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What's the down side to truck arms for a Modified?

Discussion in 'The Hokey Ass Message Board' started by Tony Travers, Nov 3, 2011.

  1. Tony Travers
    Joined: Jan 11, 2010
    Posts: 63

    Tony Travers
    Member

    Howdy
    Looking at building a 27T modified and liking Chevy style truck arm rear suspension. Mocked up it sits best at around a 93" wheel base and I should be able to bob the frame behind the body and get it low with coil overs but the arms would only end up about 36" or so long. Forgetting things like exhaust clearance is there a downside to truck arms? and at this length? I realise the longer the arm the better they ride.
    Cheers
    Tony
     
  2. No real downside, as you probably won't get enough travel to worry about pinion angle change. I'd make sure the crossmember that the arms attach to is well braced to the frame, put the front ends as close together as practical, and make sure you have a very solid panhard bar mount; as the truck arms will load the bar pertty hard in corners.
     
  3. need louvers ?
    Joined: Nov 20, 2008
    Posts: 12,911

    need louvers ?
    Member

    Just remember that those have to be installed in a triangulated manner. In other words the front mounts have to be pointed together in a fairly narrow "V" shape to articulate properly. The only big thing that I see as a disadvantage would be packaging. Most "T" frames are so narrow that setting them up properlyand keeping them hidden is going to be a bit of a challenge.
     
  4. hrm2k
    Joined: Oct 2, 2007
    Posts: 3,656

    hrm2k
    ALLIANCE MEMBER

    This might help a little. while not a modified, it is still a T coupe that has a chevy truck arm style rear suspension. It was originally done with air bags / shocks but has been changed over to coil springs

    [​IMG]
     
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  5. oj
    Joined: Jul 27, 2008
    Posts: 6,247

    oj
    Member

    You won't have room for trailing arms. Nice idea, but a modified is just too low for them. To see thru the windshield your ass will be at the same place as the trailing arms. Do a scale drawing, sit on the floor with your back against a wall and mark the top of your head - it'll be about 34", from the floor to the top of your windshield it'll be about 30". You need to get your ass below the top of the frame rail - like where the trailing arm runs.
     
  6. Tony Travers
    Joined: Jan 11, 2010
    Posts: 63

    Tony Travers
    Member

    Anybody else have an opinion?
    Cheers
     
  7. Tony Travers
    Joined: Jan 11, 2010
    Posts: 63

    Tony Travers
    Member

    Howdy
    These arms look really heavy. Do they flex at all like the original GM arms? And around about how long would they be? I see you've mounted them low to fit OK.
    Cheers
    Tony
     
  8. hrm2k
    Joined: Oct 2, 2007
    Posts: 3,656

    hrm2k
    ALLIANCE MEMBER

    Tony, The arms do not flex at all that I can tell. They are made from I beams. They are attached to the K member that holds the trans tail shaft. I am going to drive the car today. I'll get a tape measure and get you some measurements.
    It might be worth your while to contact " Clark" here on the HAMB. He built this car and many others that have the same suspension. I'm sure he can give you a lot of useful information
     
  9. lakeroadster
    Joined: Nov 6, 2008
    Posts: 604

    lakeroadster
    Member
    from *

    When one rear tire goes up (or down) and the other doesn't this creates torsional twist in the I-beams, and yes they do twist. A non boxed I-beam twists relatively easy, especially one with a drilled web.

    If you use a tube or a pipe for the truck arms they are much stronger in regards to torsional twist. Then a heim style joint is needed that will allow torsional rotation due to rear axle movement.

    If you think the truck arms will be to low why not mount them to the top of the axle and use a rear tranverse Model A spring for support? (Which looks totally bad ass on a modified!)

    What you end up with is a split wishbone design very similar to Henry's factory configuration. And those worked great, just need to beef up to match whatever horsepower you are planning.
     
  10. chrisntx
    Joined: Jan 20, 2006
    Posts: 1,791

    chrisntx
    Member
    from Texas .

    If you look under the trucks these came on you will see they were not pointed together in a fairly narrow "V" shape.
    I drove a 66 Chevy pickup for 17 years plus another 17 years in a 39 Ford with the same arms
     
  11. Chris, I think he was simply comparing the setup with a straight ladder bar type suspension. What makes the truck arm work on the street is the angle of the arms, and the closer together you can get the mounts, the better the suspension flex (ever seen a stock model T go over ruts?). GM trucks used relatively huge front bushings to allow for more flex, as the mounts were fairly far apart (10-12" IIRC).
     
  12. thunderbirdesq
    Joined: Feb 15, 2006
    Posts: 6,809

    thunderbirdesq
    Member

    That Ibeam will actually flex quite a bit, that's why it's used. Just like the back-to-back Cchannel that the factory used, when it flexes/twists slightly, it takes some of the stress off the mounts. they also used LARGE rubber bushings at the front to reduce strain on the mounting points. I copied that 2x3 ibeam design under HRM2k's car under my '34 pickup except I used a transverse leaf spring and it worked VERY well. Hooked up nice, rode decent, very stable under load and at highway cruising speed. Highly reccomended if you can fit it under the vehicle. My arms were 36" long from the axle centerline to the front bushing center.
     
  13. need louvers ?
    Joined: Nov 20, 2008
    Posts: 12,911

    need louvers ?
    Member

    Thanks exwestracer, you stated exactly what I was saying. I've seen these mount in parallel before and the owner wondered why he was ripping mounts off the axle every week... Nothing quite like turning the rear axle housing into a large, very non-compliant sway bar!
     
  14. yup!
     
  15. Tony Travers
    Joined: Jan 11, 2010
    Posts: 63

    Tony Travers
    Member

    Howdy
    I was at a junk yard this morning and looking at stuff as you do and I saw trailing arms like these on a Mitsubishi 4 wheel drive. They have big complaint bushes at the back and a pin and bushes up front so will should bind any which way. They are around 36" long and would seem to be an idea economy bolt in set up. They have sold millions of these things world wide so I guess you could call it proven engineering.
    Cheers
    Tony
     
  16. Tony Travers
    Joined: Jan 11, 2010
    Posts: 63

    Tony Travers
    Member

    Howdy
    I was at a junk yard this morning and looking at stuff as you do and I saw trailing arms like these on a Mitsubishi 4 wheel drive. They have big complaint bushes at the back and a pin and bushes up front so should not bind any which way. They are around 36" long and would seem to be an idea economy bolt in set up. They have sold millions of these things world wide so I guess you could call it proven engineering.
    Cheers
    Tony

    [​IMG]
     
    Last edited: Dec 30, 2011
  17. sololobo
    Joined: Aug 23, 2006
    Posts: 8,071

    sololobo
    Member

    Very interesting info, I am such a nerd on suspension issues. Our group engineer Terry and X-derilict are our suspension cats. They talk the stuff at meetings and I just shake my head and mutter oh yeah. But I hardly know what they are saying, I just drive the utility wagon with supplies for our pit position. Posts like these do turn the light a little for me, but it is still pretty damn dim! ~sololobo~
     
  18. mrconcdid
    Joined: Aug 31, 2010
    Posts: 1,157

    mrconcdid
    Member
    from Florida

    Heres a pic of a 68/69 C-10 chevy truck
    Frame and all, this is what I plan to use. I cut this out complete so I could keep the crossmember and the correct angels.
    it may give you come ideas on your set up.
    MrC
     

    Attached Files:

  19. Mitsubishi, huh? Those are the EXACT same design used on the Land Rover Discovery...which I guess you are familiar with...:D
     
  20. Tony Travers
    Joined: Jan 11, 2010
    Posts: 63

    Tony Travers
    Member

    No but I'll go and look ...
     
  21. Hackerbilt
    Joined: Aug 13, 2001
    Posts: 6,238

    Hackerbilt
    Member

    ...AND, if I remember correctly, the same design was used on the Mercedes G wagon.
    Not gonna find too many G wagons lying around, but it does help verify that this design is workable and reliable when multiple manufacturers use pretty much the same exact design.

    I'm pretty sure that these manufacturers all use these arms in a basicly parallel configuration...so those extra bushings at the axle really offer a lot of additional flex!
     
  22. Mr48chev
    Joined: Dec 28, 2007
    Posts: 26,511

    Mr48chev
    ALLIANCE MEMBER

    Good point, I think that is one of the things that caused some issues with my T bucket I built in the early 70's with a truck arm style suspension. The arms didn't angle in at all and even with soft bushings in the front caused it to bind up a bit.

    That is a pretty interesting setup Tony, I am going to have to stick my nose under some of those at the local wrecking yards.
     
  23. need louvers ?
    Joined: Nov 20, 2008
    Posts: 12,911

    need louvers ?
    Member


    Ya, the actual amount that the arms are spread at the front isn't critical, they don't have to be touching. With the huge bushings in the ends, you could get away with them being about a foot apart before you ran into bind issues. Go much farther apart than that though, and you loose the center of you imaginary triangle, and the bind that will result has to manifest itself somewhere. That is going to be weekest link in the system, it usually will be the joint in the system with the thinnest wall strenghth, and that in turn is usually you rear axle.

    Just a quicky on triangulation of suspension systems while I'm thinking about it. Go grab a 45 degree drafting triangle. Also grab a square piece of sheetmetal, plastic, or anything that is relatively bend resistant and square and about the same size. Now take the drafting triangle and hold the straight edge twords you with you hands out at either end of the straight section, and the point away from you. Imagine that your hands are the wheels, and the straight section is the axle. Put the point down on a table (do not let the wife or girlfriend see this, and don't explain if they do!) and move it through all different movements all directions and see how easy it is to move about. Make some engine or suspension noises if you want, remember your not going to explain your actions anyway. Congrats! You have just figured out how Henery's torque tube rear worked and how a triangulated three or four link, or truck arm suspension works!

    Now do the same with the square... Goes up and down just fine, but doesn't want to go side to side with out flexing, right? You just figured out how overly split wishbones, ladder bars, or hair pin suspension works, and why it just isn't a great idea IN MOST CASES. Yes, it was used a bunch in race cars throughout the forties, fifties and sixties. But you do have to remember that race cars had two things the avaerage driver type car doesn't; A very narrow range of movement to do what it needed to do, and daily, sometimes hourly maintinance.

    Hope that helps the thought process a bit. (Note, Mr48 chevy, I didn't mean to get that long winded with you. I just realized I wrote all that within your quote, and I'm just too damn lazy to retype it and post it all by itself. My lone use of a smilie, ever...:))
     
  24. Tony Travers
    Joined: Jan 11, 2010
    Posts: 63

    Tony Travers
    Member

    Well I went back to the junk yard and picked up these the trailing arms of an air bagged Range Rover 4wd. They're a composite material designed to twist enough for a 4 wheel drive they have big rubber bushes at the front and the rear is rubber mounted. I also scored a pair of Jag coil overs which are only 13" long. The shocks fit straight onto the arms using an existing mounting point. The arms are about a metre long (40") and the front pivots will be in line with the front uni joint.
    [​IMG]
    [​IMG]
    Should work OK.
     
  25. oj
    Joined: Jul 27, 2008
    Posts: 6,247

    oj
    Member

    Thats an interesting frame you are using, what is it?
     
  26. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,062

    Rusty O'Toole
    Member

    The only odd thing I noticed on my old Chev trucks was a tendency to for the rear to "walk" to the left on washboard roads. The angle of the sway bar would "jack" the rear sideways. This was only noticable on a rough gravel road when the rear was bouncing up and down fast. And only when the box was empty.
     
  27. Tony Travers
    Joined: Jan 11, 2010
    Posts: 63

    Tony Travers
    Member

    '29 Willys rails
     
  28. Tony Travers
    Joined: Jan 11, 2010
    Posts: 63

    Tony Travers
    Member

    Howdy
    I know this is going back awhile but just curious why you swapped out the Air Bags for coil springs?
    Cheers
    Tony
     
  29. hrm2k
    Joined: Oct 2, 2007
    Posts: 3,656

    hrm2k
    ALLIANCE MEMBER

    I found that I didn't enjoy hitting the snubbers on just about every ride around Baltimore. I over filled the bags to compensate. While overfilled, they contacted the frame and rubbed a hole in one of the bags. I put new bags in and had the same thing happen. The only solution I saw was coil springs so I changed it over.......best decision I made so far.
     
  30. 55willys
    Joined: Dec 7, 2012
    Posts: 1,601

    55willys
    Member

    If you check out "26 modified" by fastrnu you will see the triangulation and pivot on the wish bone. It doesn't bind and the rear spring keeps it centered. No need for a panhard bar.
     
    Last edited: Dec 14, 2012

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