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  1. #1
    Join Date
    May 2014
    Location
    Leeton NSW Australia
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    6

    Default Low profile trailer chassis

    I've seen some great trailer ideas here, so thought I'd get comments on one I'm planning.

    I'm working on a single axle bolster trailer rated at 1600kg GVM/ATM. I want it as flat as possible to keep the centre of gravity down. It's main job will be to carry a slide on camper that I'm making.

    The one shown in this picture is basically what I have in mind chassis wise.
    http://www.sportsfish.com.au/forum/u...0trailer02.jpg

    The part I'd like comments on is where the drawbar bends to become the chassis rail
    trailerchassis.jpg
    What would you do to make this stronger? I don't want to put the drawbar under the chassis rail, as that would make it higher than I want it to be.

    My current plan is to use 100x50x3 RHS with a wedge cut out to allow it to bend to the desired angle then weld it back together again. To compensate for the cutting, I'm thinking a 5mm thick diamond shaped plate welded along each side of the RHS, extending 250mm each way from the bend and 90mm high at it's tallest point, plus another 5mm thick plate underneath the RHS reaching about 300mm each way from the bend.
    I'm planning a 100x50x3 crosspiece between the chassis rails 50mm behind the bend, and another one linking the sides of the aframe 700mm in front of the bend, and bracing under the sides of the afame as commonly seen on caravans.

    Any thoughts on this plan?

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  3. #2
    Join Date
    May 2011
    Location
    Murray Bridge SA
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    3,339

    Default

    Have you thought about curving the side rails as per some boat trailers?
    What you are planning sounds OK, but I would put the cross member over the join (if you have to go that way), so that the cross member straddles the cut out, less welds involved also.
    Kryn

  4. #3
    Join Date
    Dec 2012
    Location
    WA
    Posts
    68

    Default

    Check out my Track bike trailer build, Ive sunk the drawbar into the chassis right back to the front spring hangers (and further) its very rigid and makes for a low height.

  5. #4
    Join Date
    May 2014
    Location
    Leeton NSW Australia
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    Default

    Thanks for your comments Kryn and Aufitt.

    I did think about getting the rails bent like boat trailer ones Kryn. The nearest boat shop is 50 odd kms away, but that wouldn't matter if it was the best way to go. I'm not sure how frustrating the variable curve might be as I attach accessories to the front of the chassis though. Putting the cross member over the chassis would seriously hinder my efforts to keep things as low as I want them, I can use a fairly large plate under it to tie it all together though.

    That's a VERY interesting build thread Aufitt. I think I'll probably go the way you have with putting the A inside the chassis rails. It gives some extra bracing to what I was planning, and I've now realised there's no way I can keep this trailer in the NSW 'under 254kg tare' category because of it's length, so a little extra framing wont matter weight wise. The work you've put into your spring hangers is amazing. Mine will just be standard there, as the extra 20mm or 25mm saved aren't that critical and I'm happy to use an overlay axle if I need to lose another 50mm or so. Upgrading my chassis rails to 3.5mm is something else I think I'll do, just a little extra to strengthen it further.

    Thanks again, and any extra ideas will be most welcome too.

  6. #5
    Join Date
    Sep 2008
    Location
    Perth
    Posts
    363

    Default

    My thoughts are - 100 x 50 x 3.0mm seems fairly heavy for your cross members. You might be able to go lighter than that. Some of the experts on here will know, or look into similar sized caravans and what they're made of.

  7. #6
    Join Date
    May 2014
    Location
    Leeton NSW Australia
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    Thanks Legion.
    Because there wont be a floor in the trailer I'm happy to use heavier material for the crossbeams. The chassis rails will be 3500 long, and there are only 4 crossbeams (one at each end of the springs, plus a front one and a back one). With a single axle and a 1600kg ATM, I figure they need to be strong. It'll be more like a boat trailer than a caravan chassis.

  8. #7
    Join Date
    May 2014
    Location
    Leeton NSW Australia
    Posts
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    Got my springs today, and found the first problem with my design.

    The springs are long and strong, but they're also tall. Including the shackles, from the bottom of the chassis to the top of the spring would be 275mm, so I'm going to have to get more creative with my design.

    An overlay axle was already part of my plan, but that's not going to be anywhere near enough with the height of the springs. I thought about lowering blocks between the spring and the axle, and I may still use them too, but I think it will still be too tall.

    The best option I can think of is to mount the springs beside the chassis rails, and flip the shackles so that they hang down from the spring eye to a mounting point at the bottom of the chassis. Something similar to what the following pictures show
    http://coloradok5.com/reviewpics/ordflipprecut.jpg
    and
    http://image.truckinweb.com/f/tech/1...ng-removed.jpg

  9. #8
    Join Date
    Sep 2008
    Location
    Perth
    Posts
    363

    Default

    Again, I'm not an expert but that seems dodgy. If it was for a road-only trailer maybe but I'm assuming since it's for a van it will go down some bumpy tracks. I would personally prefer my springs underneath the chassis rails.

    Did you look into a dropped axle? That's what I used on my boat trailer, 3" from memory.

  10. #9
    Join Date
    May 2014
    Location
    Leeton NSW Australia
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    What problems do you foresee with the side mounted springs idea Legion?

  11. #10
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    Sep 2008
    Location
    Perth
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    I would imagine there's a large shock loaded bending moment induced in that pin and bracket. If I went down that path I would heavily over design it and check it intermittently. I'd personally definitely prefer the springs vertical force component to be aligned with the rails' vertical cross section.

  12. #11
    Join Date
    Dec 2012
    Location
    WA
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    68

    Default

    Quote Originally Posted by -Ripper- View Post
    Got my springs today, and found the first problem with my design.

    The springs are long and strong, but they're also tall. Including the shackles, from the bottom of the chassis to the top of the spring would be 275mm, so I'm going to have to get more creative with my design.

    An overlay axle was already part of my plan, but that's not going to be anywhere near enough with the height of the springs. I thought about lowering blocks between the spring and the axle, and I may still use them too, but I think it will still be too tall.
    Are they Off road springs? Maybe go to slipper springs instead. and lay your chassis rails flat, since you are chasing low profile.

  13. #12
    Join Date
    May 2014
    Location
    Leeton NSW Australia
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    Thanks for your comments guys, I'm progressing, but slowly.

    Quote Originally Posted by Legion View Post
    If I went down that path I would heavily over design it and check it intermittently.
    The design I've come up with will be considerably stronger than the factory made ones shown at those links Legion.

    The shackle mount will basically look like an offroad mount but inverted, and with the side bracing going right across the pivot point and extended down to allow it to bolt into a cross member. The cross member will go right across the trailer under the chassis rails, thus putting the load into the bottoms of them rather than the sides. The bolts (high tensile) will be horizontal, so they'll be carrying the weight on their shear strength (plus clamping friction) not on their threads.

    The front spring hangers will be made from square hollow section 100x100x9 and 250mm long. This is just the right size for the bushes to fit snugly inside, strong enough, and the length will allow 4 well spaced bolts to attach it to the side of the chassis rail. There'll also be a reinforcing plate inside the rail to further distribute the weight. Another advantage of having the hangers that length is that almost half their length can remain square and only the rear part will need the bottom cut out to allow the springs to flex. Trying to draw presentable picture of these parts would take me too long, so I'll just post photos of them when they're done.

    Quote Originally Posted by Aufitt View Post
    Are they Off road springs? Maybe go to slipper springs instead. and lay your chassis rails flat, since you are chasing low profile.
    They are off road springs Aufitt, and the more parts I get together the more I think this is going to sit way higher than I was planning. Flattening the chassis rails would gain me 50mm if I was putting the springs under them, but I'm going to gain about 100mm by inverting the shackle beside the chassis. Slipper springs would gain me a similar amount, but I suspect the ride might be a bit harsh as they're almost 600mm shorter than the springs I'm using, and the slippers are 25mm narrower too.

    My next miscalculation showed up when my hub drums arrived. They're 6 stud ones and the correct PCD, but the centres are made to fit Landcruiser hubs (104mm?) which are quite a bit bigger than the centres of older 6 stud wheels off 2wd commercials. I thought about buying new Sunraysia type wheels which have big centre holes even though they're only 14" rims, but I'm wanting an older style to the trailer, and Sunnys wouldn't fit that. With a bit of digging I found the same parallel bearing hub drums have 80mm centres when they're bored to suit ford wheels, so my solution was to take them to a machinist who can turn the centres down to the 92.5mm that I need for them to fit my wheels. My father has a lathe, but it's not big enough to turn a 10" brake drum on, I'll use it for part of the shackle mount fabrication though.

    Tomorrow I order the bulk of the steel for the chassis. A caravan I looked at which is a similar length has 2.5mm chassis rails, and a 3mm A frame in 100 x 50 RHS, so just to be on the safe side I'm going a bit thicker (3 for chassis and 4 for A frame).

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