pivoting frames and mounting campers

dhackney

Expedition Leader
#16
Physics, etc.

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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/buildup_album_03/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?
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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.
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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
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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?


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.
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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.
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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?
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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.

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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/forum/showthread.php?t=5949&page=3

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

Expedition Leader
#17
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.

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dhackney

Expedition Leader
#18
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.
 
#20
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
 

whatcharterboat

Supporting Sponsor, Overland Certified OC0018
#24
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 .
 

kerry

Expedition Leader
#25
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?
 

dhackney

Expedition Leader
#26
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'.

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
 

whatcharterboat

Supporting Sponsor, Overland Certified OC0018
#27
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.
 

DiploStrat

Expedition Leader
#28
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. :Wow1:
 
#29
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