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Thread: pivoting frames and mounting campers

  1. #11
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    was just thinking about something firmly fixed to the body that comes down alongside the chassis rails but is no way attached to them.

    And maybe tapered away from the chassis at the bottom so there is no sharp wear area on the chassis. It only has to guide the front of the camper


    Quote Originally Posted by kerry View Post
    That's what I was thinking. On dump trucks I've seen a piece of U channel welded to the dump body frame that comes down around the chassis frame. In my case, there is the additional problem of the hardwood spacer. It is held in place by a piece of U channel on the bottom of the service body frame. ?But that channel is only about 3/8 of an inch deep. So if the body moved upward more than that amount that hardwood spacer could easily become displaced. I'd have to weld on some extensions to that 3/8 channel to keep that wood in place.
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    Quote Originally Posted by whatcharterboat View Post
    BTW Remember we are talking about sprung U bolts here rather than the l shear plates associated with normal u bolts.

    But wouldn't a spring loaded U bolt design make it all the more important to have a shear plate in the design?

    Rationale:
    1. Spring loaded U bolts can only grip as tight as the spring pressure
    2. Rough terrain offers the opportunity for the load to "walk" forward or back on the frame as the U bolts loaded and unloaded

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    But wouldn't a spring loaded U bolt design make it all the more important to have a shear plate in the design?

    Rationale:
    1. Spring loaded U bolts can only grip as tight as the spring pressure
    2. Rough terrain offers the opportunity for the load to "walk" forward or back on the frame as the U bolts loaded and unloaded



    Quote Originally Posted by whatcharterboat View Post
    Doug,

    No (at least IMO anyway). On Kerry's truck, the body is fixed at the rear of the chassis preventing any fore / aft movement. Remember Toner's advice.The shear plates in this case would also restrict free vertical movement where it is needed and by being bolted top and bottom of the shear plate (like the one in the drawing) it doesn't look like it would be effective in preventing sideways movement. At least not as effective as the one's Kerry and I have been discussing.

    Remember he isn't trying to stop a tray with a 20T load on it from slamming into the back of the cab, > he's trying to allow for frame flex.

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    Quote Originally Posted by whatcharterboat View Post
    Doug, Do these docs that recommend the shear plates for use with Ubolts make any mention of fixing the body at the rear? Or are they recommended for the use of Ubolts all the way from front to back? This makes good sense if this is the case. Could you check please, if it's convenient? Interested to know? Not that we are likely to ever use ubolts anyway, but it's good info to post up and I still believe what we are talking about will greatly help Kerry and others with similar ex work trucks.

    John.
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    Quote Originally Posted by whatcharterboat View Post
    Sorry Just answered my own question. It say in the fine print of that doc that a minimum of 3 attachment points (using u bolts??) per side is recomended. No mention of fixing anywhere.

    If you think about how everything will move> looking at the drawing , as the chassis drops away from the body with sprung ubolts and the shear plates in place the body will try and move backwards, , Well if it's fixed at the rear and therefore unable to move,the attachment points of the shear plates will just destroy themselves.

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    Quote Originally Posted by whatcharterboat View Post
    Doug,

    No (at least IMO anyway). On Kerry's truck, the body is fixed at the rear of the chassis preventing any fore / aft movement. Remember Toner's advice.The shear plates in this case would also restrict free vertical movement where it is needed and by being bolted top and bottom of the shear plate (like the one in the drawing) it doesn't look like it would be effective in preventing sideways movement. At least not as effective as the one's Kerry and I have been discussing.

    Remember he isn't trying to stop a tray with a 20T load on it from slamming into the back of the cab, > he's trying to allow for frame flex.
    John,

    Sorry for the confusion. I was not referring specifically to Kerry's installation, only to the general concept of using spring loaded U bolts.

    Other's who happen onto this thread may not be following the specific application (Kerry's) and I wanted to get the shear plate concept out on the table before somebody built using all spring loaded U bolts with no solid attachment anywhere.

    If it was me, as you suggest, I'd add the type of extended channel/guide you and Kerry are talking about with any spring loaded U bolt system so that the payload would stay latterally located. But again, I am qualified to tie my shoes but not qualified to engineer or recommend anything related to this.

    For people in the states, there are LOTS of shops that put work payloads on medium duty trucks in the USA. It would pay to talk to a few about what they've learned over the years.

    Doug

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  2. #12
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    Default Unimog pivot systems as they relate to the FG

    Background:
    The Mercedes Benz Unimog has been produced for over 60 years. It began as a universal power implement/chassis primarily targeted to the agricultural market. It has evolved into a multiple-product line offering that covers everything from its agricultural origins to municipal implement platform to ultra-capable heavy off-road trucks.

    Forums:
    There is a Unimog specific topic area on ExPo here: http://www.expeditionportal.com/foru...splay.php?f=26

    There is a Unimog topic area on BenzWorld here: http://www.benzworld.org/forums/unimog/


    Relationship to the Fuso FG:
    Some people consider the FG to be a "poor man's Unimog." This is somewhat of a misconception, as the FG is much more of a road truck than an über-capable off-road vehicle such as a classic Unimog. About the closest you can get with that analogy is the current generation UGN series Unimogs.

    The current generation UGN (U500 in the US) Unimogs are somewhat similar to the Fuso FG. The previous generations of Unimogs, what we all think of when we think of a Unimog, are less so. The current UGNs have ladder frames and standard drivetrains.

    The previous generations used torque tubes to completely isolate the driveline from the chassis. The classic Unimog chassis itself was designed to be very flexible. All components were mounted with 3 or 4 point pivot systems: motor, cab, payload, etc.

    The Fuso FG is much closer to a UGN than a classic, old-style Unimog, to which it shares only distant comparitive features (diesel power, 4x4, etc.). Differences between an FG and a UGN include, but are not limited to: 33k GVW, portal axles, lockers, 20/22.5 tires, SRW and a higher frame rail height. The frame on a UGN is still twisty, but the lack of torque tubes means much less isolation between the driveline and the frame. The UGN is less capable off-road than the previous generation. The UGN still uses a 3 or 4 point pivot system for its payload.

    Classic and current generation Unimogs use 3 and 4 point pivot systems to mount components (motor, cab, etc.) and payload(s). Payloads are supported and isolated using a 3 or 4 point pivot system.

    Some 3rd party expedition campers such as Unicat also use 4 point pivot systems, regardless of if they use a Unimog chassis or not. There is much cross-over in discussion between the Unimog chassis 4 point pivot system and 3rd party, aftermarket systems.


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    Quote Originally Posted by lehel1 View Post
    hi again

    our unimog camper has a torque tube that runs thru all three camper side mounting plates on our 4 point mounting system. i'll see if i can explain this alittle,

    the front and rear mounts are on a center bolt, so if you can imagine you have a top cross plate 3 feet long or so mounted to the camper bottom, the other half monuted to the truck frame. i beleive there been many pic's of this shown on this site. now the center mount is a fixed mount across to both truck frame members. o.k. so three mounts are there, now there is a tube welded from the rear camperside bracket to the center camperside bracket, then another tube continuing from the center bracket to the front camperside bracket. well, i'll get one of you to post pic's of this.

    anyhow, one thing i noticed is most custom 3 or 4 point mounting systems i've seen so far seem to be at the extreme ends of the truck frame. our unimog frame which is the stock setup from unimog has the rear and front mounts in alittle over two feet in from each end.

    our current plan is to use our 14' flatbed as the top frame supporting the camper fully flat from front to rear, and mount a 4 point system closer in from each end on the truck frame. this perhaps will bring the front mount almost 2.5 feet or alittle more back from the cab right where the reinforcement plates are for the step down frame, the back one will be in from the end almost 4 feet (which includes the 2 additional feet added because of the 14 foot flatbed) putting it very close to the rear spring shackles. with an existing camper frame i beleive there won't be a need for the mounts to be futher toward the extreme ends.
    our concern at the moment is any up and down movement of the upper frame vs the twist factor of the truck frame between the three mounts.

    pics will tell more and i'll work with one of you soon to get these posted on here

    cheers lehel and laura

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    Quote Originally Posted by charlieaarons View Post
    It's been stated in this thread that Unimogs use a rigid mounting in the center and a transverse pivot at both ends.
    Unicat, at least, which started with Unimogs but now uses all sorts of medium and HD chassis, uses transverse pivots at both ends and a trunnion system at the center which allows fore/aft motion. Furthermore, the right and left female parts of the trunnion are separated so if the two frame members wish to flex differentially in the vertical axis, they can do so.
    I use the interior of the male tubing which is mounted to the camper to store my sewage hose.

    Charlie

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    Quote Originally Posted by Robthebrit View Post
    I have some notes somewhere regarding 3point vs 4point mounts from when Eddie built my camper mog.

    A mog typically has a spider frame which 3 point mounts to the frame and gives a rigid platform which you can use for whatever. Initially my box was going to be put on this spider frame but the platform is about 8 inches higher than it needs to be. Eddie was a mechanical engineer and decided to mount camper shell directly to the frame to get the center of gravity that much lower, this is pretty much what unicat and those folks do but I don't know how they decide where to put their mounts.

    He ultimately decided on a 4 point mount which is rigid in the center and pivoted at front and back. He did a load of tests figuring out where to put the pivots, weather to use 3 or 4 points and where to place the center bar. His reason for moving the pivots around was to find the place that had the least movement in the pivots so he never exceeded them and stressed the box. This is very specific to how a mog frame, specifically a long wheel base 416 and the info may not be applicable to anything else, regardless I'll dig it out and share it.

    When emailing with him at the weekend I as how much thought he put into the reverse problem of the load stressing the frame/truck. He generally put no thought in to because he was under gross weight and the mogs GVW rating is for an absolute worst case scenario, something 20 degrees of frame twist, off camber, 45 degree hill, locked diffs with a single traction wheel. The only thing he did to the frame was to modify the front cross member as its a common place for a 416 frame to break due to the engine, the suspension mount and the power steering all being in the same area.

    [snip]

    Rob
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    Quote Originally Posted by charlieaarons View Post
    Unicat uses a transverse pivot in the front and rear and in the center a trunnion that allows fore/aft pitching with a cylindrical rubber bushing. It obviously can't pitch when the frame is in the rest position because the pivots only allow transverse motion. In other words, the frame can twist below the camper but the camper stays steady. These pivots/trunnion(s) are mounted directly to the main frame, there is no subframe.

    Charlie

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    Quote Originally Posted by kerry View Post
    I don't think those kinds of mounting systems could be used with commercial campers like Bigfoots or the Texson I have because the camper itself is not built strongly enough to be anchored directly to the truck frame. Some kind of secondary bed system would be necessary. There's a guy in upstate NY with an FG and a standard delivery truck box converted to a camper. I wonder how his has been holding up. I doubt it has any kind of pivot system.
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    Quote Originally Posted by Robthebrit View Post
    That's the trick, a stress free mount does not require the box to be strong. My camper is made of fiberglass and foam panels. The three/four point mounts themselves are quite big but between them they provide a torsion free platform (3 points make a plane) on which you can much mount the more fragile shell.

    Doug's Fuso is a great example, its big foot camper that is properly mounted and camper has not suffered any stress. By itself a big foot is not that strong, if that camper was mounted directly to the fuso frame is would have broken apart. The flex is even worse for a unimog (on a mog the engine, trans, cab and everything else is 3 point mounted to allow the frame to flex), my camper has no signs of stress but again if my shell was mounted to the frame it would get torn apart.

    The only way to mount to the frame is to have a stiff frame to begin with (I think the eco roamer is going down this path) or to have a box that is so strong it will stop the frame from flexing, ultimately forcing a flexible frame to be rigid with a box will cause to something to fail/warp/crack.

    Rob
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    Quote Originally Posted by kerry View Post
    I understand that the box does not need to be that strong. All I'm saying is that the floors of commercial truck campers are not designed to have 3 or 4 mounts directly attached to them as the sole weight carrying points. They need some kind of frame or bed underneath.
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    Quote Originally Posted by ntsqd View Post
    I see value in more than three points for mounting a potentially heavy or high inertia structure to a ladder frame in that the point loads are reduced. Most of what I've read in these body builder's links are pointed at making the loads distributed loadings rather than point loads. From a design perspective having the loading introduced to the whole top of the frame rail, as is commonly done with delivery box vans, makes for a more economical frame design. Point loads require that the frame be stronger or that the payload maximum be reduced because they are essentially a bridge spanning the distance between the loading points.

    What I don't like is that no matter how you do it, more than three points means that you are introducing some torsion or bending into that structure. After three points the odds of any further mounting points always being co-planar aren't real good. Can always design in some compliance to a joint, but then how much load carrying is the joint doing?

    Considering this, I'm wondering about a sub-frame that accepts the point loads and distributes them to the truck frame. Cleverly done I suspect that the CG hit would not be significantly more than just the height of the distribution portion of the sub-frame. There is no reason that the joints themselves need to be on top of this sub-frame. Depending on desired "articulation" and the clearances required for that, they could be below the level of the top rail of the frame itself.

    anyway, Food for Thought.
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    Quote Originally Posted by Robthebrit View Post
    I just went to measure my 416 frame for a reference..

    Its a C section frame, single piece with a bend in the middle, 6.5 inches high, 2 1/8 wide and about 9mm thick. Its difficult to get any info on what a mog frame is made from because Mercedes say if you break the frame you scrap the truck (I think its illegal to fix frames in Germany). The gross weight of a 416 is about 14500 pounds.

    Attached are pictures of the camper mog mounts, all the weight of the camper is really on 6 points because the front and rear pivots are the cross members so the load is spread to both sides, the center mount spans the frame. The front and back are pivots with a rubber bushing. The center mount is somewhat rigid but it can pitch forward and back on a rubber mount which you can't see it in the picture, its above the bar. The center mount is just in front of the rear axle, the front mount is about 3 feet in front of the center and the rear mount is about 3 feet behind the center and behind the rear axle.

    You can see the floor of the camper is somewhat structural, it's made of 1 inch of marine plywood but the mount isn't really just a point. The camper side of the mount, which doesn't twist, is a piece of C section about 4 feet wide. The camper is really mounted on 3 bars and not 4 single points, the term 4 point mount refers to how it connects to the frame. In theory I could put a metal plate on the mounts and then put a more fragile shell on the plate (this is kind of what the original unimog spider frame does).

    Edit: I'll measure a 1300L frame later on, from looking its way bigger than a 416 frame and its gross weight is still fairly low at something like 16000.

    Rob
    Note: I can't copy the links for the photos over. This post is from the Broken Frame thread. http://www.expeditionportal.com/foru...t=24225&page=7


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    Quote Originally Posted by Robthebrit View Post
    I forgot this image (you can't add to the attachments in an edit)..

    This is the frame from the parts manual, my camper attaches to the cross tube immediately to the rear of the bend and the cross member behind the axle/spring mounts. The sway bar goes through the tube in front of the axle/srping mounts and the center rigid mount is directly above this.

    Note: I can't copy the links for the photo over. This post is from the Broken Frame thread. http://www.expeditionportal.com/foru...t=24225&page=7

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    Last edited by dhackney; 03-31-2009 at 08:00 PM.
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  3. #13
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    post from the broken frame thread: http://www.expeditionportal.com/foru...t=24225&page=7

    I will try to answer as many questions as I can in batch mode.

    a) We had to use a rear pivot point on our 3 point frame because we needed to lock the front of the camper to the garage. If we'd used a front pivot the camper would have contacted the garage or we'd have had to raise the camper to clear it.
    b) Excellent photos of the legendary mog 4 point system. It's a lot easier to understand with photos. I think for those of us who are not engineers, it's like two 3 point pivot frames locked together and mounted back to back on a limited motion teeter-totter. We were unable to use a system of this type due to the camper/garage interference situation. The garage is rigidly attached to the frame. Our 3 point pivot frame is rigidly attached to the frame at the aft wall of the garage. If we put the entire payload on a new subframe and install it on a bigger truck we will probably use a four point system of this type to avoid having another very long distance between the fore and aft load points on the frame.
    e) I am in 100% agreement on the superiority of a building full of factory engineers with teraflops of computing power and a test track out back compared to anything we'd do under the shade tree. That is the fundamental reason we bought the FG rather than an aftermarket/3rd party system - it was the only factory 4x4 on the U.S. market.
    e) As pointed out, a 3 point pivot frame that is attached on a cross member is not really a 3 point frame since it is distributing the load onto the main frame rails at two points. I had never realized that before. I feel better about myself and our Fuso already...
    f) The biggest challenge with our 3 point pivot frame design (and I suspect Michel's) is that the rigid front mounts are a very long ways from the rear pivot mount. That means the entire load of (in our case) the camper, most of the external storage boxes and the pivot frame itself is being applied to two widely separated longitudinal points. To make matters worse, from the frame's standpoint, the rear pivot point is way back there where the frame tapers down and its section modulus is low. I think this is where we over-stressed the frame, by applying forces at the extremes of the dimensions of the frame.
    g) I think the FG is a great chassis. The only problems we've had with anything even remotely connected with Mitsubishi Fuso are things we modified or changed.

    Doug

    Note: post edited for relevent content

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  4. #14
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    Here is a kind of related picture...

    When we moved house we had a POD with the loose ends, when they bought the pod full there was no issue but when the guy came to pickup the empty he backed off the driveway and high centered the truck.

    Check out the pictures and the twist on the flat bed, this is what you would call a rigid frame truck. This amount of twist would pop the sides on any fiberglass shell.

    [Getting him out was something different... neither of my mogs could budge him they just dug all 4 wheels into the ground, only had one strong enough strap so couldn't pull with both. The guy up the street has a small-ish cat front loader and that managed to pull him free]

    Rob
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    You don't inherit the world from your parents, you borrow it from your children.
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    1979 Unimog 416 Expedition Camper
    1974 Unimog 421
    2004 Dodge Ram 2500, 4x4, Double Cab, Cummins Turbo Diesel
    2006 25' Airstream International CCD
    2009 Harley Davidson

    Sugarloaf, Boulder, CO

  5. #15
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    Default Goals, Philosophy, etc.

    RE: more thoughts on camper box suspension
    1. The more you spread the load across the length of the frame the better; this implies the more load points you have, the better
    2. Simple is good. Add simplicity until you have the fewest components accomplishing the goal possible.


    Your goals are:
    • Provide location between the camper box and the frame on all possible axis of movement, e.g., lateral, longitudinal and axial. That's a fancy way of saying the camper box stays where it's supposed to on top of the Fuso.
    • Provide for independent movement of the Fuso FG frame that does not load, stress or twist the camper box.


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    Quote Originally Posted by kerry View Post
    Why are expedition camper builders more focused on complex mounting systems compared to the commercial markets?
    My somewhat flip answers would be:
    1. Peer pressure
    2. Too much time on our hands
    3. Being much, much, much (try infinitely) too overly focused on the truck prior to departure and subsequently discovering what it is all about
    4. Internet access to ExPo
    5. Trying to build a truck that is ideal for the entire planet instead of the 80-90% we could access with a VW Westphalia
    6. Building to handle conditions that we "might" encounter rather than those we are certain to encounter


    Based on almost two years of living out here, building a pivot frame and breaking the truck frame, my answers would be:
    1. With the FG frame, you really do need to allow for frame flex stresses on your payload if you are going anywhere other North American paved or graded roads.
    2. If you come out here, you will absolutely encounter conditions that will put severe stress on anything rigidly bolted to an FG frame, even on market town roads.
    3. Most (OK, almost all) North American Commercial Off The Shelf (COTS) campers are not built to the strength of a typical service body. If you are going COTS, you need to isolate that relatively fragile camper from the FG frame somehow.



    Quote Originally Posted by kerry View Post
    But my guess is that expedition FG campers are only rarely exposed to those kind of extreme frame flex inducing conditions.
    Real world, full-time overlanding is not about seeking out extreme frame flex inducing conditions. It is very different from weekend or vacation sport 4x4, etc. Most of your time out here is spent on market town roads going from one interesting place to another. Having said that, some of those market town roads would not qualify as roads in the U.S. Things are different out here in that regard.

    Quote Originally Posted by kerry View Post
    Is it wise to build for those conditions or do as the commercial body installers do? Or, if frame flex is something that should be considered under all conditions and not just extreme conditions, why aren't commercial truck body installers using pivot mounts of some kind?
    Like any good consultant (I'm still a card-carrying member of the guild), my answer is "it depends."

    If you know for certain you are staying in North America and you'll be on graded BLM roads, graded gravel or pavement, I don't' think you need to worry much unless you are using a COTS camper.

    On the other hand, if you are coming out here with a payload that is less than battleship over-engineered, strong and capable, then you should address the issue. You will definitely, without doubt, be put into conditions out here that will take the FG frame to its limits. If you don't want your camper taking those stresses, isolate it from the torsional rotation of the frame.

    If I had to choose, I'm a lot happier about breaking our truck frame, something you can get fixed literally anywhere in the 3rd world, than our camper, which would have been very challenging to repair.

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    Quote Originally Posted by DiploStrat View Post
    Pivot frames are interesting, expecially for rock crawlers.

    But pivots do nothing about washboard and, in my experience, washboard is far more of a problem for long range/term overlanders than twisting.

    Washboard is fairly rare in South America but it is THE issue in Africa. (To be fair, South America can produce a pot hole or two, but that is not really the same.) I defer to our Australian friends, but it is my impression that the road trains are pretty good washboard makers. I know nothing about Asia.

    So do spare a thought to your suspension - as in it needs to be softer and better damped than you think. And if so, it will absorb a lot of twist.

    FULL DISCLOSURE: These comments based on experience with many vehicles, not/not including a Mitsubishi truck.

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    Quote Originally Posted by DiploStrat View Post
    washboard. (aka tôle ondulée, corrugations, etc.)



    About one metre (3 feet) peak to peak and about .3 metre deep. The killer is the absolute regularity of the spacing of the peaks and valleys. (The conventional wisdom is that washboard is caused by the tramping of heavy vehicle tires. I am inclined to believe this as the roads in the Central African Republic actually have very little washboard – but then they only see a dozen trucks per day and sometimes only one or two per month. Cameroon, on the other hand, has heavy traffic and the washboard is awful. I would love to hear from our Australian cousins on this.)

    N.B. Like all bad roads, washboard is notoriously hard to capture in an image.

    Eventually, every single bolt on the car begins to loosen.

    This is a different problem from grinding through deep holes, etc. Those really do torque your frame.



    Ideally, you must be able to do both.

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    Quote Originally Posted by GroupSe7en View Post
    To quote the Bard: "The heart-ache and the thousand natural shocks; The frame is heir to.."

    I've been following the various frame flex; mounting systems; disaster in the making; etc. threads with great interest.

    Is it not true that:
    • frame flex is designed in as part of the suspension
    • the frame flexes after the active suspension has exceeded it's asymmetrical limit
    • rigid structures mounted to the frame are at risk from torsional loads transmitted to them from the frame
    • the frame is not at risk from the rigid structure
    • the stresses on the frame are induced by the load the frame must carry
    • these stresses are mitigated by the active suspension
    • the combination of frame strength; suspension capacity; and road environment determine the gross weight range the vehicle can operate within
    • vehicles the operate within those three parameters have no worries (this is why many commercial 'boxes' can be mounted directly to the frame)
    • vehicles that overload or operate in environments outside of the design specs are at risk (that would be us)
    • every time the suspension fails to isolate the frame from the environment, stress is introduced into the frame/load.
    • since any system can only handle a finite amount of stress - the frame eventually fails

    It is not flexing that kills frames, it is the stress the load puts on the frame when the suspension is overtaxed that kills them. For instance, the stresses induced by washboard style roads (an environment outside the frame/suspension's design envelope)

    The key to maintaining the integrity of the frame is to change the inputs into the design envelope equation. If we increase the value for the operation environment (rough roads, etc.) we must increase the value for either the suspension or decrease the load (or both).

    The purpose of mounting systems is to protect the 'box' - it has nothing to do with protecting the frame.

    The system must be balanced within the design envelope. Flexible frame mounts are required for systems where the operating environment exceeds the capacity of the suspension to isolate the frame/load from the road.

    Twisting kills the rigid structure - pounding kills the frame.

    Iandraz, I think your build should address these issues.

    Cheers,
    Mark
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    Quote Originally Posted by DiploStrat View Post
    Mark,

    Thank you for a neat job of summing things up. Here is a tangential/parallel review:

    -- Most of us (that would include me) are more familiar with dealing with rough roads in a Land Cruiser/Rover/Blazer SUV platform. These vehicles have strengths and weaknesses, but the most important point is that they are relatively small and rigid. Such flexing and twisting as they do has little effect on the body or cargo. Everything changes when you move to a pickup/truck chassis carrying a separate cargo.

    -- Weight and space are always issues as the overlander (as opposed to a weekend camper) is planning travel outside his/her country of origin, probably into less develop parts of the world, and thus needs to carry more heavy stuff (fuel, water, parts, tools, food, batteries, etc.). The amount of stuff, and thus weight, can spiral alarmingly.

    -- Similarly, the overlander is probably thinking of a trip that measures in weeks and months, rather than hours and days. Thus camper space and comfort and facilities (how long are you comfortable with a squat toilet?) become real issues. Thus the volume of the camper expands and, as everyone realizes, it is much harder to build a large structure that is strong and light.

    -- As Mark pointed out, there are several systems at play. The easiest is the engine/drive train; they are all pretty good. Unlike the rock crawlers who love to break axles, an adequately sized overland vehicle rarely breaks much. Modern emissions controls, etc., are more of an issue for fuel availability or the annoying halt that follows the sudden failure of some electromagic device. Availability of replacement tires (or resistance to flats) is more of an issue than “off road” traction. While 4x4 is probably essential for strength, most overlanders rarely use 4x4 except for getting to a nice campsite off the road. A quick review of this forum, will reveal, however, that this is precisely what most members worry about. This can lead to an unpleasant epiphany. I have seen it happen in as little as 1,000 km of Saharan washboard.

    -- The least understood and studied is suspension. A pity, because, as Mark noted, the right suspension does several essential things: 1) It protects the cargo (human and material) from impact and vibration, and 2) It also allows for the vehicle to keep the wheels on the ground when grinding through mud holes, etc. Many of the companies working in this area focus on getting the vehicle up to allow the mounting of big tires while ignoring the issues of free travel and damping. Again, fine for the local mud hole or the Rubicon trail – dangerous for overlanding.

    -- In a “perfect” world, an overland vehicle would have a totally compliant suspension tied to a perfectly rigid frame. “Underneath” the frame, everything could move in every desired direction so that nothing would pass to the frame. (Think Citroen DS.) “Above” the frame, nothing would move, so that any cargo, of any shape, size, or strength, could be attached without worry about damage.

    -- In the real world, the suspension has limits, at which point it passes vibration, impact, and torsion to the frame. In turn, real world frames are designed to bend rather than break, and thus the cabin and cargo must either reinforce the frame or be isolated from it. Or a bit of both.

    -- Most of the “cargo” that overlanders want to carry is a camper. Most commercial campers are designed to plug and play with a variety of trucks and to be used on good roads. Most commercial campers are not designed to be strong enough to strengthen the frame on which they are mounted, rather they depend on the frame to support the camper. A pivoting sub frame is one clever, proven method of tying a relatively fragile camper. The problem is that a pivot system can be a bit like a dump truck as opposed to a flat bed. The flat bed spreads weight cross the entire frame, while the dump body concentrates it at three or four points and further concentrates it when the wretched mess dumps. Put your rear pivots too far back on the frame, dump while moving, etc., you can see how the forces can multiply. A pivot subframe must be designed to distribute, not concentrate its forces on the factory frame.

    So, as they say in West Africa, “what to do?”

    -- There is no perfect answer, everything is a compromise.

    -- The goal is to keep the forces and loads, at every point, below the limits of each of your “systems”; suspension, frame, cabin/cargo.

    -- One obvious point – you need a bigger truck. Look closely at the history of companies like Earthroamer and Provan. They have to warranty their products and they lose money and sales if they fail. Both companies have steadily upped the size of the truck that they use, WITHOUT increasing the size of their camper. There is a message here.

    If it were cheap or easy, the commercially available overland campers would not be so $$%%# expensive.
    ----------------------------------------------------------
    -------------------------------------------

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  6. #16
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    Default Physics, etc.

    --------------------------------------------------

    Quote Originally Posted by kerry View Post
    I have a question for the more engineer minded amongst us. Looking at this picture of Doug's pivot frame:

    http://www.hackneys.com/mitsu/photos...3/image001.htm

    It looks to me as if the load of the camper is concentrated at the three points of the pivot frame with the rear load being carried at the pivot bolt right at the very back. Does this set up in any way increase the possibility for frame flex compared to a commercial truck box which would mount directly on top the frame rails?
    ----------------------------------------------------------------

    Quote Originally Posted by ntsqd View Post
    Kerry, It will allow for more frame flex, which is the whole point. Trying to make a ladder frame that long rigid enough to not flex under the expected loads isn't really feasible. Mogs use this approach, three point loadings, quite successfully. It's the old 3 points define a plane thing. Adding a fourth point or more usually isn't a problem in theory, but we've all sat on that 4 legged barstool with one slightly shorter leg.
    -----------------------------------------------------------------

    Quote Originally Posted by kerry View Post
    I understand how it allows for frame flex on uneven terrain. It seems to me that under those conditions, the frame is 'twisting' to accommodate the uneven terrain. What I was thinking was the the long distance between the load points allows for vertical flex on rough roads which, all other things being equal, are conditions under which frame flex isn't necessary. I don't have any idea as to how both conditions could be dealt with in a single design
    ----------------------------------------------------------------------

    Originally Posted by kerry
    I have a question for the more engineer minded amongst us. Looking at this picture of Doug's pivot frame:

    http://www.hackneys.com/mitsu/photos...3/image001.htm

    It looks to me as if the load of the camper is concentrated at the three points of the pivot frame with the rear load being carried at the pivot bolt right at the very back. Does this set up in any way increase the possibility for frame flex compared to a commercial truck box which would mount directly on top the frame rails?


    Quote Originally Posted by FusoFG View Post
    I don't think the 3 points increases the possibility of frame flex because for the allowable load (about 8000 lbs) the frame is supposed to flex to allow the wheels to stay on the ground.

    That's why the frame is riveted and bolted together and not welded into one rigid piece.

    The problem with just 3 mounting points is that concentrating the load in just 3 places may exceed the capacity of the frame at one of those points. Even though it's less than the maximum payload.

    The Mitsubishi body builder manual says that you have to distribute the load over the frame and not concentrate it in just a few places.

    That's why, depending on the load, a 4 point mount is better than a 3 point mount. It spreads the load over more of the frame.

    Unimog uses a 4 point frame.

    If you think about it, a 3 point mount is really only a 2 point mount. The 2 flexible mounts at one end are really in one place horizontally along the frame. And the 3rd pivot is at the other end of the frame.

    A 4 point mount, like the unimog, would distribute a third of the weight at one end, a third of the weight in the middle and a third of the weight at the other end.

    Myself, Darrin and others have used 3 point mounts, but my load is under 4000 lbs, less than half the maximum payload. Plus in Darrin's case, he used a sub frame that spread the load over the entire frame much like a normal truck body would do.

    Maybe if you are closer to or above the maximum payload, a 4 point mount is better.

    I can't tell from the picture where the break is either, but in addition to using a 3 point mount and exceeding the maximum payload, Doug also extended the frame.

    If his extension was more rigid than the rest of the factory frame it could have created a stress point or hot spot where the normal flexing of the frame was concentrated and caused a fracture from repeated bending at one spot.
    -------------------------------------------------------------

    Quote Originally Posted by kerry View Post
    By frame flex, I had in mind what you call a hot spot of normal flexing. I agree that a 3 point pivot is really 2 points and that a mount halfway between the ends would reduce that hot spot of flexing. Looking at the build photos and the break photo, it appears the break is halfway or so between them. I'd guess that with the pivot mounts at each end, the 'hot spot' of flex would be somewhere between the ends of the spring shackles.
    ---------------------------------------------------------------

    Quote Originally Posted by Bajaroad View Post
    A question for those of you that have torsion-free sub-frames, whether 3-point or 4-point.

    I understand the idea that the truck frame twists independently of the camper frame on uneven ground. But I was wondering if there is an affect on the stability of the camper while driving on the road, especially in the case of the 3-point frame.

    In other words, if I take a hard turn on the road, does the camper flex outward on the axis of the sub-frame pivot?

    Or maybe the shear strength of the camper walls keeps everything rigid?
    -------------------------------------------------

    You will experience roll / flex of the camper / frame assembly in response to turns, road tilt, off-camber 4x4 use, etc.

    We installed 50/50 Fox racing shocks on the frame to quiet its motion down and it made a huge improvement in handling and feel of the entire rig.

    ---------------------------------------------------

    Quote Originally Posted by kerry View Post
    Bruce posted pictures recently on a thread here showing his air bag based pivot system and shocks to dampen up and down as well as side to side
    movement.

    The pictures are here:

    http://www.expeditionportal.com/foru...?t=5949&page=3

    He recounts the motion of the camper while driving.
    --------------------------------------------------

    Quote Originally Posted by Tom_D View Post
    On a typical three point system, if the two points of contact are on the middle of the frame and the single contact is on the rear end, the camper stays quite level while the rear wheels can twist with the ground. My camper moves over rough and rutted dirt it is very stable on the highway with no shocks. I know of no Unimogs that need shocks either.

    Tom
    -----------------------------------------------------

    Quote Originally Posted by charlieaarons View Post
    A 3 or 4 point mounting system is not a "suspension". It doesn't matter if my camper weighs 20 or 100 tons, it wouldn't lean outwards on turns (relative to the frame) unless the frame rails flexed assymmetrically. It should merely allow the frame to flex without putting torsion or stress on the floor.
    My advice: don't do what you mentioned in the last post, designing a true suspension for the camper. You will be asking for instability. Copy Unicat's system.

    Charlie
    -------------------------------------------------------

    Quote Originally Posted by haven View Post
    Avi Meyers has a short movie on the Unicat Americas website that illustrates the motion of a torque-free subframe. The movie shows Avi driving his International 7400 Unicat over rocky, rutted, and sandy terrain in North Africa.

    http://www.unicatamericas.com/video/international.mov

    Like FusoFG says, you can clearly see the camper stay parallel to the rear axle as the cab of the truck stays parallel with the front axle. The camper doesn't flop around on the frame of the truck, it just moves in unison with the rear axle.

    Chip Haven
    ---------------------------------------------------------

    Quote Originally Posted by charlieaarons View Post
    I was speaking "metaphorically" to point out that the Unicat mounting system only responds to frame rail assymmetric motion, not inertial or gravitational loads on the camper. Incidentally it is hinged in the front and the rear with the "fixed" tubular mounting just above the rear axle. There is no subframe except for steel bars embedded in the camper floor tapped for mounting bolts for the brackets.
    Definitely stronger camper boxes exist, like globalexpeditionvehicles, with internal framing and aluminum skin. They are likely heavier as well.

    Charlie
    ------------------------------------------------------

    Quote Originally Posted by Robthebrit View Post
    What Charlie says is true for all properly configured multipoint/pivot mounts, nothing special about the Unicat system. The pickup beds on my other unimogs have a 3 point mounts too.

    As Unimogs go the U500 is not that bendy, its designed as a semi rigid frame. The UHNs (U3000,U4000 and U5000) still use the ladder type frames as used in the older 1300s and 406/416s. You can get a huge amount of twist across the frame, multi point mounts are essential and on these mogs everything is mounted in a 3 point system including the cab, engine and tranny.

    Rob
    -------------------------------------------------------

    Quote Originally Posted by Bajaroad View Post
    You Mog guys are preaching to the choir - I'm on board with the properly designed 3-pt system.
    My point is that a properly designed 3-pt system is not necessarily adequate for a weight bearing flatbed and the bed will respond significantly to inertial or gravitational loads depending on the load and shear strength of the flatbed. I've done the FEA analysis on my flatbed frame sitting on 3pts - a camper box on top makes all the difference.

    A 4-pt system that you describe should reduce that response by about half if the two outsides mounts are in the middle. That's probably the best approach for my flatbed, but I want the subframe to follow the cab as much as possible, so I'm going to stick with 3pts and a supplimental air springs.

    Just to argue . . . I will say your subframe does respond to inertial loads, it's just that the response is too small to notice. Continue to increase the inertial load and the response increases more or less linearly.

    Thanks for the discussion.
    -----------------------------------------------------

    Quote Originally Posted by Bajaroad View Post
    Why would the optimum twist be at the step-up. The modulus is highest at the step-up.

    My question was about the location of the axis of rotation of the twist - see attached. If the 3pt pivot point is not on this axis then it will have a laterial component (sideways movement) when the frame twists. I worry this will place stress on the two stationary points.
    ---------------------------------------------------------

    Quote Originally Posted by egn View Post
    In reality you will never have a rotation exactly at this point because the twist is never fully symmetric. And you also get some bending. So you have to give some room for lateral movement in the bearings anyway.
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  7. #17
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    Default Statistics, Sample Sets, etc.

    From non-forum private communications:

    Of course, we have no way of knowing if any of this will work in the real world.

    ** Statistically, the sample sets are so small right now, and the test conditions so varied, that it is very hard to draw overall conclusions. Don & Kim Green have a non-pivot mounted camper box on their FG and have taken it all over the world. Sample set of one. Joe Blow builds an Alaska Camper on a PuffnStuff 4x4 3550 and drives it to Wyoming and back. Sample set of one. It is hard to spot real trends, rules or standards from what is out there right now. About all you can say for certain in world overlanding is that the old round nose MB former German army/border patrol/police radio trucks work well, last long and are about the easiest things to get fixed anywhere in the world. Beyond that, IMO, there just isn’t enough data to support any broad statements.

    ---------------------------------------------
    Last edited by dhackney; 03-31-2009 at 07:38 PM.
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  8. #18
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    Quote Originally Posted by Robthebrit View Post
    Check out the pictures and the twist on the flat bed, this is what you would call a rigid frame truck. This amount of twist would pop the sides on any fiberglass shell.
    Based on our travels, you will experience that amount of road surface offset out here in the developing world, even if you are living the typical market town road existance.

    That is a very sobering photo for anyone who thinks they can avoid all flex issues by moving to a "rigid frame" truck. Wow.

    Thanks for posting these shots.
    -------------------------------------------

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  9. #19
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    .
    Quote Originally Posted by UK4X4 View Post
    Its a promo vid, but shows some pretty good shots of the frame flex- camper movement and some close ups of the box mounts., installing and being machined

    If your thinking of frame mounting a box on some rubber hocky pucks ,this may change your mind.

    http://www.youtube.com/watch?v=YgmF8...eature=related
    -------------------------------------------

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  10. #20
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    Thanks Doug - that was an outstanding bit of collection and organization.

    It's great that this topic has a home. I'm sure it will be visited by many non-FG people.

    Cheers,
    Mark

    Porsche Cayenne S "TropiCayenne"
    Honda Ridgeline - go ahead and laugh
    Ducati Elefant

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