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Thread: pivoting frames and mounting campers

  1. #21
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    Smile Paging WhatCharterBoat

    We need some comments by someone who does this for a living.
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    DiploStrat

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  2. #22
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    If someone at my house is skillful enough to take cellphone pictures and then post them I will do it.

    Charlie
    Unimog U500 with Unicat camper; diesel BMW X5 35d, diesel BJ40 Landcruiser and diesel M37

  3. #23
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    Quote Originally Posted by charlieaarons View Post
    If someone at my house is skillful enough to take cellphone pictures and then post them I will do it.

    Charlie
    Charlie,

    If you need help posting them, email them to me and I'll put them on our server and post them for you.

    Doug
    -------------------------------------------

    web: http://www.hackneystravel.com/
    blog: http://www.autopsis.com/
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  4. #24
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    Paging WhatCharterBoat
    We need some comments by someone who does this for a living.
    Fred Sorry mate. I'm in the middle of designing a medium scale wind turbine installation for a remote village in Alaska and addressing all the environment issues whatever they maybe. Just a uni assignment but it's Dday and I'm panicking a bit.

    You've got a great handle on what happens under a truck in the real world anyway (going by what I read in your last big post) and I intend to add more to this discussion soon.

    Back to it .

  5. #25
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    Not quite sure how to formulate my question/comment but I've been thinking about these issues in relationship to my truck. It has a 13' service body on it. The camper sits in the back 8' of this service body. At the moment the camper is attached with turnbuckles at the back bumper and front of the floor section of the camper. This puts the attachment points to the service body about 8' apart. This means that the amount of flex that the camper has to absorb is the amount passed thru the service body in the last 8' of the truck frame. Since the service body is bolted and welded at the back of the frame, the frame rails will have to be level relative to each other at the back.
    Does anyone know the maximum amount of flex that will occur in that back 8' of the frame. Is the flex uniform throughout the frame are accentuated at any points such as just before or after the drop down sections or at the back?

    I've seen pictures of Aussies with truck campers on tray tops on Fuso's. (I think there are some in the thread on the top of this page). They look like standard campers and standard tray tops. How do those rigs survive the stresses of flex? There's a member here with an Avion camper on an older FG mounted on a flatbed. I wonder how long that camper has been on there and how it is been surviving?

  6. #26
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    Quote Originally Posted by kerry View Post
    Not quite sure how to formulate my question/comment but I've been thinking about these issues in relationship to my truck. It has a 13' service body on it. The camper sits in the back 8' of this service body. At the moment the camper is attached with turnbuckles at the back bumper and front of the floor section of the camper. This puts the attachment points to the service body about 8' apart. This means that the amount of flex that the camper has to absorb is the amount passed thru the service body in the last 8' of the truck frame. Since the service body is bolted and welded at the back of the frame, the frame rails will have to be level relative to each other at the back.
    Does anyone know the maximum amount of flex that will occur in that back 8' of the frame. Is the flex uniform throughout the frame are accentuated at any points such as just before or after the drop down sections or at the back?

    I've seen pictures of Aussies with truck campers on tray tops on Fuso's. (I think there are some in the thread on the top of this page). They look like standard campers and standard tray tops. How do those rigs survive the stresses of flex? There's a member here with an Avion camper on an older FG mounted on a flatbed. I wonder how long that camper has been on there and how it is been surviving?

    Kerry,

    I can't begin to answer your excellent questions about the Aussies. John (whatcharterboat) may be able to shed some light on that when he comes up for air from his university obligations.

    For the first questions about frame flex amount transmitted to the service body, I don't think anyone could answer that one. It would be dependent on the specific characteristics of your service body (metalurgy, design, etc.).

    If your camper is reasonably easy to take off, perhaps you could remove it and run some tests, or do them the next time you had it off for some reason.

    A laser level or even a laser pointer is very handy for this purpose.

    When I measured things on our truck, I marked index points on the truck while it was parked on a flat, level concrete pad. Then I took it out and put it into the positions I wanted to measure.

    Perhaps you could do the same and then deflect the suspension to maximum articulation and measure how much your particular truck/service body deflects in the aft 8'.

    Quote Originally Posted by kerry View Post
    Since the service body is bolted and welded at the back of the frame, the frame rails will have to be level relative to each other at the back.
    This assumes there is no flex in the service body across the width of the frame rails, even under maximum suspension loading/deflection.


    Doug
    -------------------------------------------

    web: http://www.hackneystravel.com/
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  7. #27
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    I am addicted. In the middle of my study and I keep saying to myself "just a quick look at expo". I certainly haven't got the time to join in but Kerry, remember what Toner said. I know what he stated about the way frames flex is exactly what happens on an FG. Read it again. It's THE key to understanding all this. Not ubolts but how a frame twists.

    Does anyone know the maximum amount of flex that will occur in that back 8' of the frame. Is the flex uniform throughout the frame are accentuated at any points such as just before or after the drop down sections or at the back?
    So there is theoretically ZERO flex at the rear if it's rigidly mounted there and why the front mounts will try to destroy themselves if its rigidly mounted at the front where all the flex is occurring. Everyone seems to be thinking that the back will flex as much as the front so a 4point pivot will in theory be ideal. They don't move like this.

    Your on the right track so far. It would be good to park it on an extreme angle and see how much the body is trying to flex I suppose.

    See ya later mate.

  8. #28
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    Default From Stephen Stewart

    Stephen Stewart is someone who has been doing this a long time; including travel to Asia, an area where I have no experience. He offers this comment:

    Body Mounting.
    Most people give very little thought to how the body of their campervan is attached to the chassis.

    Imagine a rigid box bolted directly to a perfectly rigid chassis. Now imagine driving along a slightly uneven road. There is no problem; the suspension absorbs the relative movement of the wheels keeping them all in contact with the ground. Of course there is limit to how uneven the road can be. Beyond a certain point the suspension will not be able to keep all the wheels on the ground. (True off-road vehicles are far better at this than on-road vehicles.) Even with permanent four wheel drive, unless you have differential lock, you will loose all traction with one wheel off the ground.

    However no vehicle has a perfectly rigid chassis. In the case of true off-road vehicles the chassis may even be designed to twist significantly as part of the suspension. In the case of on-road vehicles the chassis will twist simply because it is not rigid enough not to. If the rigid body of a campervan were bolted directly to the chassis then it would be twisted each time the vehicle was driven over rough ground and the body would soon buckle or crack. For this reason the body of a conventional campervan is usually mounted on rubber blocks. However these may well be insufficient to prevent transfer of "twist" from the chassis to the body when the vehicle is driven over rough roads.

    The real solution to this problem (found for example on all Unimog campervans) is a torsion-free sub-frame. This is in effect a second chassis mounted above the real chassis at either three points or more commonly two pivots at right angles. These type of mounting rely on geometry, not elasticity, to avoid the transfer of twist. However the torsion-free sub-frame found on Unimogs is expensive, heavy and raises the cabin by 200mm.

    Nearly as good as a torsion-free sub-frame, and a great deal cheaper, is to mount the cabin of the campervan on two parallel rails that rest on the chassis. At one end (often the rear) they are bolted directly to the chassis. At the other end they are attached with springs, that may allow as much as 100mm of vertical movement!

    Conventional campervans, driven for long distances over corrugated roads, are often torn apart by a combination of twist and vibration. Making the body stronger may reduce the damage done by vibration (for example making the furniture of thicker wood and fixing it to the floor, walls and roof) however this will also make the body more rigid and thus more susceptible to damage by being twisted.

    A good overland campervan should have a strong body (cabin) attached to the chassis by an appropriate torsion reducing mounting.

    From:http://www.xor.org.uk/silkroute/equipment/choosevan.htm

    N.B. The "compromise" in his approach is in the requirement for STRONG cabin. The stronger your cabin, the less need for the full on floating frame. And vice versa. There is no free lunch.

    Follow the link for some really good information on overland camper issues and some pictures from the real world of overlanding.
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    DiploStrat

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  9. #29
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    Default call for pics

    Doug, thanks for taking the lead on this useful compilation. I found these two pics in my collection. If anyone has other pics to help demonstrate the concepts discussed in this thread, I'd encourage them to post. For those that have a pivot frame, I'd encourage them to provide materials of construction and dimensions if possible. The more data we can assemble the better.

    4 point.gif
    fauxchassis.jpg

  10. #30
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    A while back I took a video with my camera strapped to my unimog frame while I drove around, to get an idea of the deflection:



    This is unloaded, so deflection will probably be much more with a load.

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