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Hot Rods Check my math, please?

Discussion in 'The Hokey Ass Message Board' started by GTS225, Aug 7, 2015.

  1. GTS225
    Joined: Jul 2, 2006
    Posts: 1,200

    GTS225
    Member

    Trying to figure out my spring rate for a pair of coil-overs on the back of my T. Being an "eyeball engineer", I'm not familiar with the methods to get it right.
    I do know this; Rear body/frame weight: 420
    fuel weight, (approx.): 75
    human weight, (200X2): 400 (20" forward of shock mounting point)

    I estimated human weight at 300, due to it being forward of the mount point. That gives me 795, divided by two, for about a 400 pound spring rate? I gotta be missing something big here, as Speedway has their T coilovers rated at 175-200.

    Any info I'm missing that's needed to correctly cipher what I actually need for coilovers?

    Thanks.....Roger
     
  2. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,118

    gimpyshotrods
    ALLIANCE MEMBER

    Last edited: Aug 7, 2015
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  3. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,283

    Kerrynzl
    Member

    Assuming the weight of the "rear body" is not including the rear end or wheels [otherwise this needs to be deducted to get sprung weight ]
    I would start at 90lb/in springs if the springs are mounted vertical. You are better off with softer springs that require a spring compressor to install them.[softer springs are usually taller]

    Laying them over changes the motion ratio.

    Don't over spring it.
    For comparison: Stock 4 leaf Eaton springs for a 67 Camaro are only 126lbs/in
     
  4. GTS225
    Joined: Jul 2, 2006
    Posts: 1,200

    GTS225
    Member

    They are mounted vertical, and the weight given is just the rear body/frame weight. Third member was sitting on the floor, attached only by the haipins and drive shaft.

    And you guys got me to thinking along a differetn tangent. If I use a 100# spring, then it's 100# to compress it one inch, it's another 100# to compress it the second inch, etc., etc. So mysuspension travel limits play into it, too.

    Thanks, guys....Two (or more) brains are better than one.

    Roger
     

  5. wsdad
    Joined: Dec 31, 2005
    Posts: 1,258

    wsdad
    Member

    Can you put 400 pounds of water in the seat and get an actual measurement? No math or guessing involved.

    Is the engine and transmission installed? Some of that weight will rest on the back springs if it sits behind the front axle.
     
  6. JOECOOL
    Joined: Jan 13, 2004
    Posts: 2,760

    JOECOOL
    Member

    I went thru this a month ago, I was trying to compare leaf and coil spring rates. There seems to not be a definate answer. I did use 110 lb coil overs on a T bucket once and it rode great.
     
  7. GTS225
    Joined: Jul 2, 2006
    Posts: 1,200

    GTS225
    Member

    The engine and trans are installed. The bucket is titled, and I've been driving it this season, but the rear ride is damn harsh. The "kit" components that we got have the rear suspension set up with coils behind the rear axle, with a shock mounted up through the center. I have no info regarding the coils that came with it.
    I'm thinking that if I get in the ballpark, so to speak, and get a pair of reasonably adjustable coilovers, then I should be able to tune it to a livable ride.

    Roger
     
  8. Don's Hot Rods
    Joined: Oct 7, 2005
    Posts: 8,319

    Don's Hot Rods
    Member
    from florida

    Anywhere between 110 and 140 will get you a decent handling bucket. I just always buy 140's and call it good.

    Don
     
  9. Mr48chev
    Joined: Dec 28, 2007
    Posts: 30,185

    Mr48chev
    Member

    Coil over springs with the coil attached to the spring as everyone who replied assumes? Or a separate coil spring in spring buckets with a regular shock running up though the middle of it? That would be very much like the Corvair front coils that I ran on the rear of my T bucket in the early 70's but that car rode pretty good. That would be similar to the Bobby Fields built chassis in a photo I snagged off the net.
    [​IMG]
     
  10. pitman
    Joined: May 14, 2006
    Posts: 5,151

    pitman

    48Chev; Used the same setup and it rode great!
     
  11. Gotgas
    Joined: Jul 22, 2004
    Posts: 6,998

    Gotgas
    Member
    from DFW USA

    Hi Roger,

    As for your method of measurement, I don't think you're too far off. Just remember that the spring is already compressed some distance at static ride height. So if it's a 140lb spring, and there is 400lbs of curb weight on it, it has already compressed around 3" at rest. At that point, it takes another 140lbs of force to compress another inch, and that can come from passengers, or luggage, or fuel, or forces transferred to the spring while driving (what you are saying is 'harsh').

    So what you'll really need to do is choose a spring rate (for comfort), and then figure out how tall the spring needs to be to give the correct final ride height. Or you could call up your kit manufacturer (or someone that knows the specifications) and ask about the rate of your current springs and then select a spring with a lower rate. Good luck!
     
  12. Andy
    Joined: Nov 17, 2002
    Posts: 4,803

    Andy
    Member

    A spring rate is for one spring. If 100# rate times 2 springs= 200# to move rear 1 inch. 795# or say 800# divided by 200 would mean the car would compress the springs 4". This is a good number.
     
  13. oj
    Joined: Jul 27, 2008
    Posts: 6,281

    oj
    Member

    The piece of the puzzle you are missing is the inches the spring compresses onto the shock. Using your example of 795lbs/2 = 400lbs total, if your shocks compress 3" (total stroke is 6") then 400lbs/3" = 133lbs per inch, that is the rate. It'll collapse the spring 3" to support 400lbs with the shock in the middle of its' travel.
     
  14. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,283

    Kerrynzl
    Member

    With all springs [I'll use linear rated as an example] they have a point of load equilibrium.
    A 90lb/in spring with 450 pounds on it will equalise at 5" of load height.
    5" is actually a good load height for a comfortable [but firm] ride.

    If you use too stiff springs the frequency of the suspension [cycles /sec ] would be like a tuning fork. This harshness will disappear at high speeds because the car hits the bumps at higher cycles/sec

    Another downside to have to stiff springs in the rear is the weight transfer across the rear during bodyroll will cause oversteer [ or the rear is loose]

    You are better off with a car that understeers slightly [pushes] and the driver can throttle oversteer [more fun to drive]

    The 90lb /in I suggested earlier is on the high side of stiffness recommended . I myself would go even lower 70-75lb/in [but that would be too controversial here]

    A 66 mustang 6cyl has 85lb/in rear leaf springs with a max load rating of 750lbs ,so they have 8.8 inches of arch before they flatten out [approx. 6inches of that load is the weight of the vehicle]

    Go soft , you won't regret it.
     
  15. GTS225
    Joined: Jul 2, 2006
    Posts: 1,200

    GTS225
    Member

    Mr48Chev posted a pic of the exact setup I have. Apparently, someone was attempting to copy that, but failed with the coil choice.

    OJ came up with the final piece. I was thinking about it today, and realized that I would probably have to get something a little taller than what I already have, but I feel our "bench building session" is getting me headed in the right direction now.
    So, it looks like I need to get a pair of coilovers with "soft" spring rates, about 3" taller than the coil/shock setup that 's currently under it.

    If this thread goes no further, I'll throw out a hearty "Thank You!" to all those that replied.

    Roger
     
  16. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,280

    Ned Ludd
    Member

    At the risk of muddying the waters after all the sound practical advice you got, one important consideration in choosing spring rate is the ride quality expressed as a natural frequency in Hz. The softer a spring, the lower the natural frequency of the system it's installed in. But also: the greater the load on a spring, the lower the natural frequency of the system it's installed in. That is, you can make a spring feel softer by putting a heavier load on it.

    Conventional wisdom is that the aim is to get as close as possible to 1Hz while avoiding the possibility of dipping into the "seasickness zone" around 0.2-0.8Hz. The necessity of making provision for variations in payload means that most luxury cars have a rear wheel-rate corresponding to a natural frequency around 1.3-1.4Hz in normal everyday use, so that it might go down to 1Hz or so when heavily loaded. An unloaded truck or a sports car will have a higher natural frequency and a correspondingly harsher ride. I'd guess that my Eibach-sprung VW Golf Mk1 daily driver is around 2.5-3Hz: not quite bone-jarring, but not the ideal tool for transporting delicate antiques over potholes, either (ask my wife.)

    As you'd expect, there is a formula for this:
    F = 3.133√(R/M)
    where:
    F = natural frequency of the suspension system
    R = wheel rate (= spring rate for live axles in bump mode)
    M = unsprung mass

    Taking your weights gives 210 + 37.5 + 160 = 407.5lbs per side (I've distributed 80% of the passenger weight to the rear, and assumed that the 420lbs is unsprung mass.) Kerry's suggestion of a 90lb/in (vertical) spring would then give you a natural frequency of 1.47Hz, which is right in the "comfortable" ballpark.
     
    Last edited: Aug 9, 2015
  17. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,118

    gimpyshotrods
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    Sure, but what's the equivalent of 90lb-in. vertical, at a 60° tilt?
     
  18. pitman
    Joined: May 14, 2006
    Posts: 5,151

    pitman

    Lynn, Lynn, city of Sin(e)!
    .866 x Vert value
     
  19. Andy
    Joined: Nov 17, 2002
    Posts: 4,803

    Andy
    Member

    .866 x vert value for relative strength of spring. The load on spring would be the inverse so load/.886. The relative rate would be rate x.866x.866
     
    Last edited: Aug 9, 2015
  20. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,283

    Kerrynzl
    Member

    Correct ^^^^^^

    Motion ratio squared.
    so it would require a 120lb/in spring
     
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  21. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,118

    gimpyshotrods
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    Bingo, and 60° would give a decent amount of body roll control.
     
  22. Andy
    Joined: Nov 17, 2002
    Posts: 4,803

    Andy
    Member

    Body roll stiffness is dependent on the spring mounting width on the frame. I think most all coilover rear suspensions need rear anti-roll bars to correct understeer.
     
  23. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,280

    Ned Ludd
    Member

    Or, a 90lb/in spring would give a natural frequency of 1.26Hz.
     
  24. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,118

    gimpyshotrods
    ALLIANCE MEMBER

    Not necessarily. There are too many variables in play to make a blanket statement like that.
     
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  25. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,283

    Kerrynzl
    Member

    With the motion ratio squared formula a 120lb/in spring laying over at 60° would have a "Wheel rate" of 90lb/1n.
    It would have exactly the same stiffness [including roll stiffness] as a 90lb/in spring standing vertical.

    You can have all sorts of levers, rockers, bell-cranks and arms controlling suspension, but in the end all the car cares about is "wheel rate"
    Example:
    My old BB Corvette racecar had 650lb front springs and a 0.66 [2/3rds] motion ratio so the wheel rate was 283lb/in
    My brothers Boss Mustang racecar had 1100lb front springs and a 0.5 [1/2] motion ratio so the wheel rate was 275lb/in
    So here is an example of a car with softer springs but having stiffer suspension.
     
    Last edited: Aug 10, 2015

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