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Thread: Spring calculation help please !!!!!

  1. #1

    Spring calculation help please !!!!!

    I have a mid 60's AC Cobra replica and am using a mid 90's 8.8 rear end. Can anyone help me or better yet, teach me to figure out what spring i need. I have all the facts IE: the left rear wheel scales at 669 Right read wheel scales at 682 for a total rear weight of 1351, I am using a stock ford geometry for this set-up with the upper and lower control arms at the stock dimensions I need a spring that is ( I assume) 250 lb.s when the spring is compressed 1" (or 10" overall compressed length for the perfect ride height).
    Help ME!!! Please!!!, why can't anyone figure this out there has to be a spring manufacturer that can help with customs.
    Sincerely,
    Mick

  2. #2
    Member JUNIOR BUILDER
    Join Date
    Mar 2006
    Location
    Illinois
    Posts
    96
    It would drive a math wizard nuts trying to figure spring rates for a car and be accurate. There are so many influencing factors, The best you can do is a good guess then adjust depending on what car handles or feels like.

    I take it you have leaf springs? That is what a Ford Cobra was equipped with. There was no mention of front springs. You know that the rear and the front should be compatible if your going around corners fast. If they aren't you front don't want to turn or your rear will slide to the outside. Is the front an A/C control arm? Or a recruited ford? Do you have roll centers for your chassis? What do you expect from the car when done? All of these questions could effect what spring rates you want and more if any suspension parts or mounting points are changed.

    If you just want a soft ride, get something from a spring manufacturer that is for a car of comparable weight. They rate springs for factory applications. from rear weights, I would guess around the weight of 65-66 mustang with 6 cylinder (pony car). If it's too stiff take out short leaf rear half. On a leaf spring the front half controls spring wrap up and the rear is for rate. As far as ride hight they can be re arched or de arched at about any good spring place that specializes in replacing them. You can do this yourself if you have a hydraulic press a big hammer and able to make a jig. If you have a Mustang two rear. I put one together just this way for my brother in law. cut rear of short spring and de arched to get ride hight. They felt a little stiff but matched up well with the front they where stock Mustang 2 spring with one coil cut. make sure you give it a good alignment after.
    The older I get the less i know for sure Dennis

  3. #3
    Senior Member DYNO OPERATOR
    Join Date
    Feb 2006
    Location
    VA Hospital, Dallas, Tx (214 302 1924) cell-972-464-7400
    Posts
    540
    I could give you an idea to start with if it were a drag car only. For street would be heavier and for track would depend on the type of track.

    If anyone can figure this for you it will be Mr Billy Shope. He is a mamber on here. You can send him a PM or you can possibly find him giving input here in the suspension section of the forum. I have not seen him for several days though.


    Bubstr gave you some pretty good advice.
    Ed
    " Let all things that hath breath, praise the Lord. Praise Ye the Lord" Psalms 150 vs 6.

  4. #4
    Senior Member DYNO OPERATOR
    Join Date
    Feb 2006
    Location
    VA Hospital, Dallas, Tx (214 302 1924) cell-972-464-7400
    Posts
    540
    " Let all things that hath breath, praise the Lord. Praise Ye the Lord" Psalms 150 vs 6.

  5. #5
    I generally stay out of threads like this, since so much depends on the goals of the owner. Yes, you're probably going to end up with wheel rates between 200 and 250 pounds per inch, but I would strongly recommend investing in a book like "Race Car Vehicle Dynamics" and following the design procedure described in Chapter 16. You'll have the satisfaction of knowing you've based your decision on a proven procedure and that the results will fall within a range corresponding to a particular class of racecar.


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