Frame flex fundamental question

GR8ADV

Explorer
So what is worse "on a FUSO" , the frame flexing as it was designed with loads under gvw or concentrated forces on the frame from a mounting system proving loading and flexing other than the designers intent. . The reason I ask is my box is mounted directly to the frame. There are about 155000 miles on the rig and it has been around. The frame was inspected and looked fine. Am I living on borrowed time? Or is frame isolation just a concept of complexity? Would love everyone's thoughts.
 

pugslyyy

Expedition Vehicle Engineer Guy
Can you be more specific when you say "mounted directly to the frame", maybe even illustrate with some pictures?

That could mean
- welded
- bolted directly
- bolted through wood or some other material
- ends prepared in some fashion to relieve stress (or not)
- etc
 

SkiFreak

Expedition Leader
This subject has been covered many times and is always one that attracts fierce debate.
It should be noted that there are so many variables here, so what works for one person may be inappropriate for someone else. The full weight of the vehicle is one significant factor here, and that must be taken into account.

I would estimate that the majority of fabrication companies that mount bodies onto trucks do not do so on trucks that go off road. The mindset of these companies is normally based around goods trucks that travel on sealed roads and they build frame attachments systems accordingly.
There are a few members of this forum that have campers solidly mounted to the chassis and report that they have experienced no frame related issues. I am in no position to dispute this and I take them at their word.

That said, it cannot be disputed that driving on the road is very different to driving off road when it come to the stresses that will be applied to the chassis.
If you plan on taking your FG into situations where there will be a need for significant articulation then I firmly believe that a fixed mount camper will exert more stresses on the chassis (focused specifically in front of the step) than the same camper if it were on a flexible mount.
I have often written that I believe that unrestrained flex is as bad, if not worse, than if the camper were solidly mounted. Too much flex will overstress any attachment points, like the riveted section of the step.
In my opinion, a spring mount system should allow for some flex, but it should also restrain the amount of possible movement. Finding the happy medium of what is an acceptable amount of free movement is the challenge.
Like ATW, I have limited my amount of movement to about 30mm separation between the chassis and subframe.

My logic here is that ATW are arguably the most experienced company in the world when it comes to mounting bodies on Fuso and Isuzu 6 tonne trucks for extreme conditions and/or off road use. They have mounted hundreds of bodies on this type of truck, which is something I do not think anyone else can claim.
If people wish to reinvent the wheel, then they are welcome to do so. For me however, I would prefer to base my designs on a known, good working example.
 

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gait

Explorer
here's just one version of all the forces and movements. I started with a meccano model.

the amount of twist absorbed between road and body depends on load, suspension stiffness, chassis stiffness, sub-frame stiffness (including body). For example, with the same suspension and same articulation a 7 tonne load would have less chassis twist than a 4.5 tonne load.

the first thing that happens as the chassis twists is the chassis rails attempt to move backwards on one side and forwards on the other side relative to the sub-frame, the sub-frame tries to twist. This may be absorbed by "u-bolt" mounts. The bolts in a rigid "bolted to chassis and sub-frame" mount may gradually work loose and/or make the little round holes a bit sort of elongated and the bolts a bit hardened or worst case sheared. The next bit is the sub-frame may be stiff enough to resist being twisted and place increasing vertical force on the mounts. U-bolts may tend to give way eventually. The spring mounts allow movement with progressively more resistance.

As well as the vertical movement the sprung mounts also have to resist fore and aft and lateral movements as well as the twisting which occurs where a fixed mount meets the chassis and sub-frame.

In my case there's a long cantilever to the rear. As that bit of the sub-frame flexes it tends to place torque on the rearmost mount and fore/aft forces on the forward mounts.

It all also varies with the vertical distance between chassis and sub-frame. What a pain the step in the chassis is.

Plus, it depends on the distance between the frontmost and rearmost mounts. Placing them some distance from the box corners is better than at the corners. eg 30mm at 4m centres will be less at 3m centres (perhaps 23mm without spending time on the calculator). Alright Owen :) 30mm at 3m centres may be 40mm at 4m .... For a whole variety of reasons my mounts are 1750mm apart with extra "inverted springs" that simply provide vertical support at the front.

The spring companies seem to be pretty helpful. Its possible with the spring forms we are talking about to create one with all the right characteristics apart from it doesn't spring back if marginally overstressed.
 

Aussie Iron

Explorer
My tray is bolted solid to the chassis (both on top of step and along rear rails) at 282,000Klms. This is the way that the body builder made it, it is also a very large body building company. Now I don't say that it is right but it is that way. I use my Canter off road (I would be at close to my legal GVM 4,495kg) and it is not unusual to see a wheel in mid air so I don't have a lot of frame flex even though I have good wheel travel. Is my chassis a problem to me not yet, is it going to be a problem time will tell. I believe that if you are putting a camper body on then spring mounting is a must to take the stress of flex out of the equation. My camp boxes are flex mounted (bolted through rubber bushes) to the tray so that if the tray twists then some of this is taken out. I don't have all the answers but I would be going with the tried and tested way that has been used by the major off road camper builders.

Dan.
 

dlh62c

Explorer
So what is worse "on a FUSO" , the frame flexing as it was designed with loads under gvw or concentrated forces on the frame from a mounting system proving loading and flexing other than the designers intent. . The reason I ask is my box is mounted directly to the frame. There are about 155000 miles on the rig and it has been around. The frame was inspected and looked fine. Am I living on borrowed time? Or is frame isolation just a concept of complexity? Would love everyone's thoughts.
During your camper rebuild, did you see any indication of component movement inside the camper body?

An example would be; shifted or cracked framing, misalignment of doors and windows, constant leaking of door and window seals, and/or broken cabinet door hinges and frames.

Torsion free mounts are just as important for protection of the mounted body as they are for the vehicle's frame. As the vehicle's frame attempts to flex and twist, any torsional forces will be transferred to the mounted body. You can't have something that's designed to remain rigid mounted to something that's designed to flex, something has to give.

Sounds like the truck frame is just fine. But with rigid 'U' bolt mounts, its important that there is blocking inside the frame 'C' channel to prevent frame deformation at the attachment points when the bolts are torqued down.

Sportsmobile Vs Pace Arrow YouTube Video, the fun part starts at 11:00.
 

DzlToy

Explorer
Simple answer: a truck frame is not a suspension component.

A rigid mounted box, such as the one the OP has is fine if mounted properly. I have owned and driven dozens of light duty and medium duty trucks over the last 20 years, everyone having a rigid mounted box or flat bed with hundreds of thousands of miles on the chassis and no problems.

Spend your time and money on a proper suspension, designed to carry your load on the terrain you travel and you will be head and shoulders above 90% of the people doing these conversions/builds.

Heavy trucks typically have little to no suspension travel and almost zero articulation. This is to carry heavy loads on road, as previously noted. In order to carry heavy-ish loads off road, you need a compliant suspension that does not allow sway or wallow. A large camper box flopping about on your chassis is a terrible design IMO.

Rigid mount the box, presumably a properly designed rigid structure in and of itself and not a $5,000 slide in truck camper designed to live life in a pick up bed and you will be fine. This Dodge truck is a perfect example, the box is not twisting or flexing relative to the cab and the suspension is doing all the work, exaclty as it should do. A frame rail is not a suspension component.

Camper Box does not need to twist.jpg
 

dtruzinski

Explorer
@dzltoy, why do you think the Unimogs use 3 pt mounting systems from the factory? I have a 2004 Fuso that "flops" around as you note on a 3 pt forward pivot system. I suspect that it relieves all of the torsional stress that would normally be transferred to the camper. But I am not an engineer, I am a high-tech guy. I am truly interested, as I am getting ready to transfer the custom camper to a 2007 and a rigid mount is much simpler than a 3 pt mount. Maybe the answer is both work and YMMV. In which case I will keep the 3-pt design. By the way flopping without some shocking mechanism is a real problem. I need to solve for this...I have literally lifted a tire/wheel on pavement due to the rapid shift in load (no shocking to slow the sway) when turning.
 

GR8ADV

Explorer
@dzltoy, why do you think the Unimogs use 3 pt mounting systems from the factory? I have a 2004 Fuso that "flops" around as you note on a 3 pt forward pivot system. I suspect that it relieves all of the torsional stress that would normally be transferred to the camper. But I am not an engineer, I am a high-tech guy. I am truly interested, as I am getting ready to transfer the custom camper to a 2007 and a rigid mount is much simpler than a 3 pt mount. Maybe the answer is both work and YMMV. In which case I will keep the 3-pt design. By the way flopping without some shocking mechanism is a real problem. I need to solve for this...I have literally lifted a tire/wheel on pavement due to the rapid shift in load (no shocking to slow the sway) when turning.
I suspect this is a suspension issue and not a mounting issue, but not having any of the facts I really don't know.
,
FWIW I believe the FUSO builders guide says mount the box to the frame via u bolts with a wood interface.
,
Obviously complete isolation from the frame would be awesome to keep the box from tipping; as long as the resulting mounting does not cause more harm than good. There is a lot of information on the portal. It is very hard to discern between real engineering and yahoos with some cool software.
,
I would really like to better understand how spring mounts attached to a rigid frame improve things. I am all ears and willing to learn.
,
FUSOS are not unimogs and their mounting methods are not relevant to each other. GL with your decision.
 

gait

Explorer
pic below is about halfway between reality and virtual, a bit useful to see what moves relative to what else. The FG has a step and a narrow chassis (for dual wheels which we don't like anyway) just to make things harder,

another useful exercise is to pick up a shoe box (with lid) and twist it. I also did same without lid due to my drop-top construct.

should I have added
the longer the chassis the more twist,
the narrower the chassis the more twist

in terms of decoupling twist and bend I'd rank them
rigid bolts
u-bolts
spring bolts
3-point

disclaimer - my box is a "rectangular shoe box" frameless fibreglass sandwich with sikaflex joins.
disclaimer - I'm incapable of thinking of independent bits, I'm a whole vehicle sort of person.

002 Meccano.jpg
 

DzlToy

Explorer
Frame Design/Spec: As mentioned, the Fuso's have narrower than normal (28-29" IIRC) frame rails and somewhat thin frames, so that is one of the reasons they WILL flex, whether you want them to or not.

I have never owned a Mog, so I cant speak to that. I would presume that Mog typically have short frames, large tires and can carry heavy, if not somewhat tall loads. We are not talking Rover or Toyota parts here. As size and mass increase rapidly, it becomes difficult to keep it all rigid. Comparing two opposite ends of the spectrum, I am getting composite panels in a few weeks for a customer build. The empty box shell (12 feet by 7 feet by 6.5 feet) will weigh well under 600 pounds with an R-20 value. It will be rigid mounted to the frame rails with no problems, according to our testing.

Now, take something like this:



I don't think you could easily make a chassis/RV box that large completely rigid, nor could it easily have a soft/compliant suspension that could carry all of that weight.

I take a page out of Colin Chapman's book and make everything as simple and light as possible. It pays dividends all around, even if its a bit extra work in the beginning.

Good luck and let us know what you come up with.
 

toylandcruiser

Expedition Leader
Simple answer: a truck frame is not a suspension component.

A rigid mounted box, such as the one the OP has is fine if mounted properly. I have owned and driven dozens of light duty and medium duty trucks over the last 20 years, everyone having a rigid mounted box or flat bed with hundreds of thousands of miles on the chassis and no problems.

Spend your time and money on a proper suspension, designed to carry your load on the terrain you travel and you will be head and shoulders above 90% of the people doing these conversions/builds.

Heavy trucks typically have little to no suspension travel and almost zero articulation. This is to carry heavy loads on road, as previously noted. In order to carry heavy-ish loads off road, you need a compliant suspension that does not allow sway or wallow. A large camper box flopping about on your chassis is a terrible design IMO.

Rigid mount the box, presumably a properly designed rigid structure in and of itself and not a $5,000 slide in truck camper designed to live life in a pick up bed and you will be fine. This Dodge truck is a perfect example, the box is not twisting or flexing relative to the cab and the suspension is doing all the work, exaclty as it should do. A frame rail is not a suspension component.

View attachment 297753
Mercedes would disagree with you. The frame is part of the suspension on a unimog. I'm sure Mitsubishi would say the same.


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GR8ADV

Explorer
pic below is about halfway between reality and virtual, a bit useful to see what moves relative to what else. The FG has a step and a narrow chassis (for dual wheels which we don't like anyway) just to make things harder,

another useful exercise is to pick up a shoe box (with lid) and twist it. I also did same without lid due to my drop-top construct.

should I have added
the longer the chassis the more twist,
the narrower the chassis the more twist

in terms of decoupling twist and bend I'd rank them
rigid bolts
u-bolts
spring bolts
3-point

disclaimer - my box is a "rectangular shoe box" frameless fibreglass sandwich with sikaflex joins.
disclaimer - I'm incapable of thinking of independent bits, I'm a whole vehicle sort of person.

View attachment 297842
All things remaining equal, yes the longer Frame will have less resistance to torsion and will result in a greater angle of twist. It does NOT mean that the torsional forces are less. The frame still needs to resist the forces.
.
In simple terms the frame must resist the torsion caused by the twisting force which is (mostly)the vertical load of the box relative to how far the vertical line through the center of mass of the box is away from the centerline of the frame. So as the box leans, the vertical line gets further and further away and the torsional force gets greater and greater. (Also visualize here how important the lower the center of mass of the box is to the equation. If it is high there is more torsion load than if it is low). Thus, not allowing it to tip is very important to torsion. If we can stop the box from leaning at all there will be very little torsion. So yes the mounting system you have shown appears to do this. Ok so if the springs are used in conjunction with a system like this I get it. But if it is fixed where you show the pivot, I don't get it. Thanks!
.
It also sounds like some of the cool mounting systems are allowing the box to flop around which would be bad juju.
.
Edit. But somewhere the box must be fixed to the frame to stop it from flopping over. The closer this mounting is to the cab the shorter the effective length of the frame and thus there is less twisting. However, remember that the forces are not reduced. Thus I wonder if this is the root cause for some frame failures with the 3 and 4 point mounting systems on the FUSOS. ??

.
 
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GR8ADV

Explorer
Mercedes would disagree with you. The frame is part of the suspension on a unimog. I'm sure Mitsubishi would say the same.
Do people actually not think of the whole truck as a system? Imagine, if you will , that you are on a steep side slope, sloping down to the right. As you drive to level ground, if there is no frame flex the left wheel would remain in the air. If you have lots of articulation the spring will extend and the wheel may touch the ground, but with no weight on it it will be just as useless and provide little value.
 
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