pivoting frames and mounting campers

Christian

Adventurer
I'm not sure that is actually what you need. You have the pivots to allow articulation of the frame, and to reduce/eliminate stress on the camper. It seems that the swaybar would simply work against this... ? I think it would be inferior to the spring loaded rail on rail systems, because they have a solid connection between the frames (well, they are spring-clamped together) allowing for good "normal" performance, but will allow some motion when the twisting stress gets too high. When this happens, some stress gets imparted to the camper, but hopefully not too much. With your pivoting system the swaybar does basically nothing to resist small motions (like you'd encounter in normal driving), but imparts high stress when you have a lot of twist.

If there is too much sway on a pivoting system, I think shock absorbers between the frame and subframe would be the best. This would damp any sudden motion between the frame-subframe but still allow full articulation and be stress free when you get crossed up offroad.

Shock absorbers have been tried on multiple builds, usually as a band-aid, I have yet to see a 100% succesfull install.
Besides shocks are for fast acting motion, not a slow lean in a round-a-bout or curve.
Furthermore, if the shocks could counteract the motion as you suggest, they would put the exact same stress on the box, as the torsion bar, simply because they would counteract the same motion, right? If they didn’t put any stress on, they would not counteract any motion.
The idea that no torsion at all may be introduced, is wrong, especially in Sitecs build, have you seen his box? It’s more sturdy than most! Torsion will always be applied from the motion of the vehicle and the box. When cornering the dynamic forces front to back are not the same.
In Sitec’s setup the theoretical question is wether the forces introduced by the torsion bar is stronger than the subframe/box can handle. In reality I’m sure Sitec’s box is plenty strong.
 

rruff

Explorer
Pivoting builds that have used shocks? I haven't seen any except for Idasho's, which was not optimized for it. You want high damping low travel shocks (if you are space limited like mine), mounted pretty much vertical and outboard. I did some searching and found some old corvette rear shocks that seemed like they'd be decent... for my build on a pickup.

I thought the major issue was the rear end shifting back and forth and juddering over irregularities or sudden steering inputs. A slow lean would not bother me... just slow down.

No, the shocks would not come close to imparting the same degree of stress. They damp sudden motions to prevent resonance and bouncing back and forth. When you really get crossed up on technical terrain, they would impart no stress at all. The swaybar is just another undamped spring and will behave like one in normal conditions, and will impart maximum stress/resistance when you are crossed up.

If the frame/box is strong and flexible enough to take that stress, then it seems like it would be much better to use a normal rail on rail spring mount.
 

Christian

Adventurer
Pivoting builds that have used shocks? I haven't seen any except for Idasho's, which was not optimized for it. You want high damping low travel shocks (if you are space limited like mine), mounted pretty much vertical and outboard. I did some searching and found some old corvette rear shocks that seemed like they'd be decent... for my build on a pickup.

I thought the major issue was the rear end shifting back and forth and juddering over irregularities or sudden steering inputs. A slow lean would not bother me... just slow down.

No, the shocks would not come close to imparting the same degree of stress. They damp sudden motions to prevent resonance and bouncing back and forth. When you really get crossed up on technical terrain, they would impart no stress at all. The swaybar is just another undamped spring and will behave like one in normal conditions, and will impart maximum stress/resistance when you are crossed up.

If the frame/box is strong and flexible enough to take that stress, then it seems like it would be much better to use a normal rail on rail spring mount.
It seems you haven’t read why the torsion bar is mounted.
It’s not juddering, it’s lean. Conventional wisdom has the box mounted at the rear with the front being able to shift in a 3 point setup. Sitec has done it opposite, and I plan to do the same. The downside is that the box is free to lean in the back, making a S-maneuver or going straight through a roundabout possibly a bit lively. The torsion bar is there to mitigate that.
A pair of small car shocks would do nothing to stop a box weighing tons do that. So you are correct, they would not impart the same stress, because they wouldn’t do anything to mitigate the problem.
I suggest going a few pages back, possibly read Sitec’s excellent build thread to get the why and how.
 

rruff

Explorer
If the torsion bar is designed to prevent lean... then you might as well hard mount the subframe to the frame, and forget about pivots. The rail on rail spring systems do in fact act like a hard mount until a certain threshold is reached and then the frame and subframe are allowed to separate.

Shocks will mitigate sway during quick maneuvers quite well if they have sufficient compression damping, which you typically find on sports cars and high end off-road truck suspensions. The undamped swaybar will not work well... unless it's so rigid that it defeats the purpose of isolating the camper from torsion. Of course car shocks will likely be insufficient for that size of truck... but you can surely find something that works.

Do you know of any pivoting builds that use shocks?
 

Christian

Adventurer
If the torsion bar is designed to prevent lean... then you might as well hard mount the subframe to the frame, and forget about pivots. The rail on rail spring systems do in fact act like a hard mount until a certain threshold is reached and then the frame and subframe are allowed to separate.

Shocks will mitigate sway during quick maneuvers quite well if they have sufficient compression damping, which you typically find on sports cars and high end off-road truck suspensions. The undamped swaybar will not work well... unless it's so rigid that it defeats the purpose of isolating the camper from torsion. Of course car shocks will likely be insufficient for that size of truck... but you can surely find something that works.

Do you know of any pivoting builds that use shocks?

You are operating in a binary mindset. If your logic should be followed, then there’s no need for a torsion bar in an independent suspension, might as well eliminate articulation.
Again, you haven’t read the description of neither install or use.
I’m not going to comment this anymore…
 

javajoe79

Fabricator
I think one of you is looking at on road manners and the other is considering what will happen when you have it crossed up, off road. On road the sway bar will do what you want it to do and that is to prevent the box from flopping around excessively. Off road it will limit the amount of separation, just like it limits articulation of suspension. That fights against the purpose of the articulating subframe in the first place. I would probably take off one of the links for off road travel where you might get crossed up and put it back on for on road travel.

If you ask me the perfect solution here would be stiff shocks and the sway bar with a disconnect for one of the links when you are off road. That way you have nice flat cornering on road and controlled movement off road. Appropriately valved shocks would absolutely slow down any motion of the box and they would do it without fighting each other and putting more twist into the subframe. Alternately you could use air bags instead of the bar and lower their pressure for off road travel. Pretty easy to do with in cab controls.

That bar seems VERY stiff. You mentioned it came off of the front suspension of a big truck?
 
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Christian

Adventurer
I think one of you is looking at on road manners and the other is considering what will happen when you have it crossed up, off road. On road the sway bar will do what you want it to do and that is to prevent the box from flopping around excessively. Off road it will limit the amount of separation, just like it limits articulation of suspension. That fights against the purpose of the articulating subframe in the first place. I would probably take off one of the links for off road travel where you might get crossed up and put it back on for on road travel.

If you ask me the perfect solution here would be stiff shocks and the sway bar with a disconnect for one of the links when you are off road. That way you have nice flat cornering on road and controlled movement off road. Appropriately valved shocks would absolutely slow down any motion of the box and they would do it without fighting each other and putting more twist into the subframe. Alternately you could use air bags instead of the bar and lower their pressure for off road travel. Pretty easy to do with in cab controls.

That bar seems VERY stiff. You mentioned it came off of the front suspension of a big truck?
I sugget you read Sitec’s build thread. Besides being a very sturdy and heavy all steel box, as far as I’ve read, he plans on disconnecting it when offroading.
 

rruff

Explorer
I think one of you is looking at on road manners and the other is considering what will happen when you have it crossed up, off road....
If you ask me the perfect solution here would be stiff shocks and the sway bar with a disconnect for one of the links when you are off road. That way you have nice flat cornering on road and controlled movement off road...
Alternately you could use air bags instead of the bar and lower their pressure for off road travel. Pretty easy to do with in cab controls.

I'm considering both. In order to make the swaybar stiff enough to work well on-road it's going to be very stiff when you really need articulation... as you said, defeating the purpose. If your subframe/box can stand stress, the spring loaded rail-on-rail seems like the best solution, since in normal road situations that keeps the frame/subframe as one unit, and only provides flex when articulating stresses are high. Plus, no need to connect/disconnect anything. But a beefy sway plus shocks, and a disconnect offroad should work well also.

I think the airbag idea is good... adjust the resistance to taste.
 

Ramdough

Adventurer
I saw a build where they had a hydraulic piston on both sides of subframe. As one extended, the other would retract. They then had a valve between them so they could adjust the resistance. On road, they closed the valve mostly…. Off-road, it was open.


Sent from my iPhone using Tapatalk
 

Sitec

Adventurer
My turn... ;) !
So, I pondered how best to fit our hab box to our truck for a long time. Rail on rail vs 3-5 point mount. I settled on the 5 point mount for several reasons. Firstly, having spent several years driving Mercedes 1617's and 2421's on some pretty average African roads. The thump and bang from the hab box flexing on the rails was one thing, but welding a box up where it was cracking was another. These trucks did get used off road. Not a huge amount, but off road none the less.
Now I'd settled on using a system with mounts I then looked into that a lot deeper. I came to the conclusion that most who use this system run a completely seperate subframe with their pivots built into the subframe... I soon worked out that this has a flaw in that all the 'pivot points' sit above the chassis (approx 300mm above the chassis twist axis). This in its own right builds in huge shear loads on the pins, chassis and subframe, which is why most designs are over engineered and require the chassis to twist along a different plane. It also adds complexity and weight to the build into which it is deployed. I have a hab box which was originaly rail on rail mounted and is already over engineered and built to be hammered over corrugated dirt roads in WA. It survived, the Isuzu that was under it fell apart after 5 years! Keep this in mind as you read on. Soooo, I have a heavy well built 12 tonne truck with a very flexible chassis (which I used as a crane truck for several years before starting this build). It loves to twist, making it surprisingly good off road. I have a solid steel contruction hab box. Why then do I need a subframe? I don't believe I do. The chassis and the hab box form the two halves of the 'subframe' required. All I need is the pivot points. By building like this, I can put the pivot points exactly where they need to be without inducing load. I have my two fixed points about a meter from the front of the hab box, utilising two existing holes in the chassis no more than 100mm from the rear hanger on the front axle springs.... right where the load is carried (this is to help keep the crawl thru doors in line with each other when under mild twist). I have the first central pivot mounted to the X member that links the two front hangers of the rear axle springs. The second pivot is mounted to the second x member which happens to be between the two rear spring hangers. This means that all 3-4 tonnes (max) of our hab box is being carried close too or at the spring points. The chassis is just there to hold it all together. Finally I mounted a third center pivot right at the back just for a little support and to stop any chassis chatter. I have built into the base of the hab box 2 bump stops along it's length on either side, allowing the chassis to twist no more than 100mm over it's usable length of 6m (which equates to 200+mm of twist at the wheels before the springs do anything. This allows the chassis to twist a little when needed but not excessively. The hab box can handle some load. The springs then do the rest. I was able to jack the rear left wheel off the ground 450mm (with blocks still in between the front springs/axle and chassis... which I remembered were still in there later on that day). While under twist, all three pivot pins could be moved easily with a light tap of a hammer (no excessive load).
Now to the anti roll bar... I looked at hydraulics, I looked at air, I looked at locking pins. Hydraulics (2 rams with a tap in between) is the way to go if $$$ was in abundance. Both hydraulic and air require hefty mounts, air bags or rams and hoses and space. Locking pins require crawling under the truck and hammering several pins home if you have managed to park in an un twisted position first up! No thanks. That left me looking at what was left of my donor truck, and it's front anti roll bar (not back). This anti roll bar is about 35mm thick, and designed to stop the cab and front of the truck from rolling about. Small loads... With this in mind, I tacked the anti roll bar mounts in place where I wanted it to be, and undertook a chassis twist test again to see that I could still get the travel I required. I stopped at 500mm purely because I ran out of jack and blocks! Everything (including hab box mounts) were still tack welded at this point, and nothing let go. Happy days.

In closing, all I'm after is to allow movement between the hab box and chassis to assist with axle travel when off road, whilst still hoping to retain an upright hab box when cornering on roads, while not having anything complicated or difficult to replace under the truck in the way of air bags, hydraulic rams, hoses, taps etc should they decide to let go. I have built in a 'get out of jail' card in that I have an accurate 100mm gap between the hab box and the chassis so if something does go drastically wrong, I can very easily slide in a pair of 100mm rails and make it rail on rail. At this point though, I don't see there being an issue. The hab box allows chassis twist giving me an off road advantage which we will use occasionally, whilst still feeling rigid and sitting firm when on the road. Wish me luck!
 

Christian

Adventurer
Sitec, I don’t think you need it, luck that is?
I know your build thread, its very impressive and well thought out.
Your point about the pivot points being in the plane of the chassis is very important, had you raised them like 200mm (8”) above that, I’m pretty sure you couldn’t have 4 points without them binding up under flex.

As you know, I will try the airbags, but my starting point is a bit different. I will integrate cab and box, so there’s no flex there.
My cab is not mounted to the chassis like yours, it has conventional cab mounts like a pickup/landcruiser/tractor. So that end is very simple and the LORD mounts I talk about is just a better and stronger version of what is already there.
As some have linked to, there was a Unimog which gave LORD mounts a bad rep here. Having read the details, I still have full confidence in them, but noticed another thing in that saga.
They experienced another problem that was not a single (albeit grave) incident; vibration! And many others with real world experience have said the same.
Vibration is just as big an enemy as twist, in the Unimog screws fell out, things fell apart because of vibration.
Resonance and vibration introduces fatigue in all kinds of places including the box itself.
Your box is a lot sturdier than what I can put on, due to weight, so mine will resonate more.
That is why I have designed what I have, and have the airbags.
 

Sitec

Adventurer
I think having the box and cab intergrated will work well. Having the front section of your 'cab box' combo mounted on fexible rubbers, with the fixed points somewhere roughly where the cab abd box are mated and then allowing the rear to twist with 1 or 2 pivots should work as needed. At worst, if one of the front rubbers does fail, you still have plenty of supports and can work around the problem. Following with interest. :)
 

joeblack5

Active member
Christian,. Maybe I misunderstand your illustration. You plan to support the cab / box weight with two lord's in the front and 2 air bags in the rear?.. I do not think that is a good strategy from the chassis perspective. The watts however nice has only the function to keep the box aligned but does not carry weight thru to the frame.. our 404 has supports about every three to four feet.. the rigid spine that runs thru just above the pivot points can twist but also helps the frame strength wise. The sub frame spine is not subjected to twist stresses from the road surface but it is going to be extended and compressed by inertia stresses when you go thru bumps on the road . Basically the chassis / frame becomes from 2 parallel beams three parallel beams with the third located above the original two. So a lot stronger from a bending perspective and of course not from a twisting perspective.
If you really want to use bags that I would put those closer to the rear axle and put the watts in the rear.

Good luck

Johan
 

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