Frame flex fundamental question

k9lestat

Expedition Leader
Besides that diary is what started me being infatuated with off road camping. I kept searchiñg different vehicles and equipment and how to's 90% of the time brought me to the portal.

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Haf-E

Expedition Leader
I think the issue with direct mounting of an expo camper box to a fuso frame is that too much of the frame's twist would be transfered to the box. The spring mounts allow for some frame flex - but a limited amount compared to a three point type mounting system.

The other issue that might need to be considered is the different expansion rates of the steel frame and the aluminum/composite expo camper. A spring mount allows for some variation between the two.

The recommendations from fuso are not necessarily for off-road use. I would trust the experience of the Australians more. The spring mounts would also be the closest design comparable to what fuso reccomends - especially if you use a number of spring mounts along each rail.

I was surprised to read about a three point mount that was set up with the single mount b
ehindvtge cab and the two mounts at the rear. That would cause the most flex I would think and would result in a lot of movement between the cab and box. Switching it around with the pivot at the rear would work much better I would think.
 

SkiFreak

Expedition Leader
I think the issue with direct mounting of an expo camper box to a fuso frame is that too much of the frame's twist would be transfered to the box.
That may be true if the suspension has no ability to move, but more importantly, it will focus the twisting to the area directly in front of the step, which can over stress that area of the chassis.

What I think some people are missing here is that you cannot consider all truck frames as being equal. The step section in the Fuso frame makes it a totally different animal, as does the thickness (or should I say thinness) of the frame rails and how far apart they are. I don't think that comparing apples to oranges has any real benefit.
From what I have been told, none of the Fuso trucks that ATW has mounted bodies on using their spring mount system have had frame failures. That in itself says something I think.

As has been mentioned, the Fuso body builders guide probably does not take into account mounting for extended off-road use.

At the end of the day, you can mount the camper box to the chassis in any way that you like, but comparing mount systems from other types of trucks to a Fuso is not really setting you up for success in my opinion.
 

gait

Explorer
...
I was surprised to read about a three point mount that was set up with the single mount b
ehindvtge cab and the two mounts at the rear. That would cause the most flex I would think and would result in a lot of movement between the cab and box. Switching it around with the pivot at the rear would work much better I would think.
totally agree - though last time I raised that here, a few years ago, it sort of got lost as it didn't fit the wisdom at the time. It was very obvious to me as I'd observed an Isuzu conversion with fixed at rear / pivot at front where owner complained of excessive body roll. In the same vein were my comments earlier about visualising twisting front and rear axles in opposite directions and observing no twist at centre. Not quite of course as the chassis rails are tapered. There is also some rotation at the fixed mounts perpendicular to the longitudinal axis - they can't be rigid.

With my 4.5mt GVM, parabolics and 3-point the amount of travel in the parabolics is such that at times they are almost flat, the amount of movement in the pivot above the chassis rail is about the +-15mm that I allowed for. 5 degrees of twist over the length of the chassis (3380mm wheelbase in my FG649E) is a lot. Try it. Even with no weight on the chassis IIRC it will lift a wheel before 5 degrees. Much like Rob Gray's unloaded test, but his chassis was much longer so it would take much more to lift a wheel. My attachments are (and will be) less than 2000mm apart. Which makes about 15mm separation between sub-frame and chassis at the pivoted end. With spring mount on opposite corners that's 7.5mm at each corner.

Loading the vehicle means more suspension spring movement, less chassis twist.

I eventually came to the conclusion the difference between springs and 3 point is all a bit academic in my setup.

I'm currently rebuilding the sub-frame with spring mounts, not because of the mounting but I had a long cantilever at the rear (chassis chopped off by previous owner) and changed from doubles to singles so extra weight of two spares broke the sub-frame. After about 30,000km on original springs and a vicious bit of track too fast.

I replaced the springs after breaking a few leaves and it was cheaper / more reliable delivery to ship parabolics from Aus than replace in UK. I'd already tried reset.

I'm also moving one spare under the rear (its high because the chassis was cut off) and I'll carry a second carcass. Apart from not wanting to bend another sub-frame it will significantly reduce pitching in the vehicle - it isn't bad, just that there was a noticeable improvements when I temporarily moved the spares into the middle of the box before welding. Not as big as the difference between breaking finger trying to turn radio on and being able to tune it but significant.

For completeness (we've covered chassis twist, roll, and pitch) attention to the roll center and roll steer helped stability in sidewinds (yaw?) - Aus roads can be relatively narrow with "B-doubles" and "road trains" which can blow an unstable vehicle off the road.

My frameless foam sandwich post tensioned drop-top box proved remarkably resilient. It took another 70,000km after Mongolian roadside weld, later patch, and slowly getting worse bent sub-frame before I had to bow to the inevitable. It helps that I'm back home.

I'm not in the least bit worried about "point loading" as there are leaf springs, shock absorbers, cab, engine and transmission, which are all attached to the chassis rails by a variety of points. There are ways of tapering or rounding things so the load is a little progressive and not a sharp point.

I suspect that loading any vehicle to its max and taking it over rough terrain is a recipe for disaster.
 

dlh62c

Explorer
Yup will do. I am in great need of getting smarter. Thanks.
GR8ADV

Your Unicell body is based on a truck cargo body designed to haul freight. These bodies are well built and are very stout. The floor is probability made out of 5/4 red oak with enough support to drive a loaded forklift on. Morgan makes a similar body here in the USA.

Considering where this truck has been; if during the renovation you didn't see any structure damage to the camper body, nor to the vehicle's frame rails, I'd load it up and hit the road. Just make sure there's support blocks in the frame 'C' channel where the rigid mounts are placed.

I don't understand why more people don't consider a truck cargo body and fitting out the interior themselves as the Greene's did. The truck and camper body have been around the world, retained its structural integrity, been renovated and can certainly do it again.

By the way, for what's its worth, I do like the placement of the windows up high as the Greene's designed it. While it limits the ability to place cabinets above the windows, you don't have to bend over to look out when standing up, as in a typical RV.
 
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GR8ADV

Explorer
GR8ADV

Your Unicell body is based on a truck cargo body designed to haul freight. These bodies are well built and are very stout. The floor is probability made out of 5/4 red oak with enough support to drive a loaded forklift on. Morgan makes a similar body here in the USA.

Considering where this truck has been; if during the renovation you didn't see any structure damage to the camper body, nor to the vehicle's frame rails, I'd load it up and hit the road. Just make sure there's support blocks in the frame 'C' channel where the rigid mounts are placed.

I don't understand why more people don't consider a truck cargo body and fitting out the interior themselves as the Greene's did. The truck and camper body have been around the world, retained its structural integrity, been renovated and can certainly do it again.

By the way, for what's its worth, I do like the placement of the windows up high as the Greene's designed it. While it limits the ability to place cabinets above the windows, you don't have to bend over to look out when standing up, as in a typical RV.
make no mistske, I am not suggesting the no-spring option is better. As discussed, having the box center of mass remain in line with the frame as it twists is the holy grail goal. U bolt mounting does not do that. I am not convinced that the spring mounts accomplish this either. But this is likely because I don't yet grasp the subtleties. iIt almost appears that these frame designs are functioning as if they are concerned more about the loading to the subframe and box (structural integrity of the box) than they are about isolation.
.
A super stiff frame, suspension with great articulation and an adjustable articulated joint strategically placed near the front of the frame to allow the front to roll independently would be just peachy. Somebody get on that....
 
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GR8ADV

Explorer
By the way, for what's its worth, I do like the placement of the windows up high as the Greene's designed it. While it limits the ability to place cabinets above the windows, you don't have to bend over to look out when standing up, as in a typical RV.
The windows seem to be placed naturally in the box for the people inside. It would not really be possible to make them lower. We do have a couple cabinets above them. I considered not putting them in given the height of the weight we would be adding.

image.jpg
 

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GR8ADV

Explorer
Ok, thanks everyone for the info. It makes sense with freedom on the rear and semi fixity on the front as shown in the diagram. Where I am/was confused is that the ATW version I looked at had the spring mount on the rear. So I thought that was what everyone was doing. It seems opposite from the Link which had this semi rigid connection at the front (rubber) and significant freedom on the rear. Spring mounts at the front to allow for connection without complete fixity makes sense. One would then just have to provide enough clearance at the rear to allow for frame flex, and mount to the box through the CG. OF course this is all well and good until one parks on a side slope. No frame flex, but what happens to the body, does it lean over?
.
I am sure the ATW and EC versions make sense as well and deal correctly with all the dynamics of a well thought out beast. I would just like to understand it better. Heck maybe that is the 4-point mounting system or something...more reading to do.
 
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GR8ADV

Explorer
GR8ADV

Your Unicell body is based on a truck cargo body designed to haul freight. These bodies are well built and are very stout. The floor is probability made out of 5/4 red oak with enough support to drive a loaded forklift on. Morgan makes a similar body here in the USA.

Considering where this truck has been; if during the renovation you didn't see any structure damage to the camper body, nor to the vehicle's frame rails, I'd load it up and hit the road. Just make sure there's support blocks in the frame 'C' channel where the rigid mounts are placed.

I don't understand why more people don't consider a truck cargo body and fitting out the interior themselves as the Greene's did. The truck and camper body have been around the world, retained its structural integrity, been renovated and can certainly do it again.

By the way, for what's its worth, I do like the placement of the windows up high as the Greene's designed it. While it limits the ability to place cabinets above the windows, you don't have to bend over to look out when standing up, as in a typical RV.
I am not sure how 'off-road' the Greenes got. For all I know they may have not done much to flex the frame. One can travel to much of the world without getting too crazy. AK to South America without leaving pavement is pretty straight fwd if one wants to. So heck, I may blow it all up with one creative trip down to Baja.
 

dlh62c

Explorer
I am not sure how 'off-road' the Greenes got. For all I know they may have not done much to flex the frame. One can travel to much of the world without getting too crazy. AK to South America without leaving pavement is pretty straight fwd if one wants to. So heck, I may blow it all up with one creative trip down to Baja.
Once a vehicle starts jumping continents, the thought of vehicle preservation should be taken into consideration.

Here's a link to a Flickr Site if you want to see pictures of the places your vehicle has been.

Should you want to get in touch with Don & Kim Greene, you can reach them though Harvest Hosts. I've asked questions regarding their build before it changed hands and they promptly responded.

They even wrote a book; Americas-Overland-The Driving-Handbook.

When will we start to see pictures of your truck and camper on the road?

Are beach front pictures of Baja soon to come?
 
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pugslyyy

Expedition Vehicle Engineer Guy
This has been a great thread with great discussion. frame issues are something that people only typically pay attention to after something bad happens. As someone who owns a Fuso with a frame that broke at least three times and was then replaced, this is a subject dear to my heart. :)

I just heard from Overland Expo that they have accepted my proposal to lead a class on "Frame Failure (or how to avoid breaking your vehicle in half)" this Fall. If you are planning on coming to Asheville in October be sure and drop in so we can all share stories!
 

Haf-E

Expedition Leader
I think having three frames break on you should make you the expert! Congrats!

I was thinking about the Fuso issue and wonder if the problem with using a three point mount on it is that perhaps it allows too much frame flex - i.e. more twisting than the Fuso was designed for since they expected a fixed bed to be installed on it. The three point mount also results in point loading where the two "outer" mounts are made. A spring mount might be the best option - restricting the frame's twisting but still allowing some to occur. It also eliminates point loading under normal conditions since the rear box lays on the frame rail normally.

Unimog frames were designed to use a three point mount and are more substantial in dimensions and are probably better steel as well. So what works on a Unimog won't necessarily work on a Fuso.

If the cabin is designed with enough rigidity the issue of torsion-free mounting by using a three point mount might not be as important - the main benefit of the three point mount is less need for rigidity in the cabin - but if you already have it then you might not need the three point mounting system as much?
 
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pugslyyy

Expedition Vehicle Engineer Guy
I think having three frames break on you should make you the expert! Congrats!

I was thinking about the Fuso issue and wonder if the problem with using a three point mount on it is that perhaps it allows too much frame flex - i.e. more twisting than the Fuso was designed for since they expected a fixed bed to be installed on it. The three point mount also results in point loading where the two "outer" mounts are made. A spring mount might be the best option - restricting the frame's twisting but still allowing some to occur. It also eliminates point loading under normal conditions since the rear box lays on the frame rail normally.

Unimogs were designed to use a three point mount and are more substantial in dimensions and are probably better steel as well. So what works on a Unimog won't necessarily work on a Fuso.

If the cabin is designed with enough rigidity the issue of torsion-free mounting by using a three point mount might not be as important - the main benefit of the three point mount is less need for rigidity in the cabin - but if you already have it then you might not need the three point mounting system as much?
Yes, without a doubt. A camper is not built for torsion or flex - all those pesky cables, water lines, gray water lines, windows, cabinets, etc - they just don't like to be torqued.

I'm an engineer, and as a (hopefully) good engineer I can say that

1. Everything breaks, and
2. The goal is to have everything break simultaneously

Good engineering = everything blows up at the same time. :)

The 3 point is great for isolating and protecting the payload (camper), but at a cost. The goal of a mounting system should be to have the frame and payload fail simultaneously (and hopefully far above any expected stress load). If the camper fails before the truck fails, or conversely the truck fails before the camper then you have made an error. :)
 
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