Dismiss Notice

Welcome To CK5!

Registering is free and easy! Hope to see you on the forums soon.

Score a FREE t-shirt and membership sticker when you sign up for a Premium Membership and choose the recurring plan.

Compilation: REAR Link suspensions, lets talk comparisons!

Discussion in 'Center Of Gravity' started by BadDog, Aug 19, 2004.

  1. BadDog

    BadDog SOL Staff Member Super Moderator Author

    Joined:
    Feb 24, 2001
    Posts:
    7,777
    Likes Received:
    11
    Location:
    Phoenix, AZ
    [ QUOTE ]
    I want to start a compilation of various topics relevant to those who are building serious off road rigs. Lots of stuff is touched on here and there, but finding it and putting it all together can be a problem. Lets try to get some of this stuff grouped together in one post. This is one such attempt among several. Lets try to keep this very focused and tech rich. NO POST PADDING! If you don't have something to add, please don't post fluff like the typical "that's cool" or similar having adding nothing relevant to the discussion.


    [/ QUOTE ]

    Lets try to compile all the data on basic REAR link suspensions here. Lets talk about (and define clearly):
    1) Conventional 4 link (triangulate upper - point on top of diff, parallel lower)
    2) Double Triangulated 4 link,
    3) Reverse 4 link (triangulated lower - point at chassis end, parallel upper)
    4) Parallel 3 or 4 link and track bar
    5) Wristed radius and track bar
    6) Wishbone upper with parallel lower
    7) Whatever I forgot…

    I can't tell you how many times I've heard people use the wrong (or ambiguous) terms when referring to a link suspension design. For instance, incorrectly calling a wishbone upper 4 link a "3 link". Lets establish definitions and stick to them so that we have a common vocabulary and frame of reference.

    Those with convenient access can post links to the "Link Calculator" spread sheets and web pages.

    Lets see if we can get all the basic "layman" descriptions of the various pieces in one place. Things like Roll Axis, Jacking, Torque reaction, "Instant Center", and Anti-Squat. Lets look at them (and describe how to find them!) for each of the primary link arrangements.

    Stull like what was found in "God of Suspensions" and other noise laden posts on PBB can be sifted out and focused on this thread.

    What are the bad characteristics of each? Things like jacking or binding and considerations/approaches to deal with them.

    What about good/bad habits of positive/flat/negative roll axis angle?

    What about good/bad characteristic and compromises of roll axis height?

    Good and bad of anti-squat?


    So, lets compare and contrast. Nothing but REAR suspension discussion on this thread. Lets keep this tightly focused and VERY tech rich. No post padding and no "that's cool" but otherwise empty posts PLEASE.

    <font color="red">
    Let's keep this technical! Don't respond with "I know a guy who runs "X" and it works great, so that's what I would run" type of stuff. Tell me what you think is best and why. Know a guy who runs it is fine for a start, but tell me what makes it so great you would do the same thing…
    </font>


    I've got lots of thoughts on these I'll add as I get time, but I'm already running out of time starting these compilation posts.
     
  2. marv_springer

    marv_springer 1/2 ton status

    Joined:
    Aug 3, 2001
    Posts:
    2,781
    Likes Received:
    0
    Location:
    Mesa, Arizona
    I've always liked the "basic" single triangulated 4 link - w/ angled uppers at the pumpkin, or angled lowers at the crossmember. Squat variables aside, these setups do a decent job of maintianing a good roll axis thru the travel range (actually angled lowers at the crossmember have less sensitivity to changes in roll during travel), although some can have some rear-steer side effects (mine) /forums/images/graemlins/ignore.gif.

    But really, I think we should learn something from the "moon buggy" 3 Link w/ track bar setup. That is that the roll center can be placed higher and more easily adjusted with this setup than any "symetrical" wishbone / 4 link. Here's an example of what I'm talking about. The roll center will traverse across the track bar - notice how much higher it stays compared to if the angled links were at the top of a "centered" pumpkin housing.
    [​IMG]
    Additionally, once you get this setup installed, adjustments to roll can be made by raising and lowering track bar mounts - which wouln't be easily done w/ a conventional 4 link.

    So... what am I saying.... /forums/images/graemlins/dunno.gif
    I guess it's that for stability in rocks/sidehills - I think the "track bar'd" 3 link has the more potential than the more traditional 4 link setups.

    Marv
     
  3. BadDog

    BadDog SOL Staff Member Super Moderator Author

    Joined:
    Feb 24, 2001
    Posts:
    7,777
    Likes Received:
    11
    Location:
    Phoenix, AZ
    Thanks Marv, good stuff and similar thoughts have crossed my mind. My first attempt (temp setup on the truggy) will likely be a reverse 4 link or parallel + track.

    Well, I guess I completely missed the boat on these "compilation" threads. I had thought that we might get some good stuff all in one place and then "pin it" as a sort of FAQ. I was going to put up some more info that I've gathered and personal thoughts on how they compare, but I guess there is not enough interest in this so I'll just let it go. At least the "link ends" thread generated something I haven't seen before from Jason which changes my thoughts a bit…
     
  4. four_by_ken

    four_by_ken 1/2 ton status

    Joined:
    Jun 9, 2001
    Posts:
    3,806
    Likes Received:
    0
    Location:
    Allenton, MI
    Hey, what happened here? No info? (well only a little)
    /forums/images/graemlins/dunno.gif
     
  5. BadDog

    BadDog SOL Staff Member Super Moderator Author

    Joined:
    Feb 24, 2001
    Posts:
    7,777
    Likes Received:
    11
    Location:
    Phoenix, AZ
    Yeah, I was a bit disappointed too. I was really hoping this would get a focused discussion going, seeing as how most discussions are general, yielding a little good tech about anti-squat here, some roll axis there, and so on. At one point I thought about laboriously typing in all my various thoughts on the matter to see if I could generate some public discussion, but even I'm finding excuses to avoid typing that much. So I just let them die…
     
  6. four_by_ken

    four_by_ken 1/2 ton status

    Joined:
    Jun 9, 2001
    Posts:
    3,806
    Likes Received:
    0
    Location:
    Allenton, MI
    No no no no no no no no

    /forums/images/graemlins/angryfire.gif
     
  7. ntsqd

    ntsqd 1/2 ton status

    Joined:
    Nov 28, 2002
    Posts:
    3,381
    Likes Received:
    0
    Location:
    So. CA
    My thots are a little different than Marv's, but my intended use is also different. I don't think there is one linkage geometry set that can do both low speed crawling and high speed desert/sand type stuff and be optimum for both. The ideals for each application are not polar opposites, but they appear to be a fair distance apart.

    With a panhard bar the roll center is located at the height of the axle pivot, but on the chassis centerline. The same is true for any 3 link where on of the arms does lateral axle location, the roll center's height is at that arm's attachment point(s) on the axle. With this design the roll center remains pretty constant regardless of the vehicle's orientation. This design certainly works on Tiny, though I wonder if a variable height RC would work better? Perhaps the added complexity is not worth whatever the gain might be.

    In a 4 link with at least one pair of converging links the roll center is not fixed at a certain height. The converging links define the roll center's height. In bump that can elevate the roll center relative to the axle a good distance above the RC's droop location.

    In chassis design books showing Short-Long Arm (SLA) front suspension geometry design the RC is typically near pavement level. One might conclude that this is desirable. It is not. What happens is that the Camber Curve is deemed more important than the RC height. Ideally the RC height would be at the height of the CG. Then the body would not roll much in the corners (weight transfer will still roll the body some). If you design an SLA suspension for this, the Camber Curve is so miserable that the car is not really drivable.
    Now take this a step further. Put the CG's height UNDER the RC's height. Look at what that does for you on a sidehill.

    Going back the the converging/partly converging 4 link, look at what having an RC that progresses towards (or even goes above) the CG does for stability when landing. With a high CG and a low RC you have an inverted pendulum, only having more than 2 tires on the ground keeps it from falling over, or trying to - really hard! As the RC and CG move towards each other this effect diminishes. Get the CG below the RC and you have a 'normal' pendulum. Pretty hard to get that to roll over.

    Is that enough typing? /forums/images/graemlins/1zhelp.gif
     
  8. ccj8008

    ccj8008 1/2 ton status

    Joined:
    Apr 2, 2001
    Posts:
    228
    Likes Received:
    0
    Location:
    Polson MT
    2 link double shackles

    What is the correct terminology of the rear setup where only 2 links are employed in combination with a shackle on both ends of the leaf spring? I like the travel distances that are available with coilover shocks. I was considering running this 2 link/double shackle setup. However, I want to remove all but a single leaf in the spring pack and use coilovers to suspend the vehicle's weight. Seems like this would provide a flexy set-up with minimal fabrication. Has anyone had experience with a setup like this?
     
  9. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

    Joined:
    Mar 5, 2001
    Posts:
    15,683
    Likes Received:
    1,392
    Location:
    642 Days to BB2018

    Sounds scary to me....

    With shackles on both ends how do you prevent the entire axle from just flopping forward or backward by a few inches at a time?...it sounds like it would allow for a lot of uncontrolled motion. I'd ask the same thing from the perspective of lateral strength. Unless you are planning to use a panhard bar, it sure seems like that axle could really deflect quite a bit sideways too....

    The reason you probably don't see that more often is that the leaf is going to restrict your travel more than a coilover will... sort of like "dumbing down" the coilovers that you'll be paying good money for.

    I would guess that weight might be another factor (leafs PLUS coilovers) and the leafs are going to bind a lot more than a well-engineered multilink suspension would.
     
  10. BadDog

    BadDog SOL Staff Member Super Moderator Author

    Joined:
    Feb 24, 2001
    Posts:
    7,777
    Likes Received:
    11
    Location:
    Phoenix, AZ
    Like Greg was getting at, what's the point?

    A single leaf is not going to control axle wrap at all, so you'll need at least 3 links. There will be 2 to locate the axle front to rear and square to the chassis, and then a third to stop rotation. You could also use something like a mono-link that provides the same restriction in "degrees of freedom", but the principle is the same. And since the single leaf on double shackles is going to be unable to locate laterally, you'll need a track-bar. At that point, you have a 3 link with track bar and coil-overs, so there is no point in having the leafs...

    I never saw much point in the floating leaf setups like that anyway. It's just not that much easier to fabricate, and you loose many of the benefits of a link suspension including fewer options for tuning and you're stuck with a lot more weight than either suspension type separately...
     
  11. ntsqd

    ntsqd 1/2 ton status

    Joined:
    Nov 28, 2002
    Posts:
    3,381
    Likes Received:
    0
    Location:
    So. CA
    I've seen trucks built with double shackled leaves and linkage. They were built that way to conform to class rules for racing. Those rules required that the OE's spring design remain (if it came with leaves it stays leaves, if coils etc.). Those rules didn't care about axle location method, only spring type.

    That would be the only reason I can see to use leaves with linkage.
     
  12. tiresmokinchvy

    tiresmokinchvy Registered Member

    Joined:
    Apr 6, 2005
    Posts:
    5
    Likes Received:
    0
    Location:
    pennsylvania
    just a quick thought from day dreaming at work . whats your opinion on a 4 link setup using air bags with limiting straps? my buddy goes to wyotech an gets 40% off air ride technologies parts think it would be worth it?
     
  13. az-k5

    az-k5 1/2 ton status

    Joined:
    Oct 15, 2003
    Posts:
    2,774
    Likes Received:
    0
    Location:
    Phoenix AZ
    Length is the biggest downfall to air bags. Also height is direcly proportional to stiffness. You cna place the back upframe on the link but then you are putting a heavy bending moment on that link. It has been done, but usually requires much more work on the links themselves (read up on engineered links for more info)
     

Share This Page