Rethinking the need for a torsion-free subframe...

[SkiFreak]

Something that gets continually overlooked is that these Fuso 4x4 trucks were never designed to compete with something like a Unimog.
Yes, the FG can be a capable off road vehicle, but its chassis is one of the weakest in this size/weight range of truck.

If the intent is to go into terrain that will involve a lot of suspension articulation then this must be factored into the camper body mounting design.
I should add... these trucks were designed to carry a load, so the default suspension and tyre setup is very stiff. The better the suspension moves the less stress will be transferred to the camper box.
A three point mount system on a FG, if not designed well, will allow excessive flexing to occur, which can be as bad, if not worse, than hard mounting the body.

If the vehicle will only do minor off road travel, hard mounting the body will probably cause little or no issues, but remember... you are not building a box truck, where flexing of the body is not an issue.
When you install fittings and fixtures inside the box, flexing can definitely become more of a problem.

There's not really any right or wrong way to mount a camper body, but how the truck will be used must be taken into consideration if you want to reduce the possibility of issues.
Personally, I think that a kinematic style mounting system is a relatively cheap insurance policy; but what would I know?

As I have said before... each to their own.

 

[DzlToy]

You will be hard pressed to find evidence of a rigidly mounted camper causing a frame or box failure for two reasons, IMO. First, few people mount camper boxes that way and 2) this market is so small already, you simply do not have a large dataset from which to pull.

If you were asking a question about a Land Cruiser or a Class A motorhome, you would get 10,000 replies. There are probably only a few hundred camper boxes mounted in FG trucks in the whole of North America, certainly not hundreds of thousands like mainstream RVs.

Energy, like water looking for a leak, takes the path of least resistance. So, in addition to designing proper mounts, whatever you deem that to be for your truck/box, you should strongly consider a "proper" suspension. Fuso builds work trucks, not comfortable trucks or fast trucks or trucks to carry 500 pounds. They build work trucks to carry heavy loads for hundreds of thousands of miles, mainly in cities or at low speeds. These trucks are simply not designed to travel cross country or tackle difficult trails. Many North American FGs do not even have a low range transfer box.

So, with a properly designed suspension, of which neither your chassis, nor your tires are components, as both are undamped springs, the energy goes to the suspension FIRST. Whatever is left goes into your mounting system and then to your camper box. If a rigid sub-frame is constructed to mitigate frame flex AND a well performing suspension is built, I believe that very little energy will ever make it to the camper. If you are concerned about stress being focused at the back of the cab, fish plate the frame to spread those stresses out. over a larger area.

 

[gait]

about 10 years ago I built with three point sub-frame on a medium wheelbase FG649. I decided on that approach after observing the amount of flex in heavier vehicles in normal road use, vertical as well as twist. I used fixed mounts just behind the step, pivot just in front of rear spring hangers. Pivot at the rear (rather than just behind or in front of step) to reduce body roll. Additional support on springs at front of box in front of step. The rear of chassis had been cut off just behind rear spring hangers by previous owner. The cantilever to two spares on the rear of house was longer than desirable. Ok for original wheels, which is what I designed for, not ok for two heavier after single wheel conversion. The sub-frame split, just in front of the pivot. I have no explanation for why there rather than behind the pivot. The vehicle weighs about 4.5 tonne loaded.

In the middle of a long trip, a bit of welding, move spares into house, the sikaflex between floor (50mm foam sandwich) and sub-frame helped hold things together.

Here's a couple of pics with the original springs. The vertical exhaust just behind the cab is a useful indicator of twist.




the springs were replace with softer parabolics. Here's a pic after the spring change.



When the opportunity presented the sub-frame was replaced. This time with 6 spring mounts over 2m of chassis from just in front of rear spring hangers. The short distance between the mounts limits the transmission of twist from chassis to sub-frame. I retained the spring supports in front of the step.

Here's a link to a one minute video, watch the vertical exhaust again particularly near the top of the dune (the dune was scalloped).

http://epicycles.com/Truck%20Blog/2017-06-08/P1100364.MP4

I realise the question is not how much the chassis twists, but rather whether u-bolts allow sufficient movement to not cause problems in chassis, sub-frame or body.

Something fundamental. The softer springs allowed more suspension travel with less chassis twist. More weight would have had similar effect. Leaf springs are progressive, the chassis twist is linear.

When the chassis twists, regardless of how a sub-frame is mounted, the sub-frame has to move relative to the chassis (or vice versa). As the assembly twists the sub-frame rail has to slide relative to the chassis rail. Attempting to constrain the movement "too much", as has been pointed out, will result in stresses appearing somewhere.

Fixed bolts will either work loose, shear, make holes oval, etc. A U-bolt allows some movement, whether its enough is a judgement. Spring mounts allow a bit more movement, whether its enough is a judgement. A three-point mount is almost free - there is twist at the "fixed" mounts, there are three dimensions to consider.

Part of allowing the chassis to flex is also keeping four wheels on the ground, articulation and traction. It may well be that u-bolts don't cause a chassis failure per se, but it may be that loss of flex reduces articulation. Just one part of articulation, springs and weight also play their part.

As to whether there are failures of u-bolt mounted sub-frames I have no idea. I've never seen one to perform any sort of analysis. Its not only whether its u-bolts its the whole design, of sub-frame, attachment points, box, weight, springs, wheelbase, etc. I have two data points. One more than I'd hoped for.

 

[Lachstock]

Came across this today, preet clean looking setup if you keep the weigth down.

 

[Buckstopper]

if you keep the weigth down

Not so sure about this approach. It looks like the bulk of the load of your camper is concentrated in one area of the frame. This type of pivot would be better with rails that were the length of the truck frame then the pivot was attached to those rails to distribute the load over the length of the frame.

 

[gait]

Not so sure about this approach. It looks like the bulk of the load of your camper is concentrated in one area of the frame. This type of pivot would be better with rails that were the length of the truck frame then the pivot was attached to those rails to distribute the load over the length of the frame.

also the pivot point above the axis of rotation of the chassis means the pivot, and thus the whole sub-frame and box, has to move sideways as the chassis twists. Not much, but once its moving it has inertia and has to stop.

 

[ControlFreak]

My body is hard mounted, it's aluminum angle iron bolted directly to the frame then the subframe of the box is made on aluminum C-channel which is welded to the aluminum angle sections. I'm about to bring it back to the shop to have a full inspection as it's been almost a year on the road and we want to see how the welds and bolts are holding up.

 

[Victorian]

Came across this today, preet clean looking setup if you keep the weigth down.

View attachment 550337

This method is not recommended by Mitsubishi. They only allow for a spring mounted subframe to spread the load.

 

[R1200GSA]

Howdy

I have zero experience in truck mounted campers but as a heavy truck mechanic I have heaps building bushfire trucks here in Oz.

These are fitted with an aftermarket tray with its own substantial chassis rails similar to what yabanja mentions. On this is mounted a water tank and all the ancillaries that go along with it.

We keep the mounting points very simple. U bolts along the chassis length similar to yabanja's method with springs between the bottom plate and the chassis plate. Conveyer belt style rubber between the two 'chassis'. I am unaware of any chassis / frame failures.

I'm not an engineer but I don't think the chassis are designed to flex to the extend that a single rear pivot allows. If this was the design brief you would imagine they would be built with a rear chassis rail setup for this. None of our fire trucks ever articulated like they did without a tray/body on it and I don't think we would want them to, given the ground they traverse and the possibility of a downhill rear wheel dropping into a hole and tipping it. Rather have less flex and the wheel not drop.

Anyways, just my thoughts. I am just starting the shopping process for a 4wd crew cab fuso and think I will mount it with U bolts but hey, with more research may change me mind.

Cheers

H

 

[KiwiMtnClmbr]

My body is hard mounted, it's aluminum angle iron bolted directly to the frame then the subframe of the box is made on aluminum C-channel which is welded to the aluminum angle sections. I'm about to bring it back to the shop to have a full inspection as it's been almost a year on the road and we want to see how the welds and bolts are holding up.

What did the inspection show ControlFreak? And if you have results/feedback/info to share, please also tell us what kind of use your rig has had and # of miles / days use you reckon it's been through. Many thanks.

 

[Zuber]

If you have the pivot point at the rear, like that photo, where is the chassis pivot center? I would guess mid-height on the chassis frame rails.

That red pivot frame has a lot of square inside corners and notches. It would have been very simple to add some large radius to the design.

 

[sixbennetts]

Following. What kind of payload weights are we talking about with the tankers? Or with a full custom made steel subframe/composite box?