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Hot Rods RPU rear spring rate?

Discussion in 'The Hokey Ass Message Board' started by Bikelessbill, Feb 14, 2019.

  1. Bikelessbill
    Joined: Oct 1, 2013
    Posts: 111

    Bikelessbill
    Member

    Wondering what coil over spring rate you guys with RPU's are running? I'm at the point of having a roller except for rear shock springs. Got a set at swap meet but had 200# rate on springs. Definitely too stiff. '29 rpu,327/350,GM 10 bolt rear. Thanks,Bill
     
    chryslerfan55 likes this.
  2. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,154

    fiftyv8
    Member
    from CO & WA

    I have no basis for suggesting 180lbs, but that is my guess.
    However I did find some stuff posted possibly on here ages ago that seemed to help make sense of it all and gives you a lead into what might help you calculate a number.

    I hope it helps...


    Coilover spring rate.

    To calculate the proper coil spring rate for the rear of your car, here is the information that you will need.

    weight on the rear tires
    weight of rear axle and tires
    distance the instant center (rear axle pivot point) is from the axle centerline
    distance the coil spring mount is from the axle centerline
    # of inches of usable shock travel

    example:
    weight of car at the rear tires 1277 lbs,
    weight of rear axle and tires 300lbs.
    the instant center measured 46.7" in front of axle centerline
    the coil spring measured 7" behind the axle centerline
    the shocks have 3" of useable travel

    take the weight on the rear tires 1277lbs. and subtract the weight of the rear axle and tires 300lbs.
    1277
    -300
    977 lbs.
    divide 977 by the number of springs (2)
    977 / 2 = 488.5 lbs.
    divide 488.5 by number of inches of useable travel (3) plus 1"
    488.5 / 4 = 122 lbs
    multiply 122 lbs by leverage ratio
    to calculate the leverage ratio you need to divide instant center distance 46.7. by total length of the instant center distance plus coil spring distance 46,7 plus 7
    46.7 / (46.7 + 7) = 0.87
    122 lbs / 0.87 = 106 lb. springs
    106 lb. spring ratio

    *this formula works for cars with the coil springs mounted vertical 90 degrees and behind the rear axle, if the coils are on an angle there are more calculations that need to be made to figure the proper spring rate*
     
    Last edited: Feb 14, 2019
    Chappy444 likes this.
  3. gnichols
    Joined: Mar 6, 2008
    Posts: 10,888

    gnichols
    Member
    from Tampa, FL

    Cool stuff. Safe to assume it works for coil overs on the front? What about leafs? Gary
     
  4. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,154

    fiftyv8
    Member
    from CO & WA

    What about leaf springs, the question was for coil overs...
    Not sure what your question is about leaf springs.
     

  5. gnichols
    Joined: Mar 6, 2008
    Posts: 10,888

    gnichols
    Member
    from Tampa, FL

    Understand. Just wondering if anyone can help. Virtually nothing is available in rodding tech about how to get the correct spring rates, on any type of spring. Heck, you can't even look at a catalog with leaf springs in it and read what the rates are. Gary
     
    HemiDeuce and fiftyv8 like this.
  6. oldolds
    Joined: Oct 18, 2010
    Posts: 3,083

    oldolds
    Member

    There is really no correct spring rate for a hot rod. Some guys want their car to ride like a Cadillac. Some think it should handle like a Ferrari. Some are happy with it riding like a 20 ton dump truck.
    Every one of those require different springs. Then you have all the different suspension designs we use and that changes things as well.
    If you are running coil overs shocks or coil springs you can order something close and change them out as needed because they have easily known spring rates. Leaf springs are a bit different because of the width, thickness and length of the each spring. I am sure somewhere there are engineering specs for those also. But a lot of variables to enter into the equations every application.
     
  7. Bikelessbill
    Joined: Oct 1, 2013
    Posts: 111

    Bikelessbill
    Member

    Thanks. Lots of good information here. Was just curious about what other RPU owners were using. Will try the spring rate formula and see what l come up with.
     
  8. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,154

    fiftyv8
    Member
    from CO & WA

    Just remember if it is too hard, then just tilt the angle of the Coilovers in at the top and the ride will soften slightly.
     
  9. gnichols
    Joined: Mar 6, 2008
    Posts: 10,888

    gnichols
    Member
    from Tampa, FL

    What? Of course there is. But if you just use any old spring that fits and "live with it", you deserve the ride you get. And if you have to buy 10 sets of coils to get the right rate, or the right length, then you deserve then the dent in your wallet for being such a dumb ass. Try to get the right one in the first place. Same goes for shocks, suspension travel, etc.. If you have 4 inches of suspension travel and buy shocks that only have 3 inches of travel then you deserve to keep replacing all the shocks you ruin. If something seems wrong, it probably is. Gary
     
  10. mgtstumpy
    Joined: Jul 20, 2006
    Posts: 8,475

    mgtstumpy
    Member

    [​IMG]
    [​IMG] AFCOIL.gif

    A friend uses 160lb in his 'A' PRU and is very happy with ride. SBC, auto.
     
  11. 250 lb springs on my RPU. The ride is not Cadillac, but it's not Peterbilt either.

    [​IMG]
     
    mgtstumpy likes this.

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