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Leaf spring design - jack squat?


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  • Leaf spring design - jack squat?

    I've been running the coilover setup on the front for a little over a year now. I ended up getting a (for the sake of conversation) 4" forward stretch with the Wagoneer 44 up front.
    My plan now is to set up the rear SOA with down-turned eyes to fight the tendency to bend right behind the front eye. If you can find a set of RE SOA springs, I'm looking to build a flatter version of that. With the eye turned down and a flat spring, axle torque will have more of a pulling effect rearward along the length of the spring than a sharp upward push - causing the spring to bend because the front eye bolt has no give. The other reason for the down-turned eye is to keep the Jeep low. I'm at an equivalent 3" lift with 35's and sit level next to a YJ with 4" lift and 33's.
    With my current SUA setup and even length springs (each leaf extends an equal distance from the center pin) I get a considerable squat when I hit the gas and the nose gets a bit light.
    I'm thinking about stacking the springs like Dodge did with pro-stock springs and have the leaves cut for a shorter spacing to the front and a bit longer spacing to the rear so I can have a bit of jacking effect when accelerating (or climbing) but maintain a good ride.
    As for the build, I'm working on getting some pictures. A huge thank you to everyone that helped me learn all the ins and outs of such a job. Your patience is appreciated and the end result is better than I had expected.
    God forgives, rocks don't
    -sons of thunder

  • #2
    Click image for larger version

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    See how the eyes are reversed? I'm looking to make a similar spring, but flat so it won't provide any more lift and should give a better ride.
    God forgives, rocks don't
    -sons of thunder


    • #3
      Hey 6spd, long time. The flatter spring should definitely ride better. As far as figuring out the squat/anti-squat characteristics that is more complicated. I have never really paid much attention to it with leaf springs. I am sure the length of the spring, mounting height of the eyes, shackle length and angle all come into play. I wonder if the link calculators would work if you made a model with a single link (e.i., the spring). One problem is the front and back of the spring should counter act each other somewhat plus the spring flex which would make it act differently from a solid link.

      What is your shackle angle at now. I did notice that when I had major shackle angle on my front SOA (about 53 degrees) i had major nose dive when braking and a lot of lift when accelerating. I changed it to 40 degrees and those behaviors were greatly reduced.


      • #4
        Are you running any kind of traction bar, bam bar, or similar on the rear? With flat leaf springs in a SOA setup, you will likely get a lot of spring wrap, at least I did. Depending on how that is arranged it can effect anti squat too.


        • #5
          Hi Jonah, good to see you're still here! I plan to put a YJ second spring on top of the stack and clamped down tight in front and with a bit of slack in the back. It provides a downward preload to fight axle wrap. With the rear of the pack a bit loose it will allow easy articulation and should be a smoother ride.
          I'll add a second main spring in the second position, then a regular stack under that. What I'm toying with is stepping the leaves a bit longer in the front and having them shorter in the back. This will allow the stack to act more like a control arm in the front but the rear of the pack will have a lower rate to give a good ride. I'm actually thinking I'd want a bit more anti squat than typical because my front end gets a bit light under low speed acceleration. I'm only running 4.10's but with the 6 speed 4.46 first gear it acts more like 4.88's on a 5 speed. I'd rather have the back push a bit on a climb than get light in the nose. - any experience in this area is welcome to chime in -
          The other reason to do this would be to counter the typical load that is placed right behind the front eye where most SOA springs bend. Also, if you think about the upward eye, any spring wrap torque will be concentrated in an upward push about three inches behind the eye. With a downward or inverted eye, any upward torque will immediately translate into a rearward pull and will be braced by the forward eye bolt.
          Another thing I'm considering is that having a flat stack SOA will bring the spring that provides lateral resistance (stability) much closer to the frame COG. This should help side hill stability as well as cornering on-road. But... given the lighter spring rate I'm aiming for as well as the lower spring frequency I may wind up with body roll. I'll have to just try it out.
          I'm looking to improve the overall ride, performance and handling characteristics without increasing my ride height.
          God forgives, rocks don't
          -sons of thunder


          • #6
            Those are some interesting ideas, I am curious to see how it works.

            For reference: I run YJ springs in SOA. I use an extra main leaf with the eyes cut off for my second leaf then the rest of the pack under that as normal. I run a ladder bar on the rear with a johnny joint/shackle set up (similar to the ruff-stuff version). Before I added the ladder I would get major spring wrap, once to the point of snapping the yoke on the pinion. The ride is great with the soft, flat springs. Compared to arched springs they are very plush. I haven't had any trouble with the nose being light on climbs and it is pretty stable on sidehills.

            I don't think the low eye on the RE springs will really change the way the torque is transferred to the chassis or the way the spring will wrap but it is an easy way to keep the ride height low and lower roll center to combat body roll, so that is good. I think this because the relationship between the axle and spring is the same, you are only changing the chassis height in relation to the spring. So the way the torque of the axle is transferred to the spring is the same. Whether the spring is fixed above or below won't matter. The spring is a lever, as you approach the end of a lever it becomes effectively stiffer. Think of a traditional bow (archery), the end of the bow arm is much smaller than near the handle yet when the bow string pulls the arm it does not bend more at the tip even with less material there. In theory if you pulled at the very end it could be infinitely small as it becomes infinitely stiff. With the leaf spring the same thing is happening, where the spring meet the eye is the end of the lever, so not much is happening there. My guess is whether it is pushing or pulling on the mounting bolt will matter very little. All that said, I haven't heard of leaf springs bending or breaking from wrap. I have broken/bent springs near the fixed eye from the axle pushing back too far on the front, often happens when the axle droops and the shackle inverts. But I have never had that problem on the rear. But like I said I still think the inverted eyes are a good idea for ride height, much easier than trying to french spring mounts into the frame.

            Stiffening the front half of the spring should increase anti-squat and help limit axle wrap. I have heard of clamping the leaves tight to help stiffen it up too, but I still think a ladder bar is the way to go. It should be easier to get anti-squat and spring wrap is controlled all while running a soft spring pack for a good ride. And of course a link suspension offers the most control, but who really wants that much control anyway...


            • #7
              Poking around a bit this morning came across this:


              Kind of interesting. Same theory you are trying, stiff front half - softer back half taken even further.


              • #8
                Finally found it
                So, combine this idea with a flipped eye mainspring like in the picture.
                Since I'm looking to stretch the rear by about 1 1/2" I'm debating on the need to make the front of the spring too stiff. The rear stretch should help fight the nose climbing.
                To move the axle back I'll have to move the gas tank back a bit. I'm working (slowly) on a new rear bumper made from 2x3x1/4" wall tube. It's going to be welded in place of the original crossmember. With a bit of trimming and folding of the tub tab seams behind the tank and relocating the skidplate rear mounts to under the new bumper, I should net about 3" of clearance. I'll also have to make a new front crossmember for the skidplate.
                Compared to the stock mainspring, the spring I am using flattens out 2" longer than stock. With a centered pin, it logically moves the pin 1" farther away (rearward) from the frame mount. That's the first inch. I 'll use a 1/2" offset on the perches for the extra 1/2". Another thing I'm doing for clearance is to use a wagoneer front 44 differential cover. They are made with a flat face that is 1/2" closer to the axle centerline than a standard round cover.
                God forgives, rocks don't
                -sons of thunder


                • #9
                  I should clarify the pulling statement. On a flat spring with the eyes up, any torque applied from the axle housing twisting will put a lot of upward force on the main spring a few inches back from the frame eye. With enough torque applied, main spring frame eye will rotate forward and allow the main spring to bend into a ^ shape at this point. This can also pull the shackle forward and allow the spring to effectively be pulled into the fold that is occurring in the aforementioned stress point (during spring wrap). Allow for shackle movement forward and the axle forcing it's way forward, the eye can rotate to the point that the main spring bends until it hits the frame or breaks.
                  Here's my theory: When the same upward force is applied under axle torque, the main spring frame eye will rotate rearward. Eye rotation will be limited because the upward force effectively translates into a pull on the main spring frame eye bolt along the top of the main spring. The shackle end of the spring will still act normally.
                  I originally thought to do a reversed eye to keep the height low. It seems that this should be an added benefit.
                  Last edited by 6spdYJ; 10-03-17, 06:59 PM.
                  God forgives, rocks don't
                  -sons of thunder


                  • #10
                    Click image for larger version

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                    What this is, is a mainspring from a Rancho 2 1/2" lift that's been cold arched in reverse to just short of the arch of a stock YJ mainspring. Since the center of the eye is 1 1/2" from the back of the spring,
                    by flipping the eye it lowers the ride height by 3". A stock YJ spring measures about 42" eye to eye. The Rancho spring flattens out to 44" so I gain 1" of wheelbase at static height. I added a diamond pointed YJ main leaf in the second position with about 1" of clearance from the eye on each end so it won't bind under flex. The diamond point profile I cut is 1/2" in from either side and 2" down the length of the spring.
                    A YJ second leaf is on top of the whole pack to effectively preload the pack to fight spring wrap.
                    The bottom leaf is from the same Rancho pack. While it it intended to act as a heavy overload to support the longer leaves in negative arch, my intent in using it is also to act as a bit of a slapper bar under acceleration at static height.Click image for larger version

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                    This is one of the clamps I made. It's pretty much a copy of some aftermarket clamps that Rancho makes.
                    I used some scrap 12ga sheet metal. The top piece is 2"x4" bent 90* at about 3/4"
                    from the short ends, so it ends up about 2 1/2" wide across the top and covers 2"
                    along the top of the spring pack. There is a slot cut in the center of each corner of the top piece at the side of the spring pack for the strap to slip through. The straps are 12" long and 1" wide with a divot punched in the dead-center. The divot is centered in one of the spring tip pad retaining holes to keep the clamp from slipping and or walking.
                    On the springs they are wrapped around at the bottom, those springs originally had clamps and pads - so it has two holes on each end. I used a Rancho pad in the original clamp hole and moved the clamp out to the stock YJ pad hole. This kept my effective stepping of the springs about 4". Click image for larger version

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                    Here you can see the spacers between each leaf that prevent excessive friction.Click image for larger version

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                    I tried to focus strength in the stack right below the point about 3" behind the eye.Click image for larger version

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                    If you look close, you can see the Rancho pad behind the clamp. Since the contact at the ends is at the pad, by moving the clamp out and the pad in, it effectively shortened the spring and helps increase the rate a bit.

                    My overall goal is to get a better ride while maintaining the same ride height. My focus is more on reducing the spring frequency than the rate. I'm trying to fight the inherent spring wrap of SOA.

                    I'd like some opinions here... What I originally set out to do is get the 1" stretch of the longer mainspring. Since I can flip my perches, I can mount them with the center pin 1/2" offset forward or backward. If I set them centered, I gain 1" of wheebase. If I flip them I can gain 1" from the spring and 1/2" from the perches. That would put my wheelbase at 99". Or I can mount the perches to move the axle 1/2" forward. This will net me a shorter 98" wheelbase, but I see it having a few advantages: Since the driveshaft I have was made with the same length of mainspring I will be using - only arched instead of flat - the driveshaft length will not need to be changed. It will also pull the axle forward, effectively shortening and stiffening the locating end of the springs while adding 1/2" to the shackle end and would likely provide the best ride. So, opinions please...
                    Last edited by 6spdYJ; 10-21-17, 08:44 PM.
                    God forgives, rocks don't
                    -sons of thunder