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leaf spring shackle angle

Discussion in 'The Hokey Ass Message Board' started by NAES, Mar 20, 2012.

  1. NAES
    Joined: Dec 24, 2008
    Posts: 490


    Hey everyone. I've been busy driving the wheels off the wagon and a big complaint I have is the rear end being really harsh. I went through a series of shocks that I have from oil to gas dampened and while they do make a difference, there is some major harshness to the ride regardless.

    The springs have a 6" dearch along with reversed eyes and 3" lowering blocks. The shackle angle is about 30* towards the front of the car. I realize loweing the car as much as I have plays a role in this as well but from a little googling I have decided that shackle angle might be playing a large part in the harsh ride also. I'm looking more like figure A than B.


    Now before I get all crazy and fab up a new shackle mount, does anybody have some input on the direction I'm considering going?

    Thanks all as usual, NAES
  2. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,325


    Shackle angle will not alter spring stiffness

    The closer the shackle is to 90 Degrees the "better" it is, The closer to the datum line [in either direction ] the more chance it will "Bind"

    On the datum line they will bind.

    Spring stiffness is more to do with the distance between the eyes [ longer = softer ]
    Re-setting springs does not alter the stiffness at all [ just the height or load ]

    I would be looking elsewhere [ bumpstops etc ]
    You have re-set the springs, reverse eyed them, and 3" lowering blocks.

    It could be something simple like the driveshaft now binding in the back of the transmission or the shocks bottoming out.
  3. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,614

    Rusty O'Toole

    If you mean 30 degrees 'A' when the weight is on the ground it sounds about right.

    Station wagons are made to carry a heavy load and came with stiff springs. Does it soften up if you put a couple of Hemi blocks in the back ? LOL.

    You may want to remove some of the leaves to soften the springs then add air bags in case you want to haul something. The short leaves stiffen the spring, the long ones carry the weight. Remove every second leaf starting from the small ones and see how you like it.

    Will also say, Chryslers rode a little hard back then especially in the back seat.

    2 more thoughts. One, old springs stiffen up with rust and worn out sliders, if yours have been recently rebuilt they should be OK .

    Second, you have lowered the hell out of it, it is probably riding on the bump stops in other words NO spring action at all, if you have not C'd the frame. Take out the lowering blocks and see if you get your ride back.
  4. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 28,805

    Jalopy Joker

    might be that you took most of "the spring" out of the spring. measure body travel with shocks not attached? full up/down. shock angle correct? sounds terrible here but, I had
    a very lowered stock frontend on a '47 Ford and found that Tokico shocks were the only ones that did any good. ended up using ones that were meant for use on the rear of an early Vette.

  5. jcmarz
    Joined: Jan 10, 2010
    Posts: 4,633

    from Chino, Ca

    How is the travel between the axle and the frame? Is the frame notched? Also, are you using standard size shocks or shorter shocks, which should be used when the car is lowered.
  6. Nope you are wrong there, Shackle angle most definitely changes effective spring rate.

    Correct, which kind of contradicts your first statement

    Actually spring stiffness has more to do with the thickness, quantity, and length of each leaf more than anything else. But all things being equal, a longer spring will be softer than a shorter one.

    It can if done improperly as you can lose the tension or stiffness in the spring

    All could be true as well, but I think given what he said and the fact that he copied that chart which is well known and well respected in the chassis business, especially for dirt cars, I think he very well may have an issue with shackle angle. Shackle length will also cause a change in effective spring rate as well, not to mention altering roll center and creating a jacking effect.
  7. 6-71
    Joined: Sep 15, 2005
    Posts: 542


    If you de-arched the spring 6",the spring will definately be longer,and change your shackle angle.You say you have a 6" de-arch and a 3" lowering block,thats a pretty drastic slam.I doubt that you have any suspension travel at long as its not hitting the frame that shackle angle should be OK. do you have a C-notch in the rear?
  8. killa fab
    Joined: Apr 21, 2011
    Posts: 101

    killa fab

    As hotroddon said above, but your diagram of shackle angle and softness/stiffness is backwards. The forward or rearward angle of the shackle effects how much up travel vs droop. Shackle angle like this- \ more droop less up travel, shackle angle like - / more up less droop. Or thats how i learned in the world of rockcrawling/off road trucks.
  9. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,325


    You are wrong, Do not confuse spring stiffness with spring "binding."

    If the spring was on a "slipper" the rate doesn't change so how do you figure out that the shackle changes spring stiffness

    A monoleaf [ for a simple example ] has the same stiffness thoughout the range of suspension movement even though the shackle angle is changing constantly.
    Multileafs are more progressive as they are loaded

    Because he has de-arched the springs, even thought it may appear longer [ because it is flatter ] the distance between the center bolt and the 2 eyes should be constant.

    The spring rate should still be the same unless the springmaker has done him "a favour" by adding a leaf [ they do this a lot ]
  10. F&J
    Joined: Apr 5, 2007
    Posts: 13,221


    Kerry: Question from my car setup. My front spring was very harsh, and it was kind of too flat. I took advice from my son and friends and added the longest additional leaf I could fit, and now it rides so adding spring :)

    Seems a de-arched "flat" spring is actually a dead spring?
  11. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,325


    It could be one of many things,

    Many springmakers when reversing the eyes and re-set spring height usually only massage the main leaf, so the second leaf could be binding with the first [ at the ends ]
    Also the spring rate could be too soft , so it is bottoming out or binding somewhere.
    Many manufacturers use soft springs that are rated [ lb's / in ] but they also have a spring "Load Rating", which is something the aftermarket doesn't do. [ They raise the "Load Rating" to compensate for low "Spring Rates" ]
    In a leaf spring the load is an indication of the shape of the arch, in a coil it is the static height .

    When you re-set the spring you are altering the "Load Rating" not the "Spring Rate"
    For example: two springs could have the same spring rate but one has a greater arch for a heavier vehicle.

    examples here:[ I'll use rear leaf springs ]

    Ford Mustang Boss 302 rear springs [ # C9ZZ-5560-J ] have a rate of 152 lbs/in and rated at a load of 550 lbs , so it will settle 3.62 inches with 550 lbs load but because in reality it would get closer to 700 lbs upon it ,the ride height would be 4.6 inches lower than static [ Boss Mustangs sit 1 inch lower from the factory ]

    Normal Ford Mustang 6 cyl passenger rear springs [ # C9ZZ-5560-A ] have a rate of 85 lbs/in and rated at a load of 700 lbs [ it has a greater arch ] so it will settle 8.23 inches with 700 lbs load , so because of the greater arch it would sit 1 inch higher than the Boss 302.

    This all makes sense when you jack the car up, you need to jack it quite a bit before you get the wheels off the ground.

    The other thing to consider with Harsh rides is the myth of "Soft Springs and Hard Shocks" [ People always put hard shocks on their vehicle to improve handling ]
    If the shock has too much resistance on "Rebound" for the spring rate [ actually the wheel rate ] the wheels will try to "Rut" or skip over bumps, putting a harsh feeling back through the vehicle.

    Adding a leaf as you did ,would cure that.

    Springs should be chosen for the vehicle usage , but shocks need be matched to the springs
  12. F&J
    Joined: Apr 5, 2007
    Posts: 13,221


    Thanks Kerry, It will take a few reads on all that, but I am really focused on the last third of that, to figure out what was going on with the other end of the car. (rear).

    Interesting info to me. Some things we have always taken as gospel, but don't really study the why's.
  13. terrarodder
    Joined: Sep 9, 2005
    Posts: 1,101

    from EASTERN PA

    I had my main leaf reverser and took 2 leafs out, I was told it wouldn't matter. Anyway doing this my main leaf got longer and the rear shackle is alway back hitting the frame so now I need to get it shorted, the eye rerolled. That would be the same as dearching the spring, right ?
  14. NAES
    Joined: Dec 24, 2008
    Posts: 490


    OK everyone thanks for all the great input. To follow up on some questions,

    There is a mile between the axle and the frame even with around a 9" drop. The car was in the nosebleed seats when I started out. There is no way I am bottoming out on the frame or bumpstops. I did have to tunnel the driveshaft to get it to clear so I'm positive I'm good.

    Shock angle is 45* leaning towards the middle of the car at the top. I'm using KYB whites at the moment and they are giving me the best control of the ones I've tried. They are stock height and I need to jack up the axle to get them to connect as the distance between the mounts is really, really far.

    Shackle angle is approx 30* forward like in diagram "A". There is no clearance issue with the frame or anything like that.

    Terrarodder- Dearching the spring actually takes some of the arc out of the spring itself. If you were to cup your hand on the table and then extend your fingers, you would see that the dearching of your hand actually makes the distance longer as well. You might have gotten a little of that in your eye reverse job.

    Thanks again all, NAES
  15. x2.
  16. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,325


    The spring rating of a leaf spring is factor of a few parameters [ there is maths for this , but we don't need to go there Yet ]
    They are:

    Leaf Width ............ Wider is stiffer
    Leaf Thickness .... Thicker is stiffer
    Number of Leaves....More leafs is stiffer
    Spring Length.........Shorter is stiffer

    With spring length it is the measurement around the spring [ the amount of steel used ] not a straight line between the eyes.

    Taking 2 leafs out has softened the spring rating, so it will now sit flatter [ Taking up it's "Load" is the term ]
    If your vehicle had the same weight loaded on it before and after you reversed it the ride now will be a lot softer [ you be the judge as to the desirability of this ]
    So you car is now a lot lower because of 2 factors : Reverse Eyes and Softer rating [ for the weight ]
    It now appears to be longer now because it is flatter

    Rolling the eyes inward is shortening the spring, so it will raise the stiffness slightly.
    If you car "Falls over" on the corners, or now has a tendency to understeer the outside front wheel entering into corners then the rear is too soft.

    If that is the case, you need to increase the stiffness [ or rear bodyroll stiffness ] then look at setting the new spring to the desired height
    Last edited: Mar 21, 2012
    AHotRod likes this.
  17. 1971BB427
    Joined: Mar 6, 2010
    Posts: 7,378

    from Oregon

    Too much shackle angle (either forward or rearward) will increase spring bind. In essence your 30 degree angle will cause the spring to try and pull on the shackle as the spring compresses, and since it can't "grow" in length it binds. Too much rearward angle causes spring bind from the spring eye trying to compress against the shackle.
    I would get that shackle closer to straight up, (90 degrees to the spring) and I bet you'll find the ride is much smoother.
  18. Re read what I wrote "the EFFECTIVE" spring rate. And that is what is reacted upon by the car, so you have to take into account what the shackle is doing, as the total is what will be felt in the car and will create the ride. You can most definitely change the ride of a car by changing both shackle length and angle. And until the angle gets extreme, it is not so much bind as it is leverage.
  19. swissmike
    Joined: Oct 22, 2003
    Posts: 1,297


    I have to agree!
  20. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,325


    What a load of Bullshit !

    Again I will say "Do not confuse spring stiffness with spring "binding."

    On a leaf spring the load applied at the centerbolt is always perpendicular to the spring eye travel and the distance from the centerbolt to the spring eyes is always constant [ only the angles change so the spring "appears" longer ]

    And the vertical loads on the centerbolt are always met by vertical resistance shared between the spring eyes,So the load on the spring eye at the shackle is still vertical and the motion ratio remains constant.
    This is not like a coil over spring that changes the motion ratio as the spring angle changes

    Any mechanical interference [ by shackles or whatever ] is actually binding of the suspension, it is not altering "the EFFECTIVE" spring rate.

    One thing I like about the Hamb is members are willing to learn [ no matter how old we are ]
    Please don't turn this thread into a face saving exercise by manipulating the interpretation of the English language.

    Well golly Enos! We can alter the "Effective" spring rate on that thar truck by jammin' lumber between the axle and frame!
  21. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,462


    Spring arches,shackle angle.................You might want to have a look at more modern SUV's with rear leaf springs .They can carry weight and ride ok.You'll find a leaf spring with very little arch.The rear shackle will be about 20 degrees over center toward the rear.Now these leaves are wider and fewer in number...but....
    If I understand the OP,his car's rear shackle is facing foward? I assume,maybe wrong,the shackle is on the rear of the rear spring? If all this is true,it's gonna ride like shit because the shackle has to travel over center.
  22. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,325


    Shackle angle does not alter the rate of suspension, but shackle angle can limit the range of movement.

    Manufacturers set the angle as the best compromise to the range of movement!

    In this example [ previously posted ] If the shackle angle was on the datum line the suspension would bind [ both front or rear along the datum line ]


    In position "A" the suspension is closer to binding under full jounce, but in position B the shackle can invert itself under full rebound and sudden jounce [ with outboard spring perches ]
    If the suspension design has inverted shackles like some trailer equalizers do, position "B" is the better because it offers a greater range of travel.

    With springs with a low arch once the spring centre has passed through the datum line the shackle angle will then start moving the opposite direction.
  23. Tell you what, I'll just defer you to this link written be a well respected Race Car Chassis Component manufacturer, and the information that is used by thousands of race car builders around the country, and you can argue the semantics all you want. The OP asked if shackle angle could cause his stiff ride and the simple answer is YES.
    David Pozzi likes this.
  24. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,325


    The company that published that article is a major player in the world of Oval Track Dirt Cars! [ they actually refered to the R/R spring in this article ]. The problem with the article is when it is written with terminology for a "Layman" to understand it can become misleading .This is where there is a misunderstanding [ as in this case ]

    The folowing statement ,and the drawing here actually contradict each other [ going by the text written the angles "A" should soften & "B" should stiffen ]

    The vertical movement of the rear spring eye causes a jacking effect. If the shackle movement forces the rear spring eye downward ,the leaf will deflect and exert an upward force on the chassis that will add stiffness to the rear suspension. Conversely, the shackle will reduce suspension stiffness if it causes the rear spring eye to move upward during suspension travel. [A]

    Please try and understand what I am writing, [ This not me trying to insult your statements or start a shitfight ] I'll try to make this reasonably easy to understand and hopefully not misleading.
    In fact I apologise for my "face saving accusations" I made before, I didn't realize you made them based on information you read elsewhere.

    On a dirt car they "dial-in" a lot of static "wedge" or "weight jack" the car so that the pre-loaded inside rear [ L/R ] transfers weight to the outside rear [ R/R ] to give even weight distribution on both rear wheels during cornering , then rear wheel diameter "stagger" steers around the corner.
    Basically a dirt car has all 4 wheels [ 3 if it lifts the L/F ] trying to corner the car.

    Now the weight of the car is "shared" by all 4 wheels , so you can pre-load the weights diagonally by altering the load heights [ hence the term "wedge" ]
    This can only be done diagonally!
    If you jacked the heights on one side only [ or one end ] it merely raises that side of the vehicle . The weights will basically stay the same on all 4 corners [ it does change very slightly because of CGH ]

    A spring is rated at "weight over distance" or "lbs/in" so altering the corner heights diagonally [ jacking ] preloads more weight in 2 springs but unloads weight of the other 2 springs [ the total cornerweights add up to the same ]

    The linear rate of the spring remains the same regardless of what pre-load it has jacked into it.
    For Example: 200 lb/in spring with 600 lbs load on it will set 3" lower ,add another 400 lbs and it will drop another 2"

    Now back to the subject of shackle angles!
    If you had one leaning on a forward angle and one leaning on rearward angle [ on the same vehicle ] when both the springs compress and lengthen along the datum line the 2 shackles will move into different heights [ one moves back and lowers and one moves back and raises ]
    This only applies when there is 2 different shackle angles on the same axle [ dirt racers use this as a chassis tuning tool , similar to weight jackers ]

    So it only effects the spring height [ or load ] it does not alter the spring rate. The only time that weight jacking alters spring rate is when proggresive rate springs are used [ the more load on the spring the stiffer it gets ]

    On a street car or drag car where the shackle angles are the same on both the L & R , altering the shackle angles will not alter the "Rate"

    Try and think about what is written,[ even my comments ] not everybody is skilled at getting their thoughts into writing
    Last edited: Mar 28, 2012
  25. kerry,good info. since the spring is anchored off the front eye, is the 90 degree line, off the datum actually an arch? or is it still a straight line ? also, and sorry to highjack the thread but, on a mono leaf isn't the spring rate changed because the taper in the spring?
  26. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,325


    90 degrees from the datum line when the suspension is loaded with static weight. The datum line is a theoretical path of travel for the rear spring eye, but in reality the datum actually changes angle because the shackle moves in an arc.

    Note: in the schematic the datum is higher at the rear [ rear spring eye is higher than the front ] , there is a very good reason for this!

    It is to induce "Roll understeer", when a car bodyrolls the outside wheel moves forward on an arc from the front spring eye. This causes the rear end to try and steer inwards on the corner or straighten the front up [ understeer ] [ similar to Honda 4WS Preludes ].
    The purpose of roll understeer is to load up the suspension with a little bit more bodyroll so it stabilizes the rear end.

    If the suspension had Roll "Oversteer" the rearend would become loose and unpredictable on corners, but the real nightmare is in a straightline when bodyroll and roll oversteer cause the rearend to sway back and forth [ the rollsteer and bodyroll work opposite to each other ]

    This is a major design fault on most homebuilt trailers and caravans ,because people think the suspension should arc rearward over bumps

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