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  1. #1
    Join Date
    Nov 2004
    Location
    Portsmouth, Hampshire
    Posts
    468

    shock spring rates

    I am finding the 450lbs shock springs at the front of my SRV8 are a little weak for my rather heavy big block but not sure how much to go up.
    It currently sits just about ok with a lot of preload and the damping on maximum. I am about 300KG weight at each wheel measured at IVA.
    I was thinking of going to 500lbs but would appreciate peoples experience in how big an increment to step up. I dont want it rock hard and crashing down the road and it certainly isnt a track car with this engine in it!
    Also I measured my springs and they are 10" long. I can easily find standard 10.5" long 500lbs and 550lbs but wondering in 0.5" really matters and I have to source 10" which are bound to cost more as specials?

    As always that for the help everyone ! Hoping for a summer of no lock down and a finished car !
    Paul

    SRV8 - 429 Ford Big Block, 4 speed toploader

  2. #2
    Join Date
    May 2016
    Location
    Cheshire
    Age
    52
    Posts
    422
    It's been a while since I did the calculations for my spring change but 10.5" 500lb springs on a 10" shock means 250lbs of preload, so keep that in mind.
    Kittihawk: Pilgrim Sumo MK3 built '93/'94 with 2.1L Stage 2 Pinto and Ford Type 9 5-Speed Gearbox

  3. #3
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    986
    I have a heavy engine (FE) and my front springs are 250 lbs/in. My manual listed them as 14" uncompressed, but I see by my notes, they were actually 12" long. The car seems firm enough, but then I have anti-sway bars fitted too.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  4. #4
    Join Date
    Nov 2004
    Location
    Portsmouth, Hampshire
    Posts
    468
    Wow 250lbs sounds way different to mine. Im pretty sure the standard SRV8 springs were something like 350lbs on the front with a small block and 225lbs on the rear which are doubled up, so 550lbs on the rear. The front is a pretty common set up using the XJ Jag arms.
    My shocks are 13" eye to eye and the springs are 10" so again your sound incredibly different.
    What kit do you have ?
    Paul

    SRV8 - 429 Ford Big Block, 4 speed toploader

  5. #5
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    986
    Well, I'm a little far away, so my kit isn't common to the UK. It's an ERA from New Britain, Ct.

    Since my car probably weighs close to yours (2500 lbs) and holds a big block, I thought I ought to mention the spring rates as they seem to be going in the opposite direction compared to what you would like to see.

    Mine uses Spax front shocks, 17.5" eye-to-eye uncompressed, 12.1" compressed.

    I also use 4 rear shocks (also Spax) with an 8" uncompressed spring of 350 lbs/in. Eye-to-eye is 13.3" extended and 10.1" compressed.

    I see in the manual that for a small block, the front springs can be the regular 250 lbs/in or 200 lbs/in. The rear springs can be 250 lbs/in as well; it states that's for mild street use.


    EDIT: It just occurred to me that the spring rate is also dependant on where the spring is located between the a-arm pivot and the center of the tire. I use Jag rear suspension, but the front a-arms are an ERA design. So maybe my spring rates (particularly the front) really don't mean all that much compared to your car.
    Last edited by Eggbert; 12-01-21 at 09:55 PM.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  6. #6
    Join Date
    Aug 2004
    Location
    Christchurch, NZ
    Age
    60
    Posts
    2,509
    You can't really compare springs across different makes, it depends upon the geometry but how much it varies, I don't know.
    Cheers

    Myles D-W

  7. #7
    Join Date
    Sep 2009
    Location
    Leeds
    Posts
    3,012
    Hi Paul,

    If i remember correctly the early SR has the wrong spring sizes on them - hence why most SR seem to ride high - one of the only things i don't like about my SR is ride height.

    I think Clarkson mentioned something about spring sizes in an old post - worth a search to find out.

    rgds
    Gary
    Conny - A Carnival Red Metallic (paint code Ref: Jag CGG/1811) Southern Roadcraft V8 ( SRV8 ) with 383ci stroker chevy.

  8. #8
    Join Date
    May 2016
    Location
    Hockley ,Essex
    Age
    58
    Posts
    442
    Quote Originally Posted by nealgs View Post
    Hi Paul,

    If i remember correctly the early SR has the wrong spring sizes on them - hence why most SR seem to ride high - one of the only things i don't like about my SR is ride height.

    I think Clarkson mentioned something about spring sizes in an old post - worth a search to find out.

    rgds
    Gary
    Your right about the SR's, they do ride a little high at the rear.
    Other than that I can't complain about how well it handles, that is with the
    light RV 8 upfront and early XJ rear end .
    I will check out the spring lengths that I have as I can't remember and need to
    do a physical check.
    What I do remember is that the shocks are spax and I know there are some out there that
    are not that keen on these.
    All of this was checked and set up by Chang Lim at Thunder Road cars at the old gun powder
    site a few years ago now.
    Chris.

  9. #9
    Join Date
    Nov 2004
    Location
    Portsmouth, Hampshire
    Posts
    468
    Yes you are correct about the rear end on the SR and yes Clarkson put shorter rear shocks on the bring it down. My focus is on the front currently as it is clearly under sprung. They are the correct Spax units which I bought from Southern Roadcraft many years ago, they use the same shock units front and rear but different spring rates of course.
    I guess My Questions really are :
    - Do people think a 50lbs step up is sensible for a car which does drive ok but is on absolute max preload? (about 10% more spring rate seems sensible but should I go 20% more or will I have a steam roller?). It should also be noted that I am not currently running the anti-roll bar as I have a challenge mounting it with my engine.

    - What do people think about fitting a 0.5" longer spring as I can source those far cheaper at 500lbs than the correct 10" ? Someone advised me you shouldnt preload a spring more than around 1"

    Thanks all
    Paul

    SRV8 - 429 Ford Big Block, 4 speed toploader

  10. #10
    Join Date
    Oct 2006
    Location
    Toton, Nottinghamshire
    Age
    48
    Posts
    900
    I went through similar quandaries a few years ago when changing my rear shocks which were too long (by 2") and too soft. I am no expert but my thinking went along the following lines:
    - As far as I was concerned the springs carried the weight of each corner of the car.
    - The hardness of the ride is more down the the shocks rather than the springs. (my simplistic view)
    - The spring rate, pounds per inch, is just that. The spring compresses 1" when loaded up with the rated pounds in weight.
    - As the shocks/springs are not vertical at normal ride height then they will need to rated higher than your corner weight due to the angle of "thrust". (I got a bit mathematical at this point messing around with Cosines etc).

    Several pages of diagrams and scribbles later it dawned on me that I could measure the current spring length when the Cobra was on its wheels, therefore how much the corner weight of the Cobra had compressed the current spring from its open length. So in your case, knowing that your 450lbs spring compresses X centimetres. It should be possible to calculate a 500lb (or 550lbs) spring should compress Y centimetres for the same corner weight. You will also need to measure how much thread under the bottom lockring that you have available on the body of the Spax shock. The higher rated spring won't compress as much as your current 450lb spring so the lockring (bottom of the spring) will be lower. So you know how much adjustment you have to play with.

    I don't think an extra half inch longer spring will make that much difference but you can factor this into your calculations, ensuring you have some extra threads left on the body of your shocks beneath where you expect the lockring to sit.

    The shocks I bought were about £20 each so I didn't really consider it would break the bank if I couldn't attain the ride I wanted. In the end it pretty much worked out as expected.

    Russ.
    Joint Area Rep - East Midlands Region
    1984 DAX Cobra
    Rover 3.5 V8 with Holley carburetor.

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