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Rear suspension: 3 link with transverse spring?

Discussion in 'The Hokey Ass Message Board' started by scott mckelvey, Jul 27, 2012.

  1. I decided that I cannot live with what's under my '31 model A right now (quite possibly literally).

    I've searched and read just about everything out there and I understand what are some "ideal" setups, etc. I did see a few specific references to a 3 link but they were different.

    Additionally, this is in part based on what is on the car already and I deem usable. So, the idea is to reuse as much of what's already there as possible.

    Currently (two attached pics but they are hard to see):

    Proposed (see attached drawing):

    • use the existing front mounts on the frame rail w/ heim joint for 1/2" bolt (single shear)
    • use only the bottom part of the rear axle brackets w/ heim joint for 1/2" bolt (single shear)
    • Modify 32-34 Ford Radius rods for the lower links by adding threaded bungs for said heim joints
    • Top link using 1" DOM w/ .120 wall and threaded 4-link bushings on the ends. 3 degrees lower and about 5.5" shorter than lower link.
    • Weld in double shear brackets for top link to both a frame crossmember and top of axle.
    • Driveshaft would be 3 degrees higher than the lower link and is about 11" shorter.
    So, my questions are as follows:

    1. Generally speaking, is there any reason I can't do 3-link with a Transverse spring and no panhard bar? I understand there may be a limited amount of sway b/c of the spring/shackles and the fact that I'm not using a dead perch.
    2. If yes, are there any issues given my specific angles and lengths?
    3. Lastly, is the hardware and mounting sufficient (Heims with single shear 1/2" bolts, modified radius rods, and DOM tubing)?

    Attached Files:

    Last edited: Jul 27, 2012
  2. The side play issue would be the main concern. With 'bones, you have a little help holding the axle in place. The 3 link is a much more "free" design. Could be a noticeable thing, depending on your driving habits.

    I'd recommend a Watts link to go with the buggy spring, but that's getting kind of complex and not really "period"...
  3. krooser
    Joined: Jul 25, 2004
    Posts: 4,587


    Can't you just fab a rear panhard bar?
  4. rottenleonard
    Joined: Nov 7, 2008
    Posts: 2,168


    Well, correct me if i'm wrong(I'm sure no one would do it unless I offer), But Isn't the main idea of a three link that you don't need a panhard rod, the triangle of the third link takes care of it....ya? Don't know why it wouldnt work.
    And the shackles on the transverse spring should negate any binding that would occur, I don't see a problem, but mayby I'm not seeing it.
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  5. a boner
    Joined: Dec 25, 2004
    Posts: 3,524

    a boner

    On my modified, the upper/center bar is next to the drive shaft, in the drive shaft tunnel, with a length about the same as the drive shaft. I also run a panhard bar. All the bars have a heim joint on each end to allow for twist.
  6. Beau
    Joined: Jul 2, 2009
    Posts: 1,857


  7. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 1,194


    3 links are one of the best ways to get enough compliance in a street situation where bodyroll cannot [ or should not ] be eliminated.

    You don't need a panhard bar if it is designed correctly with a triangle shaped 3rd link [ 3 mounting points on the rear-end and 4 mounting points on the chassis ]
    If drive tunnel clearance is minimal use a triangulated center link under the rear-end center and 2 parallel upper links on outsides

    This method lowers the roll-center height [ a good thing ] and was very successful in the "Lotus Cortina" in the 60's
    They used it for "Road Racing" and didn't need a panhard bar or any lateral location device.

    Attached Files:

  8. NV rodr
    Joined: Jul 23, 2006
    Posts: 151

    NV rodr
    from Reno, NV

    A watts link or modified watts link will make that work. Full articulation with lateral forces being controlled with the watts.
  9. gnichols
    Joined: Mar 6, 2008
    Posts: 9,479

    from Tampa, FL

    I've been thinking that something like this would be a nice to fit into a rod chassis. It's made for a Pony car, but seems adaptable. The Watts link is hard to see, but it's on the back side of the diff. Gary

    Attached Files:

  10. NV rodr
    Joined: Jul 23, 2006
    Posts: 151

    NV rodr
    from Reno, NV

    I think the "Grumpy Jenkins" version was the first successful one that immediately comes to mind
  11. pitman
    Joined: May 14, 2006
    Posts: 4,014

    from Hampsha

    I was thinking of the setup that Kerrynzl mentions above. Then no parallelogramming
    or side deflecting. You'll want to evaluate the best upper differential bushing type, perhaps a 'shouldered' urethane or hard rubber in double shear style of mount. I'd consider simply splitting the upper link as in his left hand diagram. If it helps the roll center too, so much the better.
    Last edited: Jul 28, 2012
  12. Jmountainjr
    Joined: Dec 29, 2006
    Posts: 666

    from Maine

    If you stay with a three link, the triangulated top link as pictured in the left picture in post #7 solves your side movement control issue. The only downfall of that design in cars with limited verticle space is the extra height of the bar connection onto the top center of the rear axle housing. If you look at that style of top link, it's very close to the top bars on a triangulated four link except the bars come together at the top of the axle instead of two attachment points on either side of the center section. That difference does change some of the roll centers. But in a light car with limited actual suspension travel, you can use a triangulated four link with a transverse spring and gain some verticle clearance over the three link.

    Attached Files:

  13. chopt top kid
    Joined: Oct 13, 2009
    Posts: 959

    chopt top kid

    Scott, Your three link setup should work fine as long as you have proper preload in your transverse rear spring(rule of thumb = spring tension holds shackles at 45 degrees). Hot rods have been running transverse springs without panard bars for decades.
    Last edited: Jul 28, 2012
  14. F-6Garagerat
    Joined: Apr 12, 2008
    Posts: 2,650


    Check my build thread below, like post 91 I think. I used a Uni Ball at the front of the bones. Way better than a heim. I have to add a torque rod still.
  15. Seen it and like it, but don't like the idea of the ball in the rear.

    both the examples that you guys showed are coil sprung, and I get why that's necessary in that application, but not with a transverse spring. Am I wrong in assumption that the spring in this case will do the locating of the axle, with the understanding that there will be some play like during hard cornering. In which case my spring will located the axle side-to-side (not the links).
    Joined: Jun 6, 2005
    Posts: 1,887

    Member Emeritus

    Just caught this post this morning, after looking at your pictures just what is wrong or what do you think is wrong with the rear installation you have now?
  17. Seen that and equivalent joints (although I'd be more inclined to use the rebuildable Rubicon version if I went that route). Certainly an option if i triangulated some ladders or rods inboard, but at that point I'm completely removing and redoing everything, including adding additional frame cross members. If I go down that road then there's no limitations and I'll do exactly what I want. The point of this exercise is to see if I can make use of what's already there, safely and cleanly.

    Like most everything on this truck, it was done quick and dirty w/ little attention to aesthetics, safety, or function.

    • it's using front hairpins, which in my opinion are not up to the task. Plus they have one measely bracket barely welded in the middle for strength.
    • they are mounted outboard on the frame rails, and bind obviously with any articulation (this truck will be on the street, not the track). aside from that, something will break in this configuration so I'd just as soon fix it now.
    • not addressed in this thread, but I already remedied it: it had narrower than stock perches w/ a stock A spring and the shackles were vertical at rest and bottomed against the perch brackets on the axle with little to no travel of suspension.

    That was an issue (see above), but it's been corrected.
    Last edited: Jul 28, 2012
    Joined: Jun 6, 2005
    Posts: 1,887

    Member Emeritus

    Scott, what you have isnt as bad as you think, maybe the pictures dont show the quality of assembly but the concept is ok. If you are limited on equipment and cash this just needs a little TLC. From your post, you just need to clean up a few issues first by replacing the spring shackle mounts to the correct span, this will make a major difference. The wish bones are positioned Ok but should have a heim end or tie rod end installed at the frame mount to counter the twist from one wheel bump. If you want to see how much deflection in the rear rods, place the vehicle on stands at ride height while you are changing your spring mounts, remove the shackles and jack the axle up to simulate a 2" one wheel bump. You will see that it should not twist very much and most twist would be accepted by the moveable end. Clean up the welds, make sure the frame bracket is gussetted.

    If you are bound an determined that you are not happy with this set up the simplest set up you can install is a split wishbone set up ( check Pete and Jake's design) that converges near center line of the car. It locates the rear axle and with the use of a transverse spring eliminates the requirement of a panard bar.

    Good luck with your project .
  19. davidfe
    Joined: Jan 1, 2006
    Posts: 135

    from Illinois


    Hope you don't mind that I corrected the color balance in the pictures.

    Interesting questions.


    Attached Files:

    • A.jpg
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    • B.jpg
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    • C.jpg
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  20. '49 Ford Coupe
    Joined: May 19, 2011
    Posts: 503

    '49 Ford Coupe

    Toss it all and get a Jag IRS. It's already been very well engineered, and looks cool, too. Just my two cents. Very simple to install, aftermarket cross-members available. Google Jaguar rear suspension.
  21. davidfe
    Joined: Jan 1, 2006
    Posts: 135

    from Illinois


    This 2nd attachment uses the DeDion tube as part of the suspension
    in the Lotus 7 Series 2-3.

    Is this adequate for the solution to Scott's concerns?

    So confused about suspension issues and trying to sort them out.



    Attached Files:

    • D.jpg
      File size:
      86.6 KB
  22. I realize the topic of stresses from outboard vs inboard mounting is covered ad nauseum on this board, so I won't dredge that up. But I subscribe to the school of thought that outboard mounting of a ladder bar or radius rod on a street driven car causes undue stress, will not articulate properly, and will fail prematurely. This, regardless of how many people have done it, for how long, and how many cars are on the road with that setup. That said, I already have heim joints, but that does not alleviate the issues described above.

    While I appreciate that input, as I mentioned above, if I go the inboard mounting route, I'll be removing and redoing everything anyways, so then it's a non-issue and a blank canvas.

    Thank you. i wish i knew how to do that. My pictures still suck (I apologize in advance), but that's way better.

    Huge help, thanks.

    Maybe we can get back on track and focus on specific questions I had about the proposed suspension: not suggestions for IRS, coil overs, 4 links, or ladder bars. Not saying I understand all of those, but I can ask about them if/when I decide to use them.
    Last edited: Jul 28, 2012
  23. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 1,194



    The Lotus 7 uses a De-dion rear end to reduce unsprung weight ,They weighed about 1200lbs so a higher percentage of the total weight was unsprung [ The spring frequency's were very light ]
    The Mark 1 Lotus Cortina used the same 3 link system on a normal rear axle housing.

    The great advantage of the 3 link is that it's a "no-brainer" because the INSTANT CENTER and the ROLL CENTER are exactly the same. If the roll center is fixed at the rear axle, the RC height remains constant regardless of suspension travel or bodyroll [ provided the wheels stay on the ground ]
    The 3 link can go forward to the center of the car [ similar to a A-bone ] and work successfully but the RC height willchange with suspension travel.

    It is more desirable to have the RC height as low as possible so that lateral forces convert into vertical forces [ downward forces through overturning moment ], Which is why Lotus mounted the A-frame underneath the rear axle housing.
    This also "killed 2 birds with 1 stone" by providing an answer to the limited space to fit the A-frame .
    The A-frame doesn't need to be full width as in the picture I posted earlier, I've seen a clubman racer the used an identical A-arm as the lowers on the front-end of his Race car.

    The springs only support vertical loads so this method would work very successfully with a crossleaf and shackles at each end [ the shackles should be closer to vertical at full compression ]

    Note: A 3-link would be very successful on the front end [ especially with a dropped beam ] using an "A" or "V" frame to the center and 2 side supports where Split bones would normally go. This would eliminate the "death wobble" that has been discussed on many occasions
    [ 3-link suspension at both ends would be my suspension of choice on an open wheel Hot Rod like a T-bucket or Hiboy ]
  24. pecdaddy
    Joined: May 23, 2010
    Posts: 196


    Heres what I did. Tie rod ends on the bones at the frame. Hiems on the 3rd link.

    Attached Files:

  25. Very interesting that's almost identical to what I had in mind. I take it that you haven't driven it? Did you get the idea from somewhere that someone had used it, b/c I haven't really found anyone else that's done it (at least not on the HAMB or from a little google searching)?

    Also, your front brackets look like the speedway ones, right? If so, I assume that you got the ones that accept tie rods, or did you modify a set that have the straight hole?
  26. Scott, I have been thinking about what to do for my 30 tudor's rear supension for over two months now, so your post comes in handy. I think your plan is going to address your concerns, I don't know the answer to your main question as I see it. What I think you are tring to get at, is your geometry correct as your plan is drawn in your first post.
    I was at this point also, how long should the arms be and where should they mount. I hate to say it but I'm just planning on taking the easy way out at this point and build a set of ladder bars that mount as close together up front as possable. I know this does not answer your question but...
  27. Well, the more I thought about it the more I felt like the right thing to do was something in addition to the transverse spring for lateral stability. I hate wishing I'd done something "right" the first time, even if I could have lived without. But I didn't like the idea of a panhard bar, still didn't like the idea of moving the ladder bars/radius rods inboard and adding a new crossmember, etc. Plus, the existing ladder bars weren't an option anyways b/c the top axle mounts would share space with the reversed eye leaf spring.

    And, the idea here was to work with as much of what I already had (that was usable and safe). So to conclude this thread, I came up with this (still alot of work to do, but you get the idea), which seems like I'll get the best of all worlds:

    • Triangulated top link:
      • I recycled one of the Speedway "rear" hairpins by removing the bracket and spreading it. So it will serve as a triangulated top link with 5/8" threaded heims with 5/8" holes, and Grade 8 bolts.
      • brackets will be 3/16" cold rolled flat stock, as double sheer on the axle and crossmember
    • Lower links:
      • the will remain outboard on the frame rails as installed by PO, as well as lower part of axle bracket. But I'll clean up the mounts.
      • 1.25" DOM .120" wall, w/ 1.25" steel bungs. Heims at each end (11/16" shank, 5/8" hole).

    Geometry should be pretty good per my original rendering. Only a 2 degree difference in angle and a 5.5" difference in length between top and bottom links. There shouldn't be any bind, and any change in pinion angle should be negligible, but also adjustable.

    Attached Files:

    Last edited: Oct 12, 2012
  28. pasadenahotrod
    Joined: Feb 13, 2007
    Posts: 11,773

    from Texas

    The most important thing in using a 28-41 Ford transverse spring is mounting it correctly. That means duplicating the mounting points for the spring shackles on the rearend. The center to center distance should be the same as stock, 49.5" for a Model A rear spring. This puts the spring under tension(probably not the correct engineering term) with the shackles unloaded being parallel to the ground. When weight is applied the shackle angle drops to a 30-45 degree angle which is ideal for this setup. The tensioned spring eliminates the need for a Panhard rod which is why Ford designed it that way.
    In 1942 Ford changed to a new design without tensioned springs adn added both front and rear Panhard bars to eliminate sidesway. This gave the cars a bit softer ride in order to better compete with their compadres at GM and Chrysler which has IFS and dual rear springs for many years.
    Joined: Jun 6, 2005
    Posts: 1,887

    Member Emeritus

    Nope dont think this going to work well, The upper mount that you have installed on the top of the pinion will not operate on the same arc as the lower radius rods. Everytime you have a suspension defection up or down the upper bar will change the rear axle pinion angle as it rotates the rearaxle to corelate to the upper control rods arc length swing.
  30. Gofannon
    Joined: Feb 8, 2007
    Posts: 348


    There's a good 3 link calculator here:

    I'm using an asymmetrical 3 link in my T roadster. T spring, English Ford Pilot rear 'bones (similar to '36 except lighter and they hang down a lot lower). I cut the 'bones off at the rear and fabbed up bushes. Split at the front but angled in the same as unsplit 'bones. The top link is shorter and mounted about half way between the rear axle centreline and the RH lower mount. The front mount for the top link has 2 hole positions, which , if the calculator is about right, will give either 70% or 100% anti-squat. As it is asymmetrical, under acceleration it will load the LH wheel which is the one that will brake traction first (using an open drive '57 Chevy rear). Jaguar once used this concept. I'm not using a panhard or anything , just relying on the transverse spring. Don't ask for pics as I have it all stripped down at the moment, and it's all unproven theory as it is a work in progress so I haven't driven it yet!

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