Need Help With Springs

[Bryce]

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.

[jfgbody]

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.

[Bryce]

Thanks!

[jfgbody]

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.

[Bryce]

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.

[jfgbody]

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?

[6spdcamaro]

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.