pivoting frames and mounting campers

[fisher205]

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]

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]

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.

[Iain_U1250]

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.

[canals1164]

after a technical fork lift test we decided to go for a three point monting!IMG_0427.jpg

[westyss]

after a technical fork lift test we decided to go for a three point monting!IMG_0427.jpg

Keep in mind that once more weight is on the truck the suspension will do more than it does in that pic and less frame flex. I cant tell but to me it looks like the suspension isnt giving at all?

[Iain_U1250]

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.

[Sigurður Ingi Jónsson]

This seems to be where the greatest brains gather to discuss better mounting of campers. I've just posted my woes in the hard camper section. I would appreciate any input from those in the know on this thread.
The post is here: http://www.expeditionportal.com/foru...ee-three-point

[Alastair D(Aus)]

There is a separate thread that has been started regarding spring mounts that would benefit with comments by the learned members here.

http://www.expeditionportal.com/foru...-mounting-help

[kookynet]

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 !