pivoting frames and mounting campers

There are 4 pivots in both your image and in that truck. Should only be three. This is basic geometry. Just because its commonly done that way (4 pivots) does not make it correct. Three pivots can never induce a bending moment or a torsion in the camper structure, but 4 can do so quite easily. Think about my three legged bar stool vs. 4 legged bar stool analogy. With the 4 legged bar stool if one leg is short or long compared to the others then it wobbles. This is the same thing, only since those "legs" are connected at both "ends" (pivot pin attachment and pivot bearing attachment) it forces something to bend.
I think he's actually right.. To a point.. Simply described, when in use a 4 point system with center pivots at the front and rear, the middle mounts (left and right) have to have the ability to move forward and backwards. When in twist, if the front and rear pivot mounts are not low enough in the chassis, they will force the mid mounts to twist...

Scenario as follows. Front right wheel and rear left wheel in a deep hole... Front center mount just behind the cab (unless bang in the middle of the chassis center line left to right and top to bottom) will move to the vehicle right. Rear center mount will move to the vehicle left.. In doing this the middle left rail pivot is forced forwards, and the middle right rail pivot is forced backwards. The only way this can be eliminated is if all mount pivot points are in the center of the chassis 'C' section. The front one would have to be where the gearbox is, the rear one is achievable, the left and right rail mounts would be roughly where the front spring hanger bolt is located.. I think that's why some of the Merc systems have a connecting arm on each center mount which allow the body to move 'fore and aft' on each side during flex....
 
Here is a probably crazy idea for a Hybrid 3 point pivot mount. Coppied this from post in another thread

We have RR bridges with 11'6" clearance, why not go with a pop up and stay low to cab height, you're probably not concerned with mpg, but that might help too.
Yeah Pop top (hybrid) hard side trailer is way to go. It's been done on same vehicle infact.

Search Bill Caid. His website shows subframe build. Similar to Unimog. More complex.



Also in concept there may be a way to do a 3pt subframe; air bag softend, Sprung and with rear central pivot point. All that and may not create much height change. Might even be inexpensive if you use camping trailer using a camper cabin.

AKA-

Lift camper up for mounting

. hire a roll back wrecker to lift your trailer. Roll it onto the stripped bare/modified bed (rear trailer facing cab)


Create a Hybrid 3 point Pivoting Sub frame using bumper pull camper's own frame and suspension system)

Your central pivot point and first mount of your 3pt subframe:

...... Cut the Tounge off the trailer and weld ball coupler back on to trailer frame just under edge of camper wall?
...... Fabricate bolt on trailer hitch too match newly located trailer coupler. Mount this to trucks chasis.(closer to end of chassis the better?)
...... Hook trailer to ball.
...... If needed; optional bump stops and / or shock absorbers can be added at this end.


Mount of hybrid two fixed point mounts further up toward cab?

..... this is an air bag softened/sprung Cabin mount (AKA- trailer tires with lowered pressure. Maybe smaller tires to reduce hight?).
..... Add a few other tie downs to lock it to the bed (around trailer axle?)
..... consider option of moving rear axle further back toward end of Trailer along with needed bracing. Moves this two point mount closer to the point of of truck chassis's ladder frame that has less flex. (usually somewhere just behind cab)

WHALA...... your done.


Granted that is over simplified explanation... but in concept.. hmmmm [Elmer Fud voice] Vewy Intawesting[voice off]

Refercence photo of 3pt subframe on Steyr camper conversion. A screen grab from Youtube.
View attachment 347784
 
In this thread I see that the 3 point subframes are more used on the bigger trucks, and especially the flexy unimogs. How about subframes on a Landcruiser 79? Is it possible to bolt a subframe directly on the four supports where the original tray is bolted on, or will the unit crack in a while? Is there someone with experience about this?
 
I cannot see a Landcruiser 79 needing one, honestly.

The section of frame is pretty short behind the cab, and I dont think those chassis flex much.
 
My plan is to stiffen the frame and mount the box on spring mounts with only a single pivot in the rear. At rest, the box will sit on the frame, not the pivot mount. The centerline of the pivot will be on the same plane as the box to frame surface. I am using a heim type mount that will allow it to pivot and twist but no front to rear or side to side movement. The suspension will handle bumps in the road. The spring mounts will allow the frame to twist only when going slow and when the suspension drops into deeper holes. I saw a video on this site of a large, truck based camper that had hydraulic struts that controlled the amount of movement between the box and frame. On the road they limit the movement. Off-road they allow the box to move freely. I like that concept and may use it too.

 
My plan is to stiffen the frame
uh, what?

Stiffen the truck chassis?


Unless you are stripping the rig down to the bare chassis to do any stiffening, you will probably be doing more harm than good.

Stiffening only portions, like the back half, will result in some significant stress points.
 
Lost me too... I hope you mean stiffen the camper frame... By stiffening it, you are adding weight... Mount it correctly and there should be no tension in the body and it can be as light as required..
 
If anyone needs a subframe and camper designed, we have an engineer on hand that can do it. Based in Germany with many years in the Expedition Truck business he knows what he is doing. I have seen campers ripped off the chassis because of faulty subframe designs. I would not gamble with that. Send me an email and I can tell you more.

Cheers,
Andreas
 
Maybe stiffening was a poor choice of words without some explanation. The truck is a cab & chassis. It has frame bracing from the factory that runs on the top and bottom of the frame rails under the cab. The box was originally bolted directly to the frame with 1/2 inch thick plates near the front and at the rear and with 1/4 inch plywood sandwiched between. Since the box will now be dynamic, I wanted the load spread as evenly as possible along the truck frame. In order to create a perfectly flat surface for the box to sit on, and to restore the stiffness that the frame had when bolted solid to the box, I made frame rails that will bolt to the truck frame. I moved the box rearward 25" which requires a frame extension. I did not want to weld to the frame so the new frame rails also allow the extension without welding to the frame. The box frame had 2x2 steel tubing welded onto the 6 inch tall C channel to act as a spacer; it was segmented to clear the irregular hump over the rear axle. I removed that from the C channel, and the new frame rails occupy the same space. It is still in progress but here are some photos.








 
What a read! That took a few days.

I also had the idea to use heim joints at the back and spring mounts from there forward. 4 per side sounds good. Decreasing the spring rate and or spring preload the farther forward the mount is. Simple and compact. I was also envisioning the design with the locator pin before I saw it in this thread. Maybe even incorporate a bronze bushing around the locator pin to avoid galling and or noise when the mount moves apart.
 
OK I have read most of the 42 pages and many of the links and have only changed my design several times, so I have decided to post a picture of my unusual situation and take input. the difference in my scenario is all mounts will be between the axles and my top of frame is already 53" tall so I cannot afford to waste any height, and there will be cabin above the rear mounted engine ( think of it as a backwards mounted cab over truck camper ) the only plus is I have about 8,000 pounds to work with / add on, but of course lighter is better ( this Rig will never be light ). IMG_1229.jpg It originally had a 1,000 gallon water tank in the middle above the frame and of course some bodywork . photo 2.jpg
 
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So I plan to have two semi ridged mounts at the rear of cabin just in front of rear spring hangers, this is where the water tank mounted. and a single center mount or pivot just behind the front spring shackles, also where the water tank mounted. I will mount outside of the frame rather than on top this way I can leave 2" air space between frame and cabin structure, also thinking the front pivoting mount may be designed with a spring override in case the frame comes in solid contact with the cabin it could pull the pivot up gaining more travel pulling against the spring. and a watts link to keep everything on center. Just a thought, I'll try to put it on paper
 
here is a primitive sketch of my "soft pivot"
the pivoting yoke on top of a dense rubber cushion, a large shaft running through cushion and frame or mount within some sort of guide tube or greased bushing or delrin liner, a compressed appropriate spring on the bottom plus flat washer and lock nut or castle nut and pin, and then the "watts link" to keep the body centered on the shaft IMG_1368.JPG
 
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