Well, I broke it.

1aquaholic

Adventurer
I asked many questions to people who I thought had the knowledge to answer them. I would be very grateful if any one here could give me a better resource so that I could clear this up.

j
 

mog

Kodiak Wrangler
The only input I can give is I have a liftgate on my truck. It's mounting extends 2 feet aft of the Fuso's frame and it is rated at 2000 pounds.
I've had close to that on it with no problems BUT that is STATIC. Off course a dynamic load is going to put much more stress on your frame.
_
You might see if there is some information in the Fuso's body builder manual HERE
See chapters ( if using this one ) 1-2-2, 1-3-5, 1-4-1, 1-10-1 (I guess the "do not install here" means outside of the rail, behind the wheel because of debris).
It looks like from the 2nd manual ( BBManual 2 ) that the FE had a rear center fuel tank as 'stock', but certainly not a large as yours. Page 11-12-4
The 2nd manual should be helpful as it gives you all of the factory frame dimensions, so perhaps your 'mod' falls into one of those ranges.
It also has all the frame modulus information which is very useful to calculate the allowable loads (for people with bigger brains then me)
 
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1aquaholic

Adventurer
Thanks MOG, I have been through those documents many times and assumed the same about the "do not install here". The only thing it really says is how to attach the extension if it's under 10 inches or over, nothing about max length or loads.

j
 

mog

Kodiak Wrangler
I'd guess that worst case you could not fill it full (if I remember from your other thread in was 110 gallons total, with most for veggie and 15 for diesel ?).
Work your way up with a fuel load that keeps it within were the stock aft Fuso tank would be, and go up in increments from there. Monitor closely for any problems or discrepancies.
Of course you could probably get away with more fuel with strictly on-road driving and reduce it when rough-road driving
 

1aquaholic

Adventurer
That is correct and very good points, that terrible feeling in my gut is a little less intense now, thank you for that.

j
 

SkiFreak

Expedition Leader
From memory, you are/were a welder, so definitely not someone I would consider "without a clue".
If you really are concerned you can always add some additional bracing now, opposed to fixing things should they go wrong down the track. Obviously, extra steel means extra weight, but there is not that much you can do about that.
As has been mentioned, on road driving will yield far less stress than off road driving, so you have to design according to how and where you plan on going with the truck. I am seriously into over engineering, and pay for that when it comes to the weight of my truck, but I work on the theory that a bit stronger is better that not really strong enough. I worry about the overall weight sometimes, but not the strength of things I have built.
It's a tough call, but if you have concerns, now is the time to do something about it; but you already know that... ;)
 

1aquaholic

Adventurer
Thanks, I would be happy to reinforce but don't have the knowledge of where and how, maybe I'll hire a engineer to take a look. I don't plan on doing heave off roading, mostly beach, snow, and dirt roads......at least for the first few years but then to south america and maybe shipping it down the road to the other continent.

Sorry pugslyyy for the high jack, just got me freaked!

j
 

GR8ADV

Explorer
The only input I can give is I have a liftgate on my truck. It's mounting extends 2 feet aft of the Fuso's frame and it is rated at 2000 pounds.
I've had close to that on it with no problems BUT that is STATIC. Off course a dynamic load is going to put much more stress on your frame.
_
You might see if there is some information in the Fuso's body builder manual HERE
See chapters ( if using this one ) 1-2-2, 1-3-5, 1-4-1, 1-10-1 (I guess the "do not install here" means outside of the rail, behind the wheel because of debris).
It looks like from the 2nd manual ( BBManual 2 ) that the FE had a rear center fuel tank as 'stock', but certainly not a large as yours. Page 11-12-4
The 2nd manual should be helpful as it gives you all of the factory frame dimensions, so perhaps your 'mod' falls into one of those ranges.
It also has all the frame modulus information which is very useful to calculate the allowable loads (for people with bigger brains then me)
I always suggest professional advice. It is not reasonable to expect proper technical solutions/advice without someone knowing exactly what you have done, how you did it, and how you plan to load your vehicle. Loading behind the rear axle completely changes the stresses within the frame. Fatigue will be the killer of a truck frame. I am not sure who on this site can model that effectively.
 

pugslyyy

Expedition Vehicle Engineer Guy
Based on observed failures and confirmed by the FEA that I've recently done, the biggest stress riser is just in front of the front spring mount, just before the crossmember.

The frame is going to bow (or probably balloon) over the rear axle. Right where you are adding load via the air bags. How are the air bags attached to the frame rail and are they tied to the crossmembers?

Steel can take a lot of stress, so deformation doesn't equal failure but I would look carefully at what is going to happen to the frame rails between the main cross members. You are definitely adding some stresses there.
 
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gait

Explorer
when I saw the photos the first time round my thought was "that's a bit unusual" and looked a bit more closely.

I figured that on my FG the spare wheel was originally hanging off the rear and despite the taper on the chassis rails there's probably enough to hold the tanks. The taper on the tanks will probably help. I also figured the tanks would probably fail before the chassis. I vascillated between "calling the baby ugly" and doing nothing. I figured that while unusual it wasn't a show stopper.

Now the pics appear again and my reaction is similar. Unusual but probably ok. For no other reason than it looks odd but ok.

There's a few things happen. One is the static load. And if the chassis can't take that we are all in trouble. The dynamic loads will depend somewhat on how the body is mounted as well as the tanks. With few mounts I suspect there's a tendancy for resonance. With lots, or a continuous piece of wood or polyurethane there's possibly less. A bit of damping.

There's a couple of modes of failure. One is the off-road large amount of twisting, relatively slow and infrequent that can lead to overstressing the chassis. Bending, twisting, breaking. Seems to be rare. Another is the fatique failure from frequent bending that's as likely to occur on as off road and takes a longish time to manifest. I suspect, given the angle and type of fracture that Pugslyyyy's failure is fatigue though at this distance I could well be wrong.

Another aspect is weight distribution. Weight at the extremities add to the moment of inertia. Too much weight at front and rear and the vehicle will pitch and yaw, too much weight up high and it will roll. Different to just looking at where the centre of gravity and where the roll centre is. I haven't thought about it too much recently but uneven weight distribution can effect roll steer and handling in high side winds.

So, what am I trying to say .... there's a risk whenever a body and other stuff is mounted on the back of a truck. Certainly the tanks look odd, but not sufficiently odd for me to ring alarm bells. Usual disclaimer of course, impossible to be definitive at this distance.

I'm a metallurgist, not an engineer, and my maths on bending moments and all that stuff is very rusty.
 

blackduck

Explorer
Ok this thread has really got me nervous, I've just got done extending my frame and building a very large fuel tank (110gl). I asked a bunch of frame guys in my area and they said I would be fine as it would counter the engine weight over the front axle, but now I'm thinking I'm going to "bow" the frame? What do you guys think?

j
Without seeing the build in detail the only thing I can deduct is youll have near on 1/2 ton of load behind the axle (which is a great pivot point working against the engine and gearbox that is reasonably well proportioned over the front axle with nothing in the middle except for the chassis and sub frame of the camper body
Chances are you might move your stress point to just in front of the rear axle
if the tank straddled the rear axle with a bias towards the front then the load would be better shared
my experience is loading and unloading oil tankers (ships not trucks) where measured balance and even distribution is the key
if we had the engine at the back and put all the cargo up front the thing would snap like a twig
best of luck
 

pugslyyy

Expedition Vehicle Engineer Guy
I would say that mine was definitely fatigue. While there were eventually failures at the 3 point mounts, the initial failure was at the frame just in front of the rear axle front spring mount, with bends to the frame between the spring mounts (the frame rail basically "unbending" there)

I've been thinking more about it, and would definitely recommend fully boxing the frame between the main crossmembers on this design. The combination of the direct load transfer via the air springs + torsion from the big tank in back make that an area of concern for me.
 

boblynch

Adventurer
I've got dual fuel tanks (50gal gallon midship and 40gal OEM rear) and dual water tanks (stock 30gal and additional 40gal rear) on my F550 camper. As was mentioned previously it's possible to vary your liquid loads based on driving situation. We normally use both fuel tanks for long highway runs and keep water tanks low. When we go off road we use only the front fuel tank and that allows us to carry more water without the extra frame stress.
 

pugslyyy

Expedition Vehicle Engineer Guy
A quick update, the new frame is coming along - has been cut and bent, will get welded up tomorrow.

The new frame rails
download_20140819_152546.jpeg

crossmember parts and pieces...
download_20140819_152539.jpeg
 
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