Broken frame

[Robthebrit]

That's the trick, a stress free mount does not require the box to be strong. My camper is made of fiberglass and foam panels. The three/four point mounts themselves are quite big but between them they provide a torsion free platform (3 points make a plane) on which you can much mount the more fragile shell.

Doug's Fuso is a great example, its big foot camper that is properly mounted and camper has not suffered any stress. By itself a big foot is not that strong, if that camper was mounted directly to the fuso frame is would have broken apart. The flex is even worse for a unimog (on a mog the engine, trans, cab and everything else is 3 point mounted to allow the frame to flex), my camper has no signs of stress but again if my shell was mounted to the frame it would get torn apart.

The only way to mount to the frame is to have a stiff frame to begin with (I think the eco roamer is going down this path) or to have a box that is so strong it will stop the frame from flexing, ultimately forcing a flexible frame to be rigid with a box will cause to something to fail/warp/crack.

Rob

[kerry]

I understand that the box does not need to be that strong. All I'm saying is that the floors of commercial truck campers are not designed to have 3 or 4 mounts directly attached to them as the sole weight carrying points. They need some kind of frame or bed underneath.

[ntsqd]

I see value in more than three points for mounting a potentially heavy or high inertia structure to a ladder frame in that the point loads are reduced. Most of what I've read in these body builder's links are pointed at making the loads distributed loadings rather than point loads. From a design perspective having the loading introduced to the whole top of the frame rail, as is commonly done with delivery box vans, makes for a more economical frame design. Point loads require that the frame be stronger or that the payload maximum be reduced because they are essentially a bridge spanning the distance between the loading points.

What I don't like is that no matter how you do it, more than three points means that you are introducing some torsion or bending into that structure. After three points the odds of any further mounting points always being co-planar aren't real good. Can always design in some compliance to a joint, but then how much load carrying is the joint doing?

Considering this, I'm wondering about a sub-frame that accepts the point loads and distributes them to the truck frame. Cleverly done I suspect that the CG hit would not be significantly more than just the height of the distribution portion of the sub-frame. There is no reason that the joints themselves need to be on top of this sub-frame. Depending on desired "articulation" and the clearances required for that, they could be below the level of the top rail of the frame itself.

anyway, Food for Thought.

[mog]

Here's an FM for parts:
Looks like the FM frame does not taper at the rear the way the FG does.

I would not suggest using an FM frame, there is no hump in the front of the frame for the 4x4 drive train, only that the metal thickness and cross section might be a starting point.

Of course your frame is being loading much different then a 'normal' truck frame. Most commercial trucks will have the bed (flat, cargo, reefer, service) mount directly to the frame along it's whole length. Not only does this distribute the weight/load more evenly, but this bed and it's related load (both empty and cargo) aid in the frame's ability to resist twisting, bending, etc. Since you are using a 3 or 4 point frame which loads the frame at 'points', a look at Unimog frames (as already suggested) would give great insight into what Mercedes has done to accommodate that in thickness, cross-section and probably most importantly material and heat treating. A great benefit of using existing frames as a reference point to calculate data for your new frame, is after the manufacture has calculated their design, it is road/field test extensively (insert photo of cool Mercedes Unimog test track), to shake out the bugs.

[kerry]

I would not suggest using an FM frame, there is no hump in the front of the frame for the 4x4 drive train, only that the metal thickness and cross section might be a starting point.

.

I didn't intend to imply that the whole frame should be used, sorry. The FG frame is bolted together in sections I believe so the back section of the frame could be replaced with the back section of an FM frame (from the bottom of the hump back to the end). In Doug's case, he extended his frame behind the drop down section. I was thinking that removing the whole FG frame from the extension backwards and replacing it with a similar length section of the back of an FM (or FH?) frame might be possible and relatively inexpensive compared to a custom frame. All this assumes that there are no problems with the front section of the FG frame. Doug hasn't mentioned any but I suppose it too could be failing. Looking at the PDF of the frame strength that Doug posted, it looks like the weakest parts are all behind the hump except for the smaller section in front where the cab hinge attaches.
I was also thinking that it might be possible to just use the long rails of an FM or FH frame and transfer the lateral sections of the FG frame over to the FM rails since it looks like the lateral sections on the FM are different than the FG's.
It's just brainstorming. Using an FM or FH frame would almost certainly reduce frame flex which while solving the cracking problem, could possibly introduce other problems from lack of flex on uneven ground.

[Robthebrit]

I just went to measure my 416 frame for a reference..

Its a C section frame, single piece with a bend in the middle, 6.5 inches high, 2 1/8 wide and about 9mm thick. Its difficult to get any info on what a mog frame is made from because Mercedes say if you break the frame you scrap the truck (I think its illegal to fix frames in Germany). The gross weight of a 416 is about 14500 pounds.

Attached are pictures of the camper mog mounts, all the weight of the camper is really on 6 points because the front and rear pivots are the cross members so the load is spread to both sides, the center mount spans the frame. The front and back are pivots with a rubber bushing. The center mount is somewhat rigid but it can pitch forward and back on a rubber mount which you can't see it in the picture, its above the bar. The center mount is just in front of the rear axle, the front mount is about 3 feet in front of the center and the rear mount is about 3 feet behind the center and behind the rear axle.

You can see the floor of the camper is somewhat structural, it's made of 1 inch of marine plywood but the mount isn't really just a point. The camper side of the mount, which doesn't twist, is a piece of C section about 4 feet wide. The camper is really mounted on 3 bars and not 4 single points, the term 4 point mount refers to how it connects to the frame. In theory I could put a metal plate on the mounts and then put a more fragile shell on the plate (this is kind of what the original unimog spider frame does).

Edit: I'll measure a 1300L frame later on, from looking its way bigger than a 416 frame and its gross weight is still fairly low at something like 16000.

Rob

[Robthebrit]

I forgot this image (you can't add to the attachments in an edit)..

This is the frame from the parts manual, my camper attaches to the cross tube immediately to the rear of the bend and the cross member behind the axle/spring mounts. The sway bar goes through the tube in front of the axle/srping mounts and the center rigid mount is directly above this.

[mog]

I didn't intend to imply that the whole frame should be used, sorry. The FG frame is bolted together in sections I believe so the back section of the frame could be replaced with the back section of an FM frame (from the bottom of the hump back to the end).....

Darn good idea!!! It might solve all the problems and using a used FM frame, it could even come it pretty cheap.

[dhackney]

Guys,

Thanks for all the great input, feedback and suggestions.

I will try to answer as many questions as I can in batch mode.

a) We had to use a rear pivot point on our 3 point frame because we needed to lock the front of the camper to the garage. If we'd used a front pivot the camper would have contacted the garage or we'd have had to raise the camper to clear it.
b) Excellent photos of the legendary mog 4 point system. It's a lot easier to understand with photos. I think for those of us who are not engineers, it's like two 3 point pivot frames locked together and mounted back to back on a limited motion teeter-totter. We were unable to use a system of this type due to the camper/garage interference situation. The garage is rigidly attached to the frame. Our 3 point pivot frame is rigidly attached to the frame at the aft wall of the garage. If we put the entire payload on a new subframe and install it on a bigger truck we will probably use a four point system of this type to avoid having another very long distance between the fore and aft load points on the frame.
c) We were in the Amazon basin, so I missed the eBay FM frame. I think it could have been a potential solution.
d) You are correct on the potential use of an FM or other donor factory frame. Our strategy would be to graft it on at the stepdown or aft of our extension. We would strengthen the step-down as per the Fuso body builders docs at the same time.
e) I am in 100% agreement on the superiority of a building full of factory engineers with teraflops of computing power and a test track out back compared to anything we'd do under the shade tree. That is the fundamental reason we bought the FG rather than an aftermarket/3rd party system - it was the only factory 4x4 on the U.S. market.
f) The Bigfoot is surprisingly strong due to its fiberglass/foam/sandwich construction. It's built like a low-end sailboat hull. However, even though it might be the strongest truck camper out there, it wouldn't last 15 minutes on the roads we've been on if it was bolted to the superflexible FG frame.
e) As pointed out, a 3 point pivot frame that is attached on a cross member is not really a 3 point frame since it is distributing the load onto the main frame rails at two points. I had never realized that before. I feel better about myself and our Fuso already...
f) The biggest challenge with our 3 point pivot frame design (and I suspect Michel's) is that the rigid front mounts are a very long ways from the rear pivot mount. That means the entire load of (in our case) the camper, most of the external storage boxes and the pivot frame itself is being applied to two widely separated longitudinal points. To make matters worse, from the frame's standpoint, the rear pivot point is way back there where the frame tapers down and its section modulus is low. I think this is where we over-stressed the frame, by applying forces at the extremes of the dimensions of the frame.
g) I think the FG is a great chassis. The only problems we've had with anything even remotely connected with Mitsubishi Fuso are things we modified or changed.

Doug

PS - John, nice ketch!

[dhackney]

The famous aggressiveness of Pirhana makes them easy to catch.

Has anyone else here noticed that Chip always knows what he is talking about?





Easiest fishing of my life...