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  1. #16
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    I think Soundman has covered the principle pretty thoroughly, but I thought I'd add my understanding of the issue.

    The point of concern is where the drawbar meets the first crossmember (crossmembers after this don't experience the same stresses). The stresses are due to vertical movements of the drawbar relative to the trailer. These counteracting movements put the top and bottom of the RHS under tension and compression alternately. If the top of the section is weakened by the action of heat affected zone at this point of greatest stress, then there's increased risk of failure, which would be a fracture along the HAZ.

    Once cracked, I'd expect there's two modes of further failure - the sides of the RHS begin to tear, and without the top of the RHS to lock the sides in position, they can move away from the vertical, massively reducing the strength of the section.

    I guess plates either side would clearly add strength, and maybe reduce the amount of stress experienced right at the join in question by reducing the flex of the drawbar in that area due to the stiffening action of the plates.

    With regard to stiffening the drawbar with trusses etc. I think Yonnee argued that having some flex in the drawbar is desirable as it reduces the peak stresses elsewhere.

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  3. #17
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    If plating the junction, I think it would be important to either taper the plates.....like in the pic a couple of posts above or continue the reinforcement quite some distance.....so that the point of stress is not simply moved from one point to another.

    Unless the drawbar is massivly over strenght it will always flex to some extent.

    cheers
    Any thing with sharp teeth eats meat.
    Most powertools have sharp teeth.
    People are made of meat.
    Abrasives can be just as dangerous as a blade.....and 10 times more painfull.

  4. #18
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    I like the way this thread has progressed.

    Probably not practical for a box trailer but I'd like to see a van with truss drawbar extended into truss frame so it's one big structure. I guess a light truss would be weak perpendicular to the drawbar but maybe a good design could incorporate clever cross members around the storage requirements. One big issue with van/camper would be trusses would make storage and access difficult.

  5. #19
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    There are a couple of off road campers and trailers that have truss drawbars that tie into a truss type side structure.

    In off road trailers there is a real problem with the drawbar bottoming mid span.

    these trus type structures allow a completly different approach from the straight flat draw bar that comes from under the trailer.

    Seems back in the 60s and 70s, there was more creativity going on in trailers....I have seen boat trailers that are basically a pair of web trusses from end to end.

    I do remember seeing a box trailer that has a straight truss for a drawbar that looked very much like a yacht spar

    many cleveer car trailers have their sides bulit as web trusses rather than relying on weight of steel alone.

    I think too that a lot of this creativity fas fostered by high steel prices and people with engineering qualifications having enough time to fabricate stuff at home.
    ANd maybee too more people with engineering qualifications had trade backgrounds back then.

    cheers
    Any thing with sharp teeth eats meat.
    Most powertools have sharp teeth.
    People are made of meat.
    Abrasives can be just as dangerous as a blade.....and 10 times more painfull.

  6. #20
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    Hmm. Taking it to it's extreme, could you make a whole trailer substructure out of lightweight space frame, e.g. out of 25mm x 25mm x 2.5mm SHS with vertical dimensions 200mm high and the whole thing drawbar/chassis triangulated/trussed in the horizontal planes?

    I guess you'd have to conduct a FEA of the design to determine it's feasibility.

    My main concern would be that local beam spans out of light SHS would be much more subject to impact than one big 150mm x 50mm x 3.0-4.0mm drawbar beam. So maybe not the sort of thing to consider for an off-road trailer.

    But some back of envelope calculations indicate there are definite weight savings to be made. 200mm of vertical space is a lot though, you'd want to be able to use that space somehow not just wasted space. All the triangle spans would make that difficult.

    Pushing out the individual members to e.g. 40mm x 40mm and the weight savings are lost. Maybe just a combo, or my local steel shop sells 35mm x 35mm in 1.6mm, 2.0mm, 2.5mm and 3.0mm so use that in various combos with heavier stuff on the bottom and near suspension supports and lighter stuff elsewhere.

    Building a space frame trailer would be a lot of welding ...

  7. #21
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    Over in the Trailers forum, I asked Yonnee why they don't do the sides of car trailers as simple (or complex) trusses. His response was that they'd be too tall and stop the doors opening, which I guess might be an issue for high-performance vehicles with low clearance.

    As for FEA, that's one of the many reasons I wish I had the motivation to learn Solidworks.

  8. #22
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    The truth of the matter is that the sides of many trailers are trusses either by accident or design.

    If you look at a car trailer with a plain straight bottom beam, some height any height..as little a 100mm and another section of metal across the top..and you have a truss.

    taper the back and front till the top rail meets the bottom at both ends and it realy starts to look like a truss.

    I have seen many car trailers that have definitely had genuine truss design incorpirated...after all there are a lot of car trailers built by people involved in motor racing and who understand and value high strength for weight.

    A great deal of strength can be gained by simply seperating two smaller members with spacers.

    for instance two sections of 50 x 50 rhs seperated by say 50mm spacers would be a hell of a lot stronger than two 50 x 50 rhs welded straight on top of each other...increase the length and number of spacers and the strength is increased.

    There is no need for complex diagonal truss designs to achieve considerable increases in strenght, stability and rigidity.

    when working in wood it is important to have diagonal members to prevent racking.....but in steel particularly if the truss webs are short and relativly fat.....sufficient racking resistance is achieved in plain square but joins.

    consider too that one of the members in a simple two cord truss can be considerably smaller than the other......say a 50 x 50 RHS and a 12mm rod with pieces of 50 x 3mm flat 75mm long as spacers.


    As for the issues of the small sections being more prone to damage......yeh well.....most race cars.....in particular off road buggies....are space frames ( a roll cage) with suspension and engine mounts and somewhere to sit......mostly the body is of very little structrual significance.
    They do this for weight and durability and let me tell you they cop a pounding.

    I think very simple trusses in different parts of trailers could considerably reduce weight and increase strenght.

    cheers
    Any thing with sharp teeth eats meat.
    Most powertools have sharp teeth.
    People are made of meat.
    Abrasives can be just as dangerous as a blade.....and 10 times more painfull.

  9. #23
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    I'm thinking of building a new trailer next year. If I used a 100-150mm drawbar and a 50mm chassis that's already 200mm of vertical space gone.

    I have a notcher and bender and already made my first trailer with a round pipe top frame. Maybe I can make the new one with round DOM tube chassis/drawbar. I can probably live with a 200mm high frame.

    Now that I'm crazy about tig it would be a lot easier than using stick for the first one.

  10. #24
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    Default trusses in trailer design

    nice discussion a while back on incorporating a truss into a car trailer which was flexing too much by a fellow member Styx.. see pic.

    and a trailer manufacturer in Queensland uses a sort of truss in some of their trailers which caught my eye.

    hope I haven't infringed copy right?
    Attached Images Attached Images

  11. #25
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    I must be missing something. I sketched up a couple of space frame chassis. The first used way too much steel and complexity and was going to have about 130m of tube. So I revised it but the second design still had too much and didn't look strong enough anyway.

  12. #26
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    Think about a cirmetrical diamond shaped plate, in equal to , or slightly less thickness of the RHS wall, placed between the 2 members. and stitch welded longitudinaly to each .

    Scollop each side of the diamond ( to cut down weight ). Round, or square off the diamond points to slightly wider than the width of the RHS to accommodate the leg of the end stitch welds.

  13. #27
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    Trusses and space frames, while possessing more than ample strength have a major drawback in the run of the mill trailer world. They are too expensive by far to produce.
    If one considers where the average, (very average!) trailer comes from, it is a mass production factory where often the boot soles of the employees are built up to keep their knuckles off the floor. Box sections and sheets, pre cut are fed to them, placed in a jig and something approximating weld is applied, next paint is applied from a garden hose and when the runs stop dripping, the whole lot is shipped off to be sold to Joe blogs at the cheapest he can get it.
    Complication adds cost and requires a higher standard of worker, which again increases cost.
    Some years ago, I built a trailer that was to be used for carting and erecting grain silos. The silos were up to 4.8M Dia and 7.5M tall. This trailer backed up, a lifting cradle was raised via onboard hydraulics, the silo strapped and chained on, then the cradle pulled down and the truck and trailer drove off into the sunset and the process reversed at the other end.
    We are talking a trailer that was the best part of 11M overall length, low to the ground and had to be light enough to run on small axles and 14" rims. That beast was a truss from end to end, as was the lifting frame. Wiring and hydraulics ran through the frame. The trailer, complete with hydraulic pack was only 1.4 tonnes in weight. It was the only trailer in Tasmania that could move and place such a wide range of silos. There are two now - mine and the one made as a direct copy. Sadly, the photos of this and much of my work from that period were thrown out, (not by me )
    Sorry if the post is long winded, but I am very proud of that trailer and it illustrates Soundman's point that much can be gained by intelligent design, but with the qualification that you need the right application to make it a viable approach because labour required and thus cost will skyrocket.

  14. #28
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    Yes, totally agree.

    But there are factors that make me interested in investigating alternatives to traditional builds:

    - it's a hobby for me, and I enjoy it
    - time isn't a factor
    - I like engineering and have a technical background
    - I like welding
    - I like designing

    so to further divert the original topic, here are two sketches I did:

    space_frames.jpg

    The top one the rectangular box is 4m x 2m x 0.25m and involves ~109m of tube without "drawbar". The bottom one the box area is 3.6m x 1.8m x 0.2m. The underside of that one is less, 1.2m wide. It's ~83m with "drawbar".

    Unless I could get it down to closer to 1kg/m and $5/m I don't see anything justifying it as an option, except I'd become a tube master. It would also require much more careful design to ensure suspension components are at nodes, and I'd only feel comfortable with some sort of FEA analysis to make sure it was strong enough.

    Cool mind exercise though.

  15. #29
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    Mmm. Waffle trailer.

    The other issue with that design is while a conventional trailer has probably less than a dozen welds that are truly critical, I expect each one of your dozens of connections is critical.

  16. #30
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    I would have more concerns with fit up than welding. I'd tig the lot and have quite a lot of confidence in my welds. And my limited space frame understanding suggests a proper design only has tension and compression at each node. Designing a suitable frame where that is the case would be the issue and possibly insurmountable in itself. Whatever goes on top wouldn't be a problem, a big box essentially with a stiff enough bottom that it is distributed evenly across nodes. But you couldn't just put a jerry can holder or a spare wheel or a jockey wheel in the middle of a span, they'd all have to be carefully placed. And the undercarriage stuff too - spring hangers or links or shocks or whatever all having to be at nodes.

    If you ever sold it on, the subsequent owners would also need to be aware of not being able to modify it by putting attachments on spans too.

    Pure space frame concept == too hard.

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