pivoting frames and mounting campers

[dzzz]

Bruce's subframe looks very substantial. In the picture it looks to be the full height of the dropped back section of the frame. He says that subframe is bolted to the original frame. It seems to me that a subframe of that magnitude would stiffen up the original FG frame to the point that a substantial part of the flexibility of the original frame would be gone, at least in the area of the chassis under the subframe.

I have a hard time picturing the subframe. Ate there more pics somewhere?

 

[kerry]

I have a hard time picturing the subframe. Ate there more pics somewhere?

In that top picture, the bottom of the shocks are attached to it. It's the straight steel line under the trailer frame with a lower curved line matching the dropping section of the original FG frame. It looks to be about 14" tall once it drops down to the lower section.

 

[ntsqd]

Anyone have any idea what kind of load rating the springs on a sprung u-bolt mount should have?

None, but an iterative solve wouldn't be too difficult; and the ideal spring rate may not be the same down the length of the rail.

I see Bruce is a Metallurgist.

He bolts and pads, doesn't weld or stiffen. Interesting.....

Welds are "On-Site Castings" (per Carroll Smith), with all of the usual traits that castings have.

 

[DontPanic42]

Here is a better shot of my subframe as it is attached to the FUSO frame. The subframe is 10" X 3" X 1/4 channel with 1' OAK strips. The air bag design is based on the spring system used on trash trucks. The subframe was designed so that the only attachments to the original heat treated FUSO frame would be bolts. All welding and additional drilling is on the subframe. Unfortunately, my travels have been curtailed for a while so I don't have as many miles on the design as I should have by this time.
Bruce

 

[kerry]

Bruce:
Do you have an opinion on the effect your subframe has on chassis flex? Am I right in thinking that your system is quite different than Doug Hackney's? His subframe is mounted to the FG chassis with pivot points. Your subframe is mounted with no pivots and your pivot system is between the subframe and the trailer frame? In essence, compared to people using truck campers, you have two subframes, the one you built on the chassis and the one that came with your trailer?

 

[whatcharterboat]

Bruce, your pivot to me looks indestructible and you seemed to have dealt with the effect the "step" has on a pivot / FG design far better than any of the others I've seen. Obviously it's high but with the sort of desert travel you do it wouldn't be so bad. would it?

Hope your travels plans for the "Road Hippo" start happening again soon.

Regards John.

 

[DontPanic42]

Bruce:
Do you have an opinion on the effect your subframe has on chassis flex? Am I right in thinking that your system is quite different than Doug Hackney's? His subframe is mounted to the FG chassis with pivot points. Your subframe is mounted with no pivots and your pivot system is between the subframe and the trailer frame? In essence, compared to people using truck campers, you have two subframes, the one you built on the chassis and the one that came with your trailer?

Kerry,
To be honest I haven't given it much thought as I don't have the mileage on my setup as Doug has so it hasn't seen the wear and tear. Your assesment seems just about on as my pivot points are on the added frame at basicly 4 points. Two (2) in the back and two (2) through the air bags up front. The front basicly floats. Quite a sight actually. The added subframe is bolted in 10 places ( 5 on each side) and sets directly on the FUSO frame for most of the length. The load is probably spread out quite evenly. My original design and the mock up I made included lengthening the frame to leave some storage space up front. As one of my design criteria was to be able to fit in a standard parking space, we shortened it up and moved the cabin forward about 18". We also tried to keep the center of gravity low so all the add ons except the cabin are kept at or below the frame.
It may prove to be some lucky "southern engineering" in the long run. Time will tell.
As you might remember my design criteria was based on desgining a vehicle that I could use with my chuck wagon cooking gigs. These might find me camping in a small parking lot like at the Ft Worth Stock Yard (bad scene) or driving up hill and down dale over dry creek beds or 'Jeep" tracks to totally dry campsites for up to 2 weeks on a ranch out in the boonies near the border of Old Mexico. More primitive but more fun. Somewhat different than Don, Doug, or Carl who have traveled long distances.
One thing that I was quite insistent about, was that no welding or drilling could be done on the heat treated FUSO frame. I didn't want hard or soft spots created from messing up the metal's structure and creating potential failure points. Also, it could void the warranty.
Hope that answer some of your question.
Bruce

PS:
John,
Thanks for the kind words. The folks that did much of the work on it do considerable work on big trucks for "bull haulers" (multi-level cross country stock trucks) so know how to build rugged. We nearly had daily get togethers kicking around ideas and changes. Lots of thinks happened on the fly. I have graduated from Research Engineering to the "cut to size, pound to fit "school. As I said , we may have just fallen into something that works with that good old "southern engineering". I do wish I had known you guys when we were starting this. Then again I might have been too intimidated and never started it. Guess it is easier when you don't know what you are doing and just go do it.
I am less than 12' so I can get to most places. When I go to the mountains though, I need to check for low tunnels and have a plan 'B' if necessary. - BAH

 

[DontPanic42]

I was finally able to get a copy of the shot of the rear pivot point. Sorry about it being so busy. From top to bottom, you can see the original trailer (caravan) frame, the new subframe with the pivot point from the trailer bolted in place, the oak interface, and the original FUSO frame with the tail light assembly attached. The combo trailer hitch is bolted to the FUSO frame.
The white blob with the handle to the far right is one of 2 fold down jacks that can be extended out to make a wider foot print if I have to park where there is a significant cross wind. Normally, I park facing into the wind but sometimes you don't have a choice.

 

[Terrainist]

Thanks for the pics and the comments Bruce, they go a long way in trying to figure things out.

Have you ever tried to induce some twist in your truck frame, say over a berm or similar, and stopped and looked to see what is going on with your frame and the caravan in regards to any twisting like that?

Does the caravan pretty much always ride on the air bags evenly because they are close together and your truck frame doesn't twist that much?

Did you do any beefing up of the caravan frame to resist any warpage like that?

Any comments are appreciated. Nice idea on the outriggers as well.

Thanks - James

 

[dzzz]

Thanks Bruce. So the rear of the frame is a hinge? With one hinge(bolt) per rail?
How much of the campers weight is carried on the air bags? How much of the camper frame is not in contact with the sub frame when at rest?
Don

 

[DontPanic42]

Thanks for the pics and the comments Bruce, they go a long way in trying to figure things out.

Have you ever tried to induce some twist in your truck frame, say over a berm or similar, and stopped and looked to see what is going on with your frame and the caravan in regards to any twisting like that?

Does the caravan pretty much always ride on the air bags evenly because they are close together and your truck frame doesn't twist that much?

Did you do any beefing up of the caravan frame to resist any warpage like that?

Any comments are appreciated. Nice idea on the outriggers as well.

Thanks - James

James, the short answer to your questions is basically - No. When I started this project in earnest in 2002 I was getting my information from magazine articles from the UK and Fuso dealers. Eventually, I found out about Carl Hunter's Bigfoot. Since the FG is frequently used in quarry operations and Carl's trip through Eastern Europe and China, I figured that the FG would handle anything I would subject it to. So, I haven't done any testing to check the amount of twist I would see. The only thing added to the caravan frame was some extra square tubing for attaching additional items or some spacers.

"Thanks Bruce. So the rear of the frame is a hinge? With one hinge(bolt) per rail?
How much of the campers weight is carried on the air bags? How much of the camper frame is not in contact with the sub frame when at rest?
Don "

The camper (caravan) only connects to the subframe just at 2 points in the rear and the 2 air bags at the front. Because of how the plumbing is arranged, the camper frame cannot bottom out on the subframe. These pictures show the spacing between the bottom of the camper and the subframe. The rusty piece of square tubing is attached to the camper frame.



This is at the front. If you look near the yellow shock there is a piece of angle on the camper frame directly over the rubber bump stop. This keeps the camper frame from dropping to far down if the bags should fail and crush the plumbing you can see in the background as well as damaging the black and grey water tanks.



A similar shot at the rear.



Hope that answers your questions. If not let me know - Bruce

 

[alan]

So at the end of the day the answer is to make the chassis rigid and rely on the suspension to do the job it's designed to do, I have not had much experience with Mitsubishi's but have seen bucket loads of Ford bronco's and F series truck in OZ with flexible chassis's and they all crack with severe off road work, constant twisting will kill anything.

 

[dzzz]

So at the end of the day the answer is to make the chassis rigid and rely on the suspension to do the job it's designed to do, I have not had much experience with Mitsubishi's but have seen bucket loads of Ford bronco's and F series truck in OZ with flexible chassis's and they all crack with severe off road work, constant twisting will kill anything.

I disagree. It's the simple answer that has many people in trouble.
The "answer" is to properly engineer the subframe and suspensions based on the specific truck's frame design. Flexible and rigid are not either good or bad. Just a design choice made by engineers. The camper needs to be attached in such a way that point loads to exceed the design. It also needs to be protected from some intended frame movements.
Perhaps what has been underestimate, at least by me, is the change happening day to day at over stressed points on the frame. It's easy to imagine that the frame breaks during one unfortunate event. But what probably happened to both FGs with the recent problems was that the metallurgy changed over a year or two of stresses beyond the point where the metal can return to its original form.

 

[dzzz]

............

The camper (caravan) only connects to the subframe just at 2 points in the rear and the 2 air bags at the front. Because of how the plumbing is arranged, the camper frame cannot bottom out on the subframe. These pictures show the spacing between the bottom of the camper and the subframe. The rusty piece of square tubing is attached to the camper frame.

Thanks again. Bruce, did you consider eliminating the subframe and using more attachment points? Say four sets of shocks spaced evenly down the frame attaching the camper frame and rail?

 

[ntsqd]

I disagree. It's the simple answer that has many people in trouble.
The "answer" is to properly engineer the subframe and suspensions based on the specific truck's frame design. Flexible and rigid are not either good or bad. Just a design choice made by engineers.

Not so much a deliberate choice as a forced decision. A ladder frame can only be so torsionally rigid, and frequently other considerations, like economy of production and weight, are given higher importance. Could a ladder frame be designed that would not torsionally deflect under the intended loading and have enough Factor of Safety to handle any unintended impact induced over-loads?
Certainly, but it would not be a truck that would be economically competitive, nor would it's payload capacity vs. GVW be very favorable.

The round tubular Tatra (and Pinzie?) back-bone frame design is nearly as simple as a ladder frame, and will be more rigid torsionally and easily it's equal in bending. It's just not going to work very well with a live axle, which complicates the suspension design.

If one were starting with a blank sheet of paper and a deep wallet I highly doubt that a ladder frame would find it's way into the design. Under the skin I would expect to find something that looked not much different from cross between a railroad truss bridge and a desert racing Trophy Truck.

The camper needs to be attached in such a way that point loads to exceed the design. It also needs to be protected from some intended frame movements.
Perhaps what has been underestimate, at least by me, is the change happening day to day at over stressed points on the frame. It's easy to imagine that the frame breaks during one unfortunate event. But what probably happened to both FGs with the recent problems was that the metallurgy changed over a year or two of stresses beyond the point where the metal can return to its original form.

The proverbial straw the broke the camel's back takes the blame when it really was every single one of those wash-board road ridges that cumulatively did the damage.