Frame flex question

I know there are a million threads about this and someone is gonna get grumpy with me, but I can't find a simple answer to my question. I am using an Earthroamer type setup as the example vehicle. If you build a strong enough camper and attach it rigidly to the cab, can't you fasten the whole ensemble hard to the frame? It would be like a van, or excursion, or most other SUVs. Their frames/bodies don't really twist, right? I can get that allowing the frame to twist increases the contact of the tires and therefore traction. Having a rigid thing like I am thinking would result in wheels being off the ground more during extreme articulation. The things I don't like about the 3 or 4 point systems is that it seems like when side sloping, the weight is going to transfer away from the slope, exactly where you don't want it. It also increases the overall height of the entire rig, weight of the whole thing, and the center of gravity. And all the weight of the camper body is now on only 3 or 4 points of the frame. It actually seems like 3 point mounting systems are a recipe for breaking a frame. So please someone answer simply, why can a Quigly or UJoint van offroad well with a big rigid body rigidly mounted to it's frame but a camper can't??
 

Joe917

Explorer
Simple Yes.
My truck is a solid frame mount. Mercedes recommended solid mount or 3-point. The main thing with solid mount is the cabin must be very strong. My truck is 25 years old, no signs of cabin failure.
 

BritKLR

Explorer
I'd say yes aswell. Our 1995 Tiger on a 3/4 ton Chevy with 180k on it has a aluminum and fiberglass body mounted directly to the frame with no fatigue issues. And, we do trails all over the west and south west and haven't had any issues. I suspect today's frames are significantly stronger and more rigid then ours. Good luck.
 

Attachments

IdaSHO

IDACAMPER
The short answer is that there is no one size fits all solution.

Every vehicle is slightly different, and come chassis' simply flex more than others.


But generally most "expo" type campers fall into one of two categories for me...

These are specific to "light" trucks.

Pickup truck, typically flatbed, that hauls a removable camper, that retains the utility of a truck.

And a true cab-chassis camper, that is 100% camper, all the time.


For a truck, simply hard mounting a camper (or flatbed), starting right behind the cab, will create a stress point in the chassis.
The chassis would still flex, as designed, from that point forward, while that point back would not.
Short of stripping the entire truck down to reinforce the entire length of the frame, this is rarely a good idea.
Many people get away with it, but for the most part, truck chassis' are designed to allow a bit of flex.
A properly designed torsion free mount is the solution.

A true cab-chassis unit (like the tiger above) is a totally different story.
The utility function of the truck are gone, and the camper is permanently mounted to the chassis.
When designed correctly (or correctly enough) the camper will add rigidity to the entire truck, including the chassis.
A legit builder will design the camper with this in mind, in order for the entire thing acts as one.
Considerably different than a truck with removable camper.
 

heimbig

OnTheRoadAtLast
I agree with IdaSHO and would add if you are not an engineer do NOT attach rigidly, or use 3 or 4 point articulation, nor shorten nor lengthen the frame. Do what the manufacturer of your truck recommends - and some common sense if you can find any.
 

lostih

Observer
I'm doing a build right now of something similarish and had planned on rigidly mounting until last week when I tested the frame flex of my Ram 5500.

IMG_4001 (1).jpg

I am reusing a shell and am unsure how much the frame I'm building will add torsional rigidity to the frame...so I'm starting to think about a three or four point mount system.
 

Terra Ops

Adventurer
The original question; " If you build a strong enough camper and attach it rigidly to the cab, can't you fasten the whole ensemble hard to the frame?"
Tiger's Bengal model has been built this way for years on Ford, Chevy, and Dodge trucks. I have yet to hear of any frame failure.
The coach is bolted directly to the cab and frame. Having owned one, it was my experience that this "rigid mount" also translated into a rigid ride. Not that it was
unbearable, but when the coach rocks so does the cab. My current flat bed camper, while not a 3 point mount has a much better ride than our Tiger did.
 

rruff

Explorer
If you build a strong enough camper and attach it rigidly to the cab, can't you fasten the whole ensemble hard to the frame? It would be like a van, or excursion, or most other SUVs.
Question is, what is "strong enough"? Wish I knew...

I'm building a camper out of homemade wet layup FG + Foamular 250 core, plus some wood in the core also. The Tundra is the flexiest full sized 1/2 ton these days, though stiffer than other trucks that use an open C frame. I did not want to make a pivot frame for the reasons you cited. I also didn't want to make a separate subframe since that would be redundant structure. I'm not exactly hard mounting it though. I'm planning to attach aluminum hat brackets to the camper and use Energy Suspension universal body mounts for the 8 bed mounting points on the frame. This will give the mounting a little torsional flex.

 

Terra Ops

Adventurer

heimbig

OnTheRoadAtLast
wow lostih that is bad. I certainly would not put a camper on there. clearly the bed is fixed pretty tightly to the frame just behind the cab and the frame is at risk to break there. I hinged my bed ONLY at the end of the frame (call it a 2 point system with full frame contact) and the bed can float above the truck frame (or really the truck frame falls away below the bed)

you can see my hinge mockup at: http://forum.expeditionportal.com/threads/84721-GOVE-Global-Overland-Vehicle-Economical-build?p=1266133#post1266133 and continue to post 57.
End result is a flat, non-twisty flat bed that rests along the entire surface of the truck frame (when frame is not twisting) as recommended by Ford.
http://forum.expeditionportal.com/attachment.php?attachmentid=145481&d=1361256035

and


westyss 'out with the old' did something similar (actually I stole from him):
http://forum.expeditionportal.com/threads/48351-out-with-the-old-in-with-the-new?p=701584#post701584
 
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lostih

Observer
Interesting. I've thought about doing the reverse...hinging at the cab and letting the rear float or at least be unpinnable if going on some wonky roads. I'm actually not going to use the flatbed in my system...it's just on there temporarily.
 

grizzlyj

Adventurer
snipped
The things I don't like about the 3 or 4 point systems is that it seems like when side sloping, the weight is going to transfer away from the slope, exactly where you don't want it.
Hi

I think the diamond shape flexing mount design like a Unimog has won't. And it doesn't really suffer much with poor side slope performance when the load has been mounted as Merc suggests.

The front and rear fixed points between bed and chassis rails only allow left to right swivelling, the centre pair, aligned across the chassis, only allow front to rear swivelling. So the bed at that centre mount can only move left to right to the exact same extent as the chassis does, ignoring the tiny amount of movement within the joint bushings.

Van type unibody designs used off road when they have a pair of "barn doors" at the rear often end up with dust entering through the seals as the theoretically stiff vehicle box flexes and opens up the gaps more than the seals can allow for.

:)
 

The Artisan

Adventurer
Question is, what is "strong enough"? Wish I knew...

I'm building a camper out of homemade wet layup FG + Foamular 250 core, plus some wood in the core also. The Tundra is the flexiest full sized 1/2 ton these days, though stiffer than other trucks that use an open C frame. I did not want to make a pivot frame for the reasons you cited. I also didn't want to make a separate subframe since that would be redundant structure. I'm not exactly hard mounting it though. I'm planning to attach aluminum hat brackets to the camper and use Energy Suspension universal body mounts for the 8 bed mounting points on the frame. This will give the mounting a little torsional flex.

I am doing something similar and using ubolts and mounting tabs for the body mounts to sit. Best of both worlds imo.
Kevin
 
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