pivoting frames and mounting campers
- Thread starter lehel1
- Start date
DontPanic42
Adventurer
James,
No - we never thought about that kind of an arrangement. When we first tried out the air bag setup, the the camper bounced around like a crazed bobble head doll. That is why we added the shocks to control the amount of movement. I am not sure that we could do what you suggest because of the way the plumbing is located. A camper like Oliver, Bigfoot ( if they reopen), or custom box with enclosed plumbing might work for that kind of arrangement.
As an aside, did Doug or Michel save the pieces containing the fractures? There is considerable information in those fracture surfaces.
It sounds this discussion is moving away from the traditional 4 point mounting and towards other designs. Air bags, dampers, monocoque camper/chassis, springs on u-bolts, 3 point mounting with heavy reinforced frames.
I'm a bit biased since this the design I'm using, but I don't see a reason to depart from the standard 4 point mounting which has been used with such great success by the European expedition vehicle manufacturers on a variety of different chassis (not just Unimogs).
Unicat in particular looks to be the most professional outfit around. It seems they have some very experienced professional engineers working on their designs. Many of their trucks go to the Africa, which has some of the roughest roads around. Looking at their website, they've designed campers for many different chassis, many of which are not designed by the manufacturer to use a pivot frame mounting. I see MAN, Unimog, Iveco Daily, Bucher Duro, Volvo, and Sprinter trucks.
Yet for all their designs they use the same 4 point mounting system. (they use the phrase "3-point kinematic attachment with main and flex mounts" which I think is a 4 point mounting, counting the middle mount as 2 points) And I don't hear of any frame failures on Unicat trucks.
Most of us don't have access to the engineering and test resources a company like Unicat does. As an engineer, I can say it takes a lot of talent, time, and money to develop designs like these. Shouldn't the 4 point mounting be the default design to go with? It seems like trying to use a brand new design is sort of a "reinventing the wheel", which is okay, as long as the resources are there to support that.
This page here is a great reference to Unicat's design process:
http://unicat.net/en/index2-Bodengruppe.html
I'm a bit biased since this the design I'm using, but I don't see a reason to depart from the standard 4 point mounting which has been used with such great success by the European expedition vehicle manufacturers on a variety of different chassis (not just Unimogs).
Unicat in particular looks to be the most professional outfit around. It seems they have some very experienced professional engineers working on their designs. Many of their trucks go to the Africa, which has some of the roughest roads around. Looking at their website, they've designed campers for many different chassis, many of which are not designed by the manufacturer to use a pivot frame mounting. I see MAN, Unimog, Iveco Daily, Bucher Duro, Volvo, and Sprinter trucks.
Yet for all their designs they use the same 4 point mounting system. (they use the phrase "3-point kinematic attachment with main and flex mounts" which I think is a 4 point mounting, counting the middle mount as 2 points) And I don't hear of any frame failures on Unicat trucks.
Most of us don't have access to the engineering and test resources a company like Unicat does. As an engineer, I can say it takes a lot of talent, time, and money to develop designs like these. Shouldn't the 4 point mounting be the default design to go with? It seems like trying to use a brand new design is sort of a "reinventing the wheel", which is okay, as long as the resources are there to support that.
This page here is a great reference to Unicat's design process:
http://unicat.net/en/index2-Bodengruppe.html
I found the Unimog body builder manual is available through this site:
https://bb-infoportal.mercedes-benz.com/portal/ar_unimog.0.html?&L=1
Part B is the main one that describes pivot frames. Wish I would have found this sooner! Note that Mercedes only recommends 3 point pivot frames for the shortest wheel bases - on all others they require the "2x3" pivot frame which I have been calling "4 point".
https://bb-infoportal.mercedes-benz.com/portal/ar_unimog.0.html?&L=1
Part B is the main one that describes pivot frames. Wish I would have found this sooner! Note that Mercedes only recommends 3 point pivot frames for the shortest wheel bases - on all others they require the "2x3" pivot frame which I have been calling "4 point".
whatcharterboat
Supporting Sponsor, Overland Certified OC0018
How many FGs have they done? Or at least the question should be > how many trucks have they done with a stepped chassis?
Jacob, the number of FGs we've built is well into triple figures. We never see frame failures as a result of body mounting. Some of these trucks are still in operation after 10 years or more and most of the trucks are absolutely pounded day after day in some of the worst conditions imaginable. These conditions are why most truck manufacturers come out here to do their initial testing of new models. Doesn't it even make you wonder just a little why we would never use a pivot mount on an FG?
The two recent frame failures (on Expo) using pivot mounts should not have happened . The owners in both cases went to extremes in terms of research, quality workmanship, materials, effort, attention to detail, etc, etc. First class construction, both of them and they should be praised for what they put together. There was just one fundamental flaw for which they can't really be blamed. They both appear to have had the same view of "If the Europeans do it with so much success on other trucks then it must be the best option for my FG too." Only thing is the Europeans aren't having successes with FGs yet.
I've probably repeated myself over and over on this. Apologies if I've offended anyone. Not my intention.
762X39
Explorer
Also, thanks for the link to the Mercedes guide, what a valuble find.
If I may offer my opinion I think this discussion may in some respects be the wrong way around.
Having followed the discussion plus the 2 or 3 others here on the subject I can't help but noticing that the reason some argue for using a 3 or 4 way pivoting system seems to be to allow the frame to act as suspension. I think this view might be wrong. I know of one truck that actively uses the frame as part of the suspension and that is Unimog.
Other manufacturers that uses riveted frames allow for flex because their chosen design would mean that a rigid frame, such as a welded one, would crack. This is an approach used in construcktion, brigde-building etc. allowing for a lighter but more flexible design.
There is a major difference in allowing some flex or using it actively!
So if we leave Unimog and their unique characteristics out, the reason that other manufacturers use some kind of pivoting system is not to allow the frame to flex, but to keep the flex from damaging whatever is put on the frame.
I hope you can follow my view on this, actively "encouraging" frame flex versus allowing for some frame flex.
In my opinion any sub-frame should eliminate torsional forces which might tear the camperbox apart, but at the same time it is very important that the way it does so does not encourage frame flex.
Disclaimer: Although I do not have any experience with Fuso's, I have experience with Unimogs, MAN's, Magirus' amongst others.
Having followed the discussion plus the 2 or 3 others here on the subject I can't help but noticing that the reason some argue for using a 3 or 4 way pivoting system seems to be to allow the frame to act as suspension. I think this view might be wrong. I know of one truck that actively uses the frame as part of the suspension and that is Unimog.
Other manufacturers that uses riveted frames allow for flex because their chosen design would mean that a rigid frame, such as a welded one, would crack. This is an approach used in construcktion, brigde-building etc. allowing for a lighter but more flexible design.
There is a major difference in allowing some flex or using it actively!
So if we leave Unimog and their unique characteristics out, the reason that other manufacturers use some kind of pivoting system is not to allow the frame to flex, but to keep the flex from damaging whatever is put on the frame.
I hope you can follow my view on this, actively "encouraging" frame flex versus allowing for some frame flex.
In my opinion any sub-frame should eliminate torsional forces which might tear the camperbox apart, but at the same time it is very important that the way it does so does not encourage frame flex.
Disclaimer: Although I do not have any experience with Fuso's, I have experience with Unimogs, MAN's, Magirus' amongst others.
I think Doug Hackney has posted in the past that MAN expedition campers have had problems with frame failures. If I'm right in remembering that, I wonder if these failures are related to pivot mounts in the same way that the recent FG failures are related to pivot mounts.
The Unimog does have a stepped chassis. It's stepped in a different way from the Fuso though - down instead of up, and it comes back up at the front.
Even on the FG, if you look at the video I made of my frame flex, you can see that there is very little side to side deflection due to the step.
I don't think we can use the frame failures of the Hackney truck and the Fuso Szulc as examples of why the 4 point mounting leads to frame failure. These trucks were both 3 point mounting which is very different and increases frame stresses by a minimum of 1/3 over the 4 point mounting. On the Unimogs the 3 point pointing is only allowed on the shortest wheelbase trucks, and even then the 4 point mounting is preferred.
I know everyone says the Unimog is "designed" for a torsion free mounting. But comparing my Unimog to my FG, I don't see what is different about the frame which would exclude a torsion free mounting. To me the frames seem to have approximately the same construction. As they say on the Unicat website "do other laws of physics apply to the Unimog?"
I'm not saying the 4 point mounting is the only way to do it, and it's not ideal for all situations. But if I had to choose someone to design a mounting for trucks traveling rough terrain, my money would be on Mercedes Benz due to their great experience and engineering resources in this field.
Even on the FG, if you look at the video I made of my frame flex, you can see that there is very little side to side deflection due to the step.
I don't think we can use the frame failures of the Hackney truck and the Fuso Szulc as examples of why the 4 point mounting leads to frame failure. These trucks were both 3 point mounting which is very different and increases frame stresses by a minimum of 1/3 over the 4 point mounting. On the Unimogs the 3 point pointing is only allowed on the shortest wheelbase trucks, and even then the 4 point mounting is preferred.
I know everyone says the Unimog is "designed" for a torsion free mounting. But comparing my Unimog to my FG, I don't see what is different about the frame which would exclude a torsion free mounting. To me the frames seem to have approximately the same construction. As they say on the Unicat website "do other laws of physics apply to the Unimog?"
I'm not saying the 4 point mounting is the only way to do it, and it's not ideal for all situations. But if I had to choose someone to design a mounting for trucks traveling rough terrain, my money would be on Mercedes Benz due to their great experience and engineering resources in this field.
charlieaarons
Explorer
Only the UHN (U4-5000) have a stepped frame. The UGN (U3-500) have a straight frame.
It's important to remember that Unicat's 4 point system has the pivots and trunnions mounted on strong, but bolted-to-the-frame crossmembers; not directly to the frame. So the load is spread to 6 areas, not 6 points.
And the frame is 9mm thick, on a U500.
Charlie
It's important to remember that Unicat's 4 point system has the pivots and trunnions mounted on strong, but bolted-to-the-frame crossmembers; not directly to the frame. So the load is spread to 6 areas, not 6 points.
And the frame is 9mm thick, on a U500.
Charlie
DiploStrat
Expedition Leader
Cure Looking for a Disease
I'm with Christian.
A pivoting/suspended/dancing sub-frame has no inherent virtue. What is the PROBLEM that you are trying to solve? In the case of overland campers, the fear is that the flexing of the truck under the large volume camper will damage either the camper or its contents, particularly partitions. So what to do?
Make a perfectly rigid camper
-- Heavy
-- Expensive
Use a truck with a perfectly rigid chassis
-- Doesn't exist OEM
-- Expensive to retrofit
-- Heavy
Use a truck with a completely compliant suspension
-- Doesn't exist OEM
-- Expensive to retrofit
-- May not be safe at highway speeds
Design a camper that can flex without damage
-- Will not be available COTS
-- May be hard/expensive
So that leaves us with compromises. The pivoting frame is one clever idea, but it may not always be the best idea. Most cars/trucks use a combination of at least three elements: suspension, chassis, body. Each is usually designed to work with the others. Thus the chassis provides the foundation for the suspension and the body reinforces the chassis.
Almost by definition, a pivoting sub frame will not stiffen the chassis. Small truck, light load, you may not care. Long truck, heavy load, you may care a lot. Even worse, if you are not very careful, your pivoting frame may actually concentrate the stress a specific points on the frame.
Suspension rates, chassis stiffness, and body stiffness must all work together or you will have a disaster - as some folks have discovered to their great expense.
This offered as an academic observation, not a proposed solution to any particular design problem. I merely note that many serious off road campers intentionally do NOT use a pivoting/suspended camper body.
I'm with Christian.
A pivoting/suspended/dancing sub-frame has no inherent virtue. What is the PROBLEM that you are trying to solve? In the case of overland campers, the fear is that the flexing of the truck under the large volume camper will damage either the camper or its contents, particularly partitions. So what to do?
Make a perfectly rigid camper
-- Heavy
-- Expensive
Use a truck with a perfectly rigid chassis
-- Doesn't exist OEM
-- Expensive to retrofit
-- Heavy
Use a truck with a completely compliant suspension
-- Doesn't exist OEM
-- Expensive to retrofit
-- May not be safe at highway speeds
Design a camper that can flex without damage
-- Will not be available COTS
-- May be hard/expensive
So that leaves us with compromises. The pivoting frame is one clever idea, but it may not always be the best idea. Most cars/trucks use a combination of at least three elements: suspension, chassis, body. Each is usually designed to work with the others. Thus the chassis provides the foundation for the suspension and the body reinforces the chassis.
Almost by definition, a pivoting sub frame will not stiffen the chassis. Small truck, light load, you may not care. Long truck, heavy load, you may care a lot. Even worse, if you are not very careful, your pivoting frame may actually concentrate the stress a specific points on the frame.
Suspension rates, chassis stiffness, and body stiffness must all work together or you will have a disaster - as some folks have discovered to their great expense.
This offered as an academic observation, not a proposed solution to any particular design problem. I merely note that many serious off road campers intentionally do NOT use a pivoting/suspended camper body.
Michel Szulc Krzyzanowski
Adventurer
I know everyone says the Unimog is "designed" for a torsion free mounting. But comparing my Unimog to my FG, I don't see what is different about the frame which would exclude a torsion free mounting. To me the frames seem to have approximately the same construction. As they say on the Unicat website "do other laws of physics apply to the Unimog?"
What is different about the frame is the size.
The frame of a Fuso FG is even less wide than of a Ford F-450.
And that has serious implications.
What is different about the frame is the size.
The frame of a Fuso FG is even less wide than of a Ford F-450.
And that has serious implications.
DiploStrat
Expedition Leader
You are making my point.
"What is different about the frame is the size.
The frame of a Fuso FG is even less wide than of a Ford F-450.
And that has serious implications. "
If your chosen truck has a smaller/lighter frame, then that would argue for reinforcing it with the body, rather than placing extra stress on it by concentrating the weight at only three or four points, some of which pivot.
None of the small Japanese/Chinese trucks that I see (Mitsubishi/Isuzu/Toyota/JMC) have any form of suspended body, rather all have simple steel cargo beds that effectively double the frame. And, of course, flex is totally irrelevant carrying bags of rice, boxes, or other general cargo. A large volume camper is an entirely different problem.
Provan, for example, who build on Chevrolet and Ford pick up chassis do the following:
-- Over the years they have upgraded from 3/4 to 1 ton models.
-- The camper is rigidly mounted with a steel base frame.
-- The camper itself is made of many welded aluminum square tubes.
-- The camper tightly bound to the cab.
Perfect? Who knows, but they have been building them for twenty five years and guarantee them for five years.
There is nothing wrong with a pivoting camper mount, unless it does not work.
"What is different about the frame is the size.
The frame of a Fuso FG is even less wide than of a Ford F-450.
And that has serious implications. "
If your chosen truck has a smaller/lighter frame, then that would argue for reinforcing it with the body, rather than placing extra stress on it by concentrating the weight at only three or four points, some of which pivot.
None of the small Japanese/Chinese trucks that I see (Mitsubishi/Isuzu/Toyota/JMC) have any form of suspended body, rather all have simple steel cargo beds that effectively double the frame. And, of course, flex is totally irrelevant carrying bags of rice, boxes, or other general cargo. A large volume camper is an entirely different problem.
Provan, for example, who build on Chevrolet and Ford pick up chassis do the following:
-- Over the years they have upgraded from 3/4 to 1 ton models.
-- The camper is rigidly mounted with a steel base frame.
-- The camper itself is made of many welded aluminum square tubes.
-- The camper tightly bound to the cab.
Perfect? Who knows, but they have been building them for twenty five years and guarantee them for five years.
There is nothing wrong with a pivoting camper mount, unless it does not work.
Exactly!
And please do not see this as critism of either skill nor knowlegde, but you cannot visually see or detemine differences in construction. Differences in steel, how attachments are made, rivets used, stresspoints etc. are not easily detected.
The original manufacturers have put thousands of hours in to these vehicles and after that they have spend thousands of hours testing them. They still do with every new model. I know some of you may have seen these accelerated tests they doo on Discovery or another "science" channel. The tinyest changes may have large effects.
So in my opinion (and I do not claim this to be true, just my opinion) you cannot adopt a system that works on one model and expect it to work on another, just like Whatcharterboat wrote above.
We have a lot of medium sized trucks here and they are all constructed like Diplostrat described, a rigidly mounted sub-frame that effectively doubles the rear frame.
As I wrote in my first post Unimog actively utilizes frame flex as part of the suspension and state doing so. Does Fuse state the same?