pivoting frames and mounting campers

fisher205

Explorer
Over the years the only time I've seen articulation like that is when someone raises their dump bed on uneven ground. I wish I had taken pictures of some of the twizzler frames that are left from that type of poor judgement.
 

dhackney

Expedition Leader
ntsqd; ok, you win ;)


my (hopefully soon to start) project will have straight 10.125 x 3.062 x .312" frame beams (257 x 77.8 x 8.0 mm for the metric guys)
it is a MDT so the frame is "only" HSS and not heat treated (as is basically standard with HDTrucks)
this means i have to spread the load over a larger area (more bolts) backed by a rather large flanged cross-brace to avoid any kind of frame webbing flex.
so my bros advice is clearly towards a 3-point (with the rear trunnion at mid frame level and as close to the rear axle as possible).
right now we are looking into a idea to may use two rubber cushioned trunnions (one integrated into the suspension mount cross brace and one towards the end of frame)
to may spread the punctual load in the rear over a larger area.
i will post a sketch later on ...

The highest frame stress point in your design will be at the back of the cab, so it will help a lot if you put the pivot there. Downside is then the cab to box junction will require a very flexible connection which can lead to more potential leakage. If you put the pivot at the rear to make the cab/box connection easy, you add maximum stress to the longitudinal point on your frame that is already the most stressed point. Everything is a compromise.

As previously mentioned, the 3 point payload suspension will put your entire payload weight onto the frame in only two longitudinal locations. If you keep your payload to a reasonable level (do not use our truck as an example of "reasonable"), you should be OK with your size and weight capacity frame.

In terms of "Will this 3 point payload suspension work with my frame and payload?" keep in mind that it's not just static load capacity, but also long-term stress and repetitive loading/unloading of the frame between those two longitudinal points. In this case, we are an excellent example of a potential negative outcome. :) If you have even the slightest trepidation, sleeve your frame now while it's still open and accessible.

Doug
 

ntsqd

Heretic Car Camper
I'm guessing that was a one-time event. That frame has been yielded and was hopefully retired after that. Nice for the driver that it didn't completely flop.


Re: sleeving a frame. I'm not convinced that this is the best option. It does add Section Modulus, so it will increase the frame's load carrying capacity or increase it's fatigue life at a lower loading level. What bothers me is that it is not an efficient use of weight and unless the frame is sleeved for it's full length there will be at least one pair of large cross sectional change zones in the frame. It would be really easy to shoot yourself in the foot if the sleeving isn't properly designed and executed.
Similarly, boxing a C-channel frame is also fraught with peril. I have seen such frames tear the cross members out of the rails in something with as short of a wheelbase as a Chevy K5 Blazer.

I am beginning to think that going with a load distribution design and avoiding point loads altogether is the better method for those unable to do a proper FEA on the frame. It seems to have worked fine for all of the US 2.5t and 5t military trucks going back before the start of WWII. It's not as fashionable or "Gucci" as a diamond or a 3-point mount, but unless the considerable engineering needed to make those work can and will be done it is the much safer and proven option. As I recall distributed loading is how Casa Azul was built, and it was originally a military/Gov't contract built truck.
 
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Iain_U1250

Explorer
The chassis on my Unimog is very flexible, and the mounting point system has to match - the jacking up one of the rear wheels was enough to flex the chassis quite a bit as shown in this photo.
DSC04181.jpg

My camper box is on the OEM 4 point bed - the centre mount is rubber mounted to the cross member between the axle mounts, the front and rear pivoting mounts allow for about 25 degrees of flex each way. The cab is fixed on the cross member between the front axle mounts, and the rear pivoting mount is mounted off the chassis.

If I stand on top of the camper, I can rock the box so that I get about 50mm of differential movement between the cab and the camper. I'm thinking of adding some shock absorbers to stop the wobble, but still allow it to flex. The OEM U1700 bed has a large shock on the bed to limit the movement, and there are OEM cab shock available as well.

A friend did some basic finite element analysis on my camper box - and it is incredibly stiff - if you don't mount it properly, either the box or the chassis will be destroyed eventually - my guess would be the chassis is a lot weaker than most camper boxes - just due to the relative section modulus - massive box verses relatively tiny chassis.
 

Iain_U1250

Explorer

Check out the flex in this Unimog - articulation between cab and camper scales off at around 400mm.

We are currently making the "sock" for the seal between the cab and camper, and need to cater for the 400mm of differential movement.

Regarding the pivoting mount - standard MB Unimog - 3 point mount on the cab, and 2x 3 point mount on the camper. Lateral fixed ( on 12mm rubber bushes) is on the cross member on the spring mounts, and lateral pivoting mounts.
 

kookynet

off beaten tracks lovers
Two line pivot subframe on a Canter FG

Hi,
Just adding my two cents.

I have been driving for two years on often bad Baja tracks a FG with a custom Two Line Pivot subframe without any issue.
The building was quite easy and quick.

here are some details http://kookynet.net/220-cristo-3-intro.html

k5541-50welding-articul-plates_v.jpg

Safe threading to all !
 

HowardH

Adventurer
Another approach

Why not completely separate the camper and its frame from the cab and its frame. Similar to a 5th wheel trailer with all the stress and articulation on the hitch pin. Or like an articulating loader or transit bus?

Each frame could be stiff wouldn't have to flex yet the pivot point would give you all the movement you need.
 

RoosterBooster

Observer
i guess i update this thread with the pivoting body mount solution i ended up with

however, after stumbling over a vintage 1952 "Diamond T" cab my medium duty truck project went a different direction then originally intended :p
(less expedition, more vintage looking long distance travel truck)

but i think the basic principles and goals stayed the same;
i still wanted (almost) un-restricted frame twist for off the pavement traction (by design the typical H/MDT rear air suspensions do not articulate well)
i also wanted a complete solid cab to camper connection (no movement between the two and no rubber bellow)
so after some thinkering i ended up building a front pivot sub-frame with rear air bag setup again ( i did the same thing when i extended the sleeper on my previous Peterbilt truck)

here are a couple pics (click on them for a larger view ;) );

donor International truck chassis (i picked it for the HSS low profile frame rails and the nice Z-leaf/ torque arm rear air suspension)

DSC_0754.JPG

DSC_0840.JPG

fabricated front pivots;

DSC_1202.JPG

to keep frame webbing flex/stress to a minimum i located them direct opposite of the SAE bell mounts and very close to the front spring shackles
(in place of the stock cab mounts)

DSC_1201.JPG

the rear of the sub-frame is located with this forget steel panhard bar (a OEM Peterbilt part that is used on integral sleeper cab trucks)

DSC_1254.JPG

the rear air bags are Firestone 2600 lbs units. my goal was to build the sub-frame as low profile as possible, so i located them outside of the frame in fabricated "shelves"

DSC_1230.JPG

again, to keep frame webbing stress to a minimum i mounted the bag shelves very close to the rear suspension mounting points and frame crossmember
here is a pic of the almost finished sub-frame including the cab and mockup seats

DSC_1403.JPG

with the aluminum body in place;

DSC_3242.JPG

the shocks are adjustable Rancho 9000 series (the King sticker is a joke to mess with diesel ricers :D)
they are set as wide as possible to give them a good leverage to fight body sway.
not visible are the bumpstops (to protect the sub-frame from bottoming out as well as over-extension)
the airbag supply is plumbed with a Peterbilt auto-leveling valve as well as a dash mounted "dump" valve
(to lower the camper for easy entry/exit as well as to drop it onto extendable legs to keep it from rocking when camping)

DSC_3284.JPG

...
 

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haven

Expedition Leader
Before leaving for a multi-month drive around Europe and Asia, Gary and Monika Wescott did some preventive maintenance on their iconic Turtle 5 camper. It turns out that their torque-free camper mount was designed and built 10 years ago by Midwest Four Wheel Drive, the company that builds the Bigfoot monster trucks. Gary and Monika paid them a visit to inspect and lubricate the camper mount.

The mount attaches the camper to the F550 frame in three places: two behind the truck cab, and a third on a crossmember behind the differential. The rear mount is pivoting, allowing the rear axle to tilt without a affecting the camper. Read a little bit about this feature and see a photo of the pivot pin in Gary's blog post.
http://turtleexpedition.com/across-the-usa-2013-3/

Putting the pivot at the rear of the truck chassis is unusual. Most three point designs put the pivoting mount at the front of the camper, and mount the camper to the frame in two locations close to the axle.
 

ntsqd

Heretic Car Camper
If you insist on having pivots I think that having the longitudinal axis pivot at the rear makes more sense. Put it at the front and you force the frame to rotate the camper box when ever it twists. Put it at the rear and now the frame is free to twist under the camper box. This does require that a cross-member capable of supporting the load be created, which could be a hindrance.

More important than the fore/aft location of the longitudinal axis pivot is the vertical location of this pivot. If it is either above or below the axis that the chassis twists about it will cause some lateral displacement of the camper box. If the other two supports do not have that degree of freedom built into their design some parts will be flexing and distorting if not built heavily enough, and if built heavily enough will be 'working' their attachment to the chassis. None of this is good for longevity.
 
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