Frame flex fundamental question

toylandcruiser

Expedition Leader
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.
Exactly


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dlh62c

Explorer
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.
How do you plan to attach the top, sides and bottom together?

What are your plans to prevent racking of the box?

Do you plan to use these panels as a structural component or will you back them up with an internal framework?

Don't focus solely on the issue of insulation. It's hard to do, but you have to look at a build as a whole and not as a list of discrete parts and tasks. Light isn't always better, the body or shelll is the framework on which the entire build is based.
 

DzlToy

Explorer
Don't want to get too far off track from the frame thread and will start a build thread if the customer wants. Simple version is composite panels will be bonded and bolted together with some internal panel structure. There will not be a tube frame or "studs" like you would have in a house. The idea is to make the box "monolithic". Think of a one cubic foot box made from thin sheet metal. It cannot rack or deform, even though the material is relatively thin, as the sheet steel attached to each piece prevents movement of the other pieces. Kind of like trying to crush an egg shell vertically in your hand. Even though the shell is thin, you can't do it.

Isulation will come from several sources, reflecting heat, i.e. preventing it from entering the living space, as well as keeping heated or cooled air inside that space. Second, will be well sealed joints and seams to prevent drafts or air intrusion. Third is to minimise window and door openings, no pop up tops, tent fabrics, screens, etc. Fourth is a plastic based foam with R-7 per inch and a special coating to tie everything together. R value could actually be in the 30's if it all works well together and acts according to our testing, but we will see.

Back to the frame flex: In the trail sloping to the right example, as given above -- your left front tire could lift as you transition from sloping ground to more level ground. This is quite common in off-roading. I have experienced it in everything from a Land Rover LR4 (almost zero articulation) to a classic Steyr-Puch Pinzgauer, again, very little articulation.

Contrast that with a trophy truck or class one buggy, both of which have HUGE suspenion travel and are relatively heavy for their size (about 6,000 pounds) The body stays level as the truck races through the desert and the suspension allows the tires to follow the terrain underneath. They are full tube chassis, so I can assure you there is little if any twist or flex. Though the wheel travel is considerably less, a race car is designed the same way: stiff chassis, fantastic suspension setup.

A more down to earth example is something like this - Toyota truck has little to no frame flex or twist. When traveling difficult terrain, the tires follow that terrain and the body remains relatively level. A truck body or camper box flopping around causes the vehicle to shift or slide and reminds me of driving a truck with a half full water tank in the bed. The water "pushes" you or "pulls" you as it sloshes around in the tank, even though the tank is strapped to the bed (assuming no baffles)



body is relatively level and though there may be less weight on one tire than others, the lockers, which every off road vehicle should have, will do their job and pull you through. Now, if you have some soccer mom AWD open t-case, traction control, open diff mall crawler and lose traction on one tire, thereby ceasing forward progress, then you are on your own.

EDIT: a tippy or tipsy vehicle is difficult to control and unstable. So, if it came down to having a frame that twisted or having a rigid tippy vehicle with no articulation, I would prefer the former.
 
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GR8ADV

Explorer
Back to the frame flex: In the trail sloping to the right example, as given above -- your right front tire could lift as you transition from sloping ground to more level ground. This is quite common in off-roading. I have experienced it in everything from a Land Rover LR4 (almost zero articulation) to a classic Steyr-Puch Pinzgauer, again, very little articulation.

Contrast that with a trophy truck or class one buggy, both of which have HUGE suspenion travel and are relatively heavy for their size (about 6,000 pounds) The body stays level as the truck races through the desert and the suspension allows the tires to follow the terrain underneath. They are full tube chassis, so I can assure you there is little if any twist or flex. Though the wheel travel is considerably less, a race car is designed the same way: stiff chassis, fantastic suspension setup.

A more down to earth example is something like this - Toyota truck has little to no frame flex or twist. When traveling difficult terrain, the tires follow that terrain and the body remains relatively level. A truck body or camper box flopping around causes the vehicle to shift or slide and reminds me of driving a truck with a half full water tank in the bed. The water "pushes" you or "pulls" you as it sloshes around in the tank, even though the tank is strapped to the bed (assuming no baffles)



body is relatively level and though there may be less weight on one tire than others, the lockers, which every off road vehicle should have, will do their job and pull you through. Now, if you have some soccer mom AWD open t-case, traction control, open diff mall crawler and lose traction on one tire, thereby ceasing forward progress, then you are on your own.
Why can'tI have this on my FUSO. ������

lets not get too far afield from frame mounting isolation to traction and lockers et al. For what it is worth I expect that in the pic the front tires are doing little or nothing to aid at this moment as you can tell (based on the Tom Brady rear tires) that there is no weight and thus little to no traction in the front regardless of the big Johnson flex.
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Edit. If the frame flexed or better yet articulated to allow weight on the front tires, then you would have some serious traction.
 
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GR8ADV

Explorer
Don't want to get too far off track from the frame thread and will start a build thread if the customer wants. Simple version is composite panels will be bonded and bolted together with some internal panel structure. There will not be a tube frame or "studs" like you would have in a house. The idea is to make the box "monolithic". Think of a one cubic foot box made from thin sheet metal. It cannot rack or deform, even though the material is relatively thin, as the sheet steel attached to each piece prevents movement of the other pieces. Kind of like trying to crush an egg shell vertically in your hand. Even though the shell is thin, you can't do it.

Isulation will come from several sources, reflecting heat, i.e. preventing it from entering the living space, as well as keeping heated or cooled air inside that space. Second, will be well sealed joints and seams to prevent drafts or air intrusion. Third is to minimise window and door openings, no pop up tops, tent fabrics, screens, etc. Fourth is a plastic based foam with R-7 per inch and a special coating to tie everything together. R value could actually be in the 30's if it all works well together and acts according to our testing, but we will see.

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As a point of reference. I have a fiberglass Unicel box. It has metal studs on 24 inch centers that go up the sides and hoop around the top of the box. Quite possibly they hoop around the bottom as well but I do not know this. The box is mounted per FUSO on beams u-bolted to the frame with wood separation. The rig has been around with over 150,000 miles. The box is like new and the windows all slide.
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The box is heavily insulated. Yesterday I stopped in at a shop in Portland. It was 95 degrees outside. The guys wanted to see inside, when I opened the door they all said, "wow and it has air conditioning too" as the cool breeze from inside the truck washed over them. Nope, that is just the insulation doing its job.
 

GR8ADV

Explorer
A truck body or camper box flopping around causes the vehicle to shift or slide and reminds me of driving a truck with a half full water tank in the bed. The water "pushes" you or "pulls" you as it sloshes around in the tank, even though the tank is strapped to the bed (assuming no baffles)
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Excellent point. This is the "dynamic " factor and these horizontal forces cause torsion. The same theory applies. The swaying or sloshing back and forth creates horizontal forces resulting in torsion to the frame. The forces are proportional to the height of the center of mass above the torsional plane of the frame. Again, the less and the lower the weight the better. It is a mess off-road when this happens and you can get real busy real fast on the highway when you have to chase this. Those who have had a moment or two know this. Thanks for pointing this out.
 

DzlToy

Explorer
That Unicell box is an excellent example of what we are going after. I saw one mounted on a diesel Ford van a while back, but the driver, i.e. clueless worker bee, knew nothing about it.

Hurry and close your box back up before all the 95 degree air gets in :D
 

gait

Explorer
don't take my meccano model too literally.

.... The forces are proportional to the height of the center of mass above the torsional plane of the frame....
moment of inertia is square of the height .....

also to be considered is roll steer - an outcome of a rigid front axle impacted by the box design.

if it helps, try picturing - if the front and rear axles are twisted in opposite directions and the chassis twists there is no twist (at least in the same sense as the axle twist) at a point about half way between the axles.
 

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GR8ADV

Explorer
don't take my meccano model too literally.


moment of inertia is square of the height .....

also to be considered is roll steer - an outcome of a rigid front axle impacted by the box design.

if it helps, try picturing - if the front and rear axles are twisted in opposite directions and the chassis twists there is no twist (at least in the same sense as the axle twist) at a point about half way between the axles.

Nothing ruins a good thread than getting too technical; but what the hell. Are we confusing the twisting load/moment ( ft lbs) and the composite polar moment of inertia of the members that make up the frame? The twisting moment is a function of the load x the distance. The polar moment of inertia is a function of the square of the radius of the member. Or in this case members. For those keeping score at home, this is why tall channels and I beams make lousy torsional members. FWIW school was a long time ago and I may be messed up...
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Given the fixity nature of the cab/engine of the frame at the cab, and the step down, I would not expect or design around the idea that the neutral twisting is at the mid point of the frame. But I am just guessing here with no knowledge of the frame design.
 

Czechsix

Watching you from a ridge
Unimog frames are quite a bit different than FUSO frames, or any other ladder frames for that matter. There's a reason that they use tube members as the cross pieces, and not channel, beam or square/rectangular tubing.....
 

dlh62c

Explorer
So we're all in agreement that a Fuso Canter's frame does and will twist?

If you don't want torsional forces transferred to the camper body, that's designed to remain ridged, some sort of torsion free mount is recommended.

Doug Hackney dealt with this on his Fuso build;
See photo in Post # 47, the camper doesn't appear to be mounted on a Fuso chassis, but possibly on a Ford or Chevy Class C; http://www.expeditionportal.com/forum/threads/2229-FG-buildup/page5
http://www.hackneys.com/mitsu/index-pivot-frame.htm
 
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SkiFreak

Expedition Leader
The biggest issue with 3 point mounts on a Fuso chassis is point loading. Another potential issue is unrestrained flexing. Doug himself has said that if he were building another Fuso based camper that he would use a spring mount system, similar to that which ATW use on their builds.
 

dtruzinski

Explorer
I think SkiFreak is on to something...both ATW and EC use spring mount systems. I noticed this on FX model I test drove. Did not flop around like my 3 point mount (I need to engineer or have engineered some mechanism to control/shock the load)
 

pugslyyy

Expedition Vehicle Engineer Guy
The biggest issue with 3 point mounts on a Fuso chassis is point loading. Another potential issue is unrestrained flexing. Doug himself has said that if he were building another Fuso based camper that he would use a spring mount system, similar to that which ATW use on their builds.
...and it is on a spring mount system now. :)
 
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