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4 link with LEAF Springs

Discussion in 'Center Of Gravity' started by pauly383, Oct 26, 2004.

  1. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    Has this been discussed ? I just read in the new Petersons , about a jeep from Ultimate adventure with a four link with leaf springs with shackles on EACH end of the springs instead of coilovers . They expalined the springs worked to support the truck without axlewrap . Someone give me some info on how this really works /forums/images/graemlins/ears.gif
     
  2. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    No different than any other link suspension really. The links locate the axle, so no wrap or related issues. And the springs with shackles on both ends do just like coils in that they provide support only. If it didn't have shackles at both ends, the leaf would want to influence axle arc and would cause binding. Any (normal) link suspension will work the same. Links locate, springs support. Doesn't matter if the spring is a coil, 1/4 elip, floating leafs (shackle at both ends), air bags, nitro shocks, whatever...
     
  3. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    Thanks Russ /forums/images/graemlins/thumb.gif
     
  4. marv_springer

    marv_springer 1/2 ton status

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    Paul,

    Said Jeep is right across the street from me @ FourWheeler's on Washington. Was over there at lunch./forums/images/graemlins/grin.gif

    Randy Ellis was a pioneer of this type of suspension back in the 90's. Matter of fact, he built the suspension on that Jeep.

    I would consider all 1/4 ellip, coil over, and air shock setups to be more modern than the "double shackle" setup.

    Marv
     
  5. sled_dog

    sled_dog 1 ton status

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    I must say I think 1/4 elip, airshox, or coilovers would be better options than this double shackle setup but it is certainly interesting.
     
  6. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    The only real drawbacks to the double shackle leaf is that it's bulky and in the way, plus it tends to limit your options for improving departure angle just like it does in normal leaf applications. If neither of these is an issue, then leaf springs have some advantages in that they are cheap/common, and they do tend to offer better control on droop and off-camber since, unlike coils and the other, they start to resist further travel and don't continue pushing all the way to the bottom. They are also relatively easy/cheap to tune by swapping in/out leafs (often gathered for free) rather than dealing with replacing coils, stacked coils, retainers/tenders and so forth. Nitro and coil-overs s are ultimately more tuneable, but you need to buy a variety of special hardware and parts to do it.

    Bottom line, just like everything else, it's about trade-offs and what you want/need.
     
  7. Paxx

    Paxx 1/2 ton status

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    So I couldn't run a 4 link setup with just a regular leaf setup because it would bind? You have to run dual shackles? That kinda sucks. /forums/images/graemlins/thinking.gif
     
  8. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    That is correct, but not sure why you say that sucks. If you don't make changes, the stock leaf springs locate the axle. If you have a 4 link also locating the axle, they will bind and fight each other. And you really can't engineer it out since links travel in fixed arcs, and leaf springs cause the axle travel a non-arc path due to arch changing and causing non-linear changes in arc radius around the front eye. It would also be impossible to get things like roll axis at the same angle and height, and so on...
     
  9. ntsqd

    ntsqd 1/2 ton status

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    It is possible to get a double fixed end traction bar to mimic the travel arc of a leaf spring, so it's likely possible to get a 4 link to do the same.
    Why you would want to is beyond me. The geometry of the links would likely be so screwed up as to be pointless. And then you'd still need to figure out how to deal with the differences in pinion angle change.

    Another disadvantage to leafs with linkage is unsprung weight. The sole advantage I can think of is lower damping rates due to the leaf's internal friction.
     
  10. four_by_ken

    four_by_ken 1/2 ton status

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    [ QUOTE ]
    So I couldn't run a 4 link setup with just a regular leaf setup because it would bind? You have to run dual shackles? That kinda sucks. /forums/images/graemlins/thinking.gif

    [/ QUOTE ]

    Why would you want to run a 4 link with normal leaf springs. No reason to. Maybe something to add in axle wrap control, but no need for 4 links.
     
  11. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    [ QUOTE ]
    It is possible to get a double fixed end traction bar to mimic the travel arc of a leaf spring, so it's likely possible to get a 4 link to do the same.

    [/ QUOTE ]
    I'm not so sure you can. Even with just a double fixed traction bar I don't think it's possible because the axle does not move in a fixed arc with leaf springs even on simple level compression/droop, particularly asymmetric leafs. And then there is the axle moving left and right due to roll axis on articulation. And roll steer just to make it interesting. I would say that getting even just a double fixed traction bar to handle arbitrary axle movement (compress/droop level, articulated to various degrees, combinations, etc.) with *no* binding would be next to (or more likely, completely) impossible. Being able to do it with a locating link suspension and trying to get it to have zero bind relative to the springs would be many orders of magnitude more difficult than a single link. Your engineering background is much stronger than mine, so maybe it's theoretically somehow possible with a sufficiently complex link arrangement or something, but I sure don't see how.

    And of course there is not much point since that would be just a glorified anti-wrap device at that point...
     
  12. marv_springer

    marv_springer 1/2 ton status

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    [ QUOTE ]
    Why would you want to run a 4 link with normal leaf springs. No reason to. Maybe something to add in axle wrap control, but no need for 4 links.

    [/ QUOTE ]

    Keep in mind that the suspension on this rig was built 5-7 years ago. 4-links were just beginning to be used and the trend was to use the springs only as springs and relieve them of their axle locating duties w/ the 4 link. Shannon Campbell built several tube frame Jeeps w/ similar setups using leaf/buggy leaf/4-link rears - and even won one of the early TTC's with such a rig.

    So this suspension design is not ideal... just was a part of the evolution of rear suspension design.

    Marv
     
  13. ntsqd

    ntsqd 1/2 ton status

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    [ QUOTE ]
    I'm not so sure you can. Even with just a double fixed traction bar I don't think it's possible because the axle does not move in a fixed arc with leaf springs even on simple level compression/droop, particularly asymmetric leafs.

    [/ QUOTE ]

    What you end up doing is getting the arc of travel of the traction bar to closely, but not exactly, follow the true arc of the housing's travel. The more travel you want from the suspension, the more of a problem this becomes.

    [ QUOTE ]

    And then there is the axle moving left and right due to roll axis on articulation. And roll steer just to make it interesting.

    [/ QUOTE ]

    Each spring is going to travel in it's specific path regardless of what the vehicle is doing around it. In articulation the springs will be distorted laterally to some degree, but that will have a minimal effect on the verticle travel path.
    If you get both bars (assuming more than one traction bar) to mimic the spring's travel arc the only bind likely will be due to the joints on the ends of the bars.

    [ QUOTE ]

    I would say that getting even just a double fixed traction bar to handle arbitrary axle movement (compress/droop level, articulated to various degrees, combinations, etc.) with *no* binding would be next to (or more likely, completely) impossible. Being able to do it with a locating link suspension and trying to get it to have zero bind relative to the springs would be many orders of magnitude more difficult than a single link.

    [/ QUOTE ]

    More that one linkage bar actually allows great flexibility in getting the same arc as the spring. It's just more complicated to sort out. Since a two bar linkage has a virtual pivot point (that moves as the linkage does) it would be feasible to design a two bar linkage that exactly mimics the spring's arc.

    [ QUOTE ]
    And of course there is not much point since that would be just a glorified anti-wrap device at that point...

    [/ QUOTE ]

    Oh so true.

    [ QUOTE ]
    So this suspension design is not ideal... just was a part of the evolution of rear suspension design.

    [/ QUOTE ]

    I don't know about the rest of you, but I'd really like to have a crystal ball so I could see where we'll be 5 years from now. I'm sure it'll be interesting.
     
  14. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    [ QUOTE ]
    follow the true arc of the housing's travel

    [/ QUOTE ]
    I can see that, and of course that would be the only way to approach it, but from a travel path drawing I saw a while back, the leaf sprung axle does not really move in anything other than a VERY rough approximation of a lumpy arc with it's center well above and behind of the spring eye. IIRC correctly, the movement was best described by a sort of series of arcs of differing radius and center point, all grafted together.

    And a 2 bar would be a bit easier since you would not need to deal with side to roll axis lateral shift as much (single center bar would have to deal with both sides in opposite directions but at a lower ultimate magnitude). When compressed, the spring tends to remain more in line with the end mounts, and when drooping, the perch moves well toward the compressed side. This, combined with the "not really an arc" issue is why I don't think you could ever do any better than a relatively close approximation. Same holds for the full link setup which utilize links with fixed real radius and roll axis that is not likely to change along with the leaf spring's theoretical "roll axis" which is moving and changing angle dynamically and dramatically based on leafs lengthening, twisting, deflecting, bending (asymmetrically) and so on.

    But, maybe I'm just making it too hard by missing something or lacking in education. <shrug> It's nothing but an academic exercise anyway. which has already exceeded any useful basis along with my interest level. /forums/images/graemlins/laugh.gif
     
  15. Paxx

    Paxx 1/2 ton status

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    [ QUOTE ]

    Why would you want to run a 4 link with normal leaf springs. No reason to. Maybe something to add in axle wrap control, but no need for 4 links.

    [/ QUOTE ]

    I was just thinking that if you used a 4 link rather than an anti wrap then if you decided down the road to go coilovers you would not have wasted time and money on the anti wrap. But I can see now why it wouldn't fly.
     
  16. juanblzer

    juanblzer 1/2 ton status

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    [ QUOTE ]
    I was just thinking that if you used a 4 link rather than an anti wrap then if you decided down the road to go coilovers you would not have wasted time and money on the anti wrap. But I can see now why it wouldn't fly.

    [/ QUOTE ]
    This is exactly what I ran into before I decided to go full on 4link for my rear. I was told by a reliable fabricator that to correctly mount links on a leafsprung ride, one would have to inboard those links and basically throw money into something that would not offer much stability
     

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