it.Any and all input would be greatly appreciated.

When I choose spring rates I do (two) calculations.

First what is the weight of the car at that axle. Divide that number by 2. this gives you F (force or weight)

Since F=kx next is to determine the amount fo droop you want or the shock has. This amount of droop gives you the minimum preload on the spring at ride height. If you have 3" of droop you dont want the spring to become unloaded. take F/x=K. K is your MAXIMUM spring rate.

Now this is where personal preference comes in. I design around a 1G bump. How much travel do you have in your shocks from ride to full compression, this is your x for the next equation. Take F times 1G, or whatever number you want to design around. F/x=K this is you MINIMUM spring rate. Now you have a range.

The only way to adjust this is to adjust your shock length and your shock length at ride height.

Thanks for the quick reply.Your reply shpuld help greatly in choosing spring rates.Ive been at this for over a month now and that was the most intelligent answer Ive recieved.Its harder than you think to find someone to help you choose a spring rate.

Thanks!

Alright,I sat down and sharpened my pencil, grabbed my calculator and worked on that math homework you gave me.I came up with some spring rates that I feel pretty comfortable with but I have a few questions about your equation.The first question I have is with determining the amount of droop the shock has.I assume you were referring to the ammount the shock should be compressed at ride height to get the shocks in their "sweet spot".So I determined that my shock travel is 4.25" and with a 60/40 setup I am required to compress the shock 40% to put it in its "sweet spot"the ammount the shock collapsed was 1.70".I used 1.70" for x when figuring maximum spring rate.Is this the correct way to figure for x. I then took the remainder of travel left to fully compress the shock (4.25-1.70=2.55).I used 2.55 for x when figuring minimum spring rate.Was this correct.Using the above numbers I arrived at a maximum front spring rate of 713#/in. and a minimum front spring rate of 475#/in. What is the best way to detirmine which spring rate to use?Should I avg. the two spring rates and use the one in the middle? I kind of feel like im missing somthing,should the springs be compressed 40% as well. I really appriciate you taking the time to help.

Quick answer before I go out with the wife for new years.

Asssuming your math is correct, the range gives you choices of what kind of ride you want. Soft gives you more of a street car and stiffer is more of a race set up.

If you dont mind posting your weight and motion ratio I can check your math. It seams like a wide range.

Im not quite sure how to measure and calculate motion ratio.I have spent the last few eavnings investigating how,but Im still not sure Im doing it right.How do you figure motion ratio?

The motion ratio is the distance between lower control arm bushings to the centerline of the spring divided by the distance from the lower control arm bushings to the ball joint.

It represents the ratio of shock travel to wheel travel. Wheel rate is (motion ratio)^2 times the spring rate.

In the rear suspension, assuming the springs are mounted on the rear end, the motion ratio is about 1:1, so the spring rate equals the wheel rate.
Hope that helped.

That helps alot.Thanks.
Now Im sure my math wasnt right.
Motion ratio:.588235294
front cornner weight:913# X 2 = 1826
rear corner weight:833# X 2 = 1666

That motion ratio is critical to figuring spring rates. For a typical GM front suspension, we use 2:1; that is, the wheel moves 2" for every inch of spring compression.

Comparing spring preload to shock travel is almost impossible... Every different rate spring will have a different natural preload height when carrying the same weight. What you need to include in your calcs is the installed height of the spring in your car. Let's say you set the car at ride height with the springs out and measure the distance between the upper and lower spring perches. Say that distance is 10". You will need to find a spring that balances the weight on that corner of the car at a 10" height. Go too stiff and the car will ride too high; so the stiffer the spring, the shorter it needs to be. You can't just cut them to get the ride height, because that changes the actual rate and throws all your math out the window...:faint:

Using your corner weight above and the 10" example, you'd need a 900lb spring 11" long, or a 450 lb spring 12" long, or a 275lb spring 14" long (this one is OUT TO LUNCH, but you get the idea...)

Yes, you observant types will note that I left out the motion ratio. Just trying to illustrate the idea...

I would suggest looking into stock diameter racing springs. They are sold by rate and free length, so it's easier to match the spring to what you need.