pivoting frames and mounting campers

Ramdough

Adventurer
Hi Ramdough. Well, all but the lock outs/hydraulic system are built. I have the two outer pivots no more than 100mm from the rear hanger of the front springs about 400mm behind the cab, keeping the cab and body faces in a fairly similar plane at all times. I have two central pivots, the first is located on the rear springs front hanger cross member and the second on the rear spring rear hanger cross member. These are all tack welded to the body and are yet to be fully welded. I've since added another central pivot at the very rear of the truck. All pivot points are on the center line of the chassis. I have then twisted the truck chassis (rear lh dual wheels 400mm off the ground) to the point where the lower lockers just touch the body to check that it all worked without any of the tack welds going bang, which it did. To double check what you mentioned about binding, I then unbolted and removed each locking bolt in the pivots and tapped each pin with the hammer to see how much load they were under. To my surprise they would tap out with ease and have minimal to no load on them. I must have got something right! :) Re the hydraulic setup, I have gone away from this due to weight, the time involved and cost to do it. I now plan to utilize 2 (poss 4) stumpy 100mm x 100mm coil springs between the body and chassis these will be sat with no or minimal tension when the truck is flat, and will only be compressed as the chassis twists. Here's a pic of the chassis twisted and the body straight. it was in this position that the pins would tap out. :)

View attachment 575883
Disclaimer: I am not the biggest expert on here, there are much smarter guys on this forum when it comes to the subframes.

So, are you say you have 3 central pivots now and a front fixed point? Are they urethane or solid bushings? A full picture would help.

Part of the problem is that if your pivots are not exactly at the neutral axis of your torsional stiffness, then they actually cause your pivots to translate sideways. It sounds like you have tried to hit the centroid with your design, but if you are off, each mount will move a different amount sideways. Also, twist is not the only direction that your frame bends. So now each pivots is trying to restrict the way your truck frame wants to bend.

Whatever you end up doing.... keep in mind that your flex test is under no load. Once you are fully loaded and flexing, you will twist and want to bow some too..... possibly in multiple places.


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Sitec

Adventurer
Disclaimer: I am not the biggest expert on here, there are much smarter guys on this forum when it comes to the subframes.

So, are you say you have 3 central pivots now and a front fixed point? Are they urethane or solid bushings? A full picture would help.

Part of the problem is that if your pivots are not exactly at the neutral axis of your torsional stiffness, then they actually cause your pivots to translate sideways. It sounds like you have tried to hit the centroid with your design, but if you are off, each mount will move a different amount sideways. Also, twist is not the only direction that your frame bends. So now each pivots is trying to restrict the way your truck frame wants to bend.

Whatever you end up doing.... keep in mind that your flex test is under no load. Once you are fully loaded and flexing, you will twist and want to bow some too..... possibly in multiple places.


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Hello. Yes. I have 3 central pivots along the 'centroid'. The chassis is the same depth until just behind the rear springs. The two front points are not fixed, they are a form of pivot, but only need a minimal amount of movement allowance due to them being the side pivots. These are the foremost outer points (that hold the body upright...) without the fixing bolt and washers in place. The bush is 60mm as that happened to be the size of the hole in the chassis. Behind the upright plate there is a 100 x 50 RHS leg with a plate on its base sitting on the chassis. This carries the body weight, the bush is purely to locate it. I went this way, because on a true 4 point mount system, the front pivot point needs to be ideally right in the middle of the gearbox somewhere... which it can't be.

Front Mt 4.jpg
 

Sitec

Adventurer
Here's the first of the middle pivot points. These are on the two cross members that support the rear axle spring hangers... You can just see the second central pivot pin in the top of the pic. I hadn't fitted it at this point. Both central pivot points are only tack welded at this point. If there was any real load these tacks would have let go straight away...

Mid Mounts 4.jpg
 
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Sitec

Adventurer
Here's the rear pivot point which sits lower in the rear of the chassis as the chassis depth is shallower in the rear. It is in line with the two pivots in front of it. I'm not looking for huge travel like some vehicles. I'm aiming for around 50mm allowable upward travel between a chassis rail and habitation box rail.. 100mm in total. This will allow the chassis to follow basic humps and bumps off road without stressing the body, but then still gives me full suspension travel as well if I really get into the rough... The body I'm using/modifying was originally built for mine spec work in Outback Australia so is already over engineered and was designed to be 'rail on rail'. I have left an accurate 100mm between the habitation box rails and the chassis rails so if I have any issues, i can easily slide a length of RHS in each side and make it 'fixed'.

Chassis Mods 36.jpg
 

Sitec

Adventurer
Re things changing when loaded, I have kept batteries, fuel tanks, air tanks, gas cylinders, rear lockers and spare wheels and lift hung off the chassis, so the body's weight will only increase with interior fitout and aluminum external sheet.. Two tonnes tops. As it is sat on 5 points spread across the chassis the weight should be spread pretty evenly across the points. Being a 12 tonne truck the box and contents would be no more than 4-5 tonnes spread evenly. The crane that used to be on it would lift 3.5 tonnes and often did, whilst putting huge pressure on a small 2' section of the chassis just behind the cab. There wasn't a mark or crack to be seen on the chassis when I removed the crane. Once i get to the point where I have to lift the body back off for final chassis prep and painting I'll do some more testing off road, and see what loads are where. I'm not saying it's totally stress free, but one thing I do know is it will have a lot less stress than some setups I've seen with a traditional 4 point mount subframe sat 150mm above the chassis, as the higher the frame above the chassis the more the central pivots want to go in opposite directions... :)
 
I'm not sure if this type of isolator http://www.isolator.com/ has been reviewed in this thread or not so I am showing it here. I't' called a helical cable isolator and it looks interesting to me.
These are insane... They are working up a design and quote for me for my new mounting system... mounting my flatbed atop an F-550. A large truck camper (5,500 lbs) mounts atop the flatbed. Let's see what they come up with.
 

mog

Mammutbändiger
"I'm not sure if this type of isolator http://www.isolator.com/ has been reviewed in this thread or not so I am showing it here. I't' called a helical cable isolator and it looks interesting to me."
These are insane... They are working up a design and quote for me for my new mounting system... mounting my flatbed atop an F-550. A large truck camper (5,500 lbs) mounts atop the flatbed. Let's see what they come up with.
You might want to do a lot more 'research' before you start reinventing the wheel (well, springs). As individual components, they look pretty cool, but I would guess (I'm not a spring/isolator/shock mount or anything expert) by their design they are more of an isolator for vibrations than a method to connect heavy components under major load.

For example 'Lord Mounts' are used quite a bit in aviation (which I am familiar with) and you could not find a better made/tested/certified company or components, BUT when they were used outside of their normal applications (but well within their certified load capacity), the results were less than stellar.

pict-2.jpg

I am in no way saying a fancy system using 'isolators', Load mounts, etc will not work, but is as the say goes "is the juice worth the squeeze?'" With a crew cab truck, no one will be riding in the camper so a plush Cadillac ride is no benefit. You just want to keep the camper from getting hammered on the rough stuff. As it is mounted on the flatbed, normal camper tiedown spring mounts would be the simple, cheap, and time-proven method. If you want a flexible/springy flatbed mount, there is no need to be fancy. All the fancy 3 & 4 point mounts are 'protecting' campers that are mounted directly to the frame, and in many cases (Unimogs) that are designed from the factory to have frame flex. For example, my 1017 which is made by Mercedes, does not have a 'flexy' Unimog type frame, but a uber-strong frame as does your F550. My flatbed (in its prior life the bed was a 16k capacity dump flatbed, so super beefy) is just mount rigid at the rear using the 'dump' pivot points on the bed and picking up the original Mercedes mounting points for the Fire/Rescue box. Then mount at the front using the factory T-bolts with springs add. So this gives it a little flex, more so it has 'off-road creds', than a true need. I did add to the front alignment rail two bolts (9/16, one on either side) that can be screwed in a couple turns to lock the bed so no flex or backed out for off-road operations. Not really necessary as it is fine (ridged) on road, but a 'cool' option. Given the size of your truck, the projected operations (more beach ops that hardcore trails), and the history you have had with getting mods done (discouraging enough to want to sell your awesome 6x6), I'd keep it simple. Of course YMMV
 
You might want to do a lot more 'research' before you start reinventing the wheel (well, springs). As individual components, they look pretty cool, but I would guess (I'm not a spring/isolator/shock mount or anything expert) by their design they are more of an isolator for vibrations than a method to connect heavy components under major load.

For example 'Lord Mounts' are used quite a bit in aviation (which I am familiar with) and you could not find a better made/tested/certified company or components, BUT when they were used outside of their normal applications (but well within their certified load capacity), the results were less than stellar.

View attachment 580923

I am in no way saying a fancy system using 'isolators', Load mounts, etc will not work, but is as the say goes "is the juice worth the squeeze?'" With a crew cab truck, no one will be riding in the camper so a plush Cadillac ride is no benefit. You just want to keep the camper from getting hammered on the rough stuff. As it is mounted on the flatbed, normal camper tiedown spring mounts would be the simple, cheap, and time-proven method. If you want a flexible/springy flatbed mount, there is no need to be fancy. All the fancy 3 & 4 point mounts are 'protecting' campers that are mounted directly to the frame, and in many cases (Unimogs) that are designed from the factory to have frame flex. For example, my 1017 which is made by Mercedes, does not have a 'flexy' Unimog type frame, but a uber-strong frame as does your F550. My flatbed (in its prior life the bed was a 16k capacity dump flatbed, so super beefy) is just mount rigid at the rear using the 'dump' pivot points on the bed and picking up the original Mercedes mounting points for the Fire/Rescue box. Then mount at the front using the factory T-bolts with springs add. So this gives it a little flex, more so it has 'off-road creds', than a true need. I did add to the front alignment rail two bolts (9/16, one on either side) that can be screwed in a couple turns to lock the bed so no flex or backed out for off-road operations. Not really necessary as it is fine (ridged) on road, but a 'cool' option. Given the size of your truck, the projected operations (more beach ops that hardcore trails), and the history you have had with getting mods done (discouraging enough to want to sell your awesome 6x6), I'd keep it simple. Of course YMMV
Super great insight and advice, and at a very good time also. The bed was originally installed dump truck style, and yes it did work... although I did notice that my camper tie downs would get hammered pretty hard, to the point that they needed to be replaced several times. As of this moment, I am thinking of doing a low 3-point design, floating the flatbed just 2 inches above the 550 frame rails. I think it is simple enough to fab and install and will likely be all the articulation that I might need "most" of the time. That said... I am not yet "firm" on the idea... I can always go back to the original dump truck mount system.

As for the isolators, I did swap some emails back and forth with them today. They are smart people for sure. And check out these photos they sent me of some of their work - look at the bases of the incredibly heavy commercial equipment. Whoa! They have all kinds of sizes and configurations to handle a magnitude of uses. I do like them and think they might work well, but I really do not have the desire to recreate the mount on this one... at least not at this moment. I will for sure be sticking with a more proven system.
 

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mog

Mammutbändiger
although I did notice that my camper tie downs would get hammered pretty hard, to the point that they needed to be replaced several times.
Are they getting hammered because of the size and weight (so a very large moment) of your campers, which is a good possibly? Something to consider is if you make a 'flexible' flatbed and then attached the camper to the flatbed with the same tie-downs, it could become worse as the dynamic movement could be increased with more 'flexibility' in your mounting system. So if you go to the trouble of a 3 or 4 point mounting system, you might want that as a direct connection to the camper. Certainly, the base of the camper is not stressed for that, so you will need an 'improved' ridge connection. As always YMMV.

And check out these photos they sent me of some of their work - look at the bases of the incredibly heavy commercial equipment.
Certainly, they can handle some beefy loads. But tilt that mount up or down 45° in the longitudinal plane or 25° in the lateral plane (something your truck will be doing off-road) and those coils will have a completely different load applied to them.
mount.jpg
 
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Ramdough

Adventurer
Has anyone sourced pivot bushings for heavy trucks? Talking 1.5” plus size pins in the front and aft pivot? Or anything close?


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shortbus4x4

Expedition Leader
Has anyone sourced pivot bushings for heavy trucks? Talking 1.5” plus size pins in the front and aft pivot? Or anything close?


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I have not but your best bet if you're in the US is agricultural or mining equipment stores for stuff that size. Grainger and Fastenal also come to mind, probably have to look through their catalogs but they have stuff like that. A good bearing house might be able to help too as they do bushings and such. Good luck getting into anything right now with the virus scare going on.
 

Joe917

Explorer
Has anyone sourced pivot bushings for heavy trucks? Talking 1.5” plus size pins in the front and aft pivot? Or anything close?


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A local heavy truck suspension shop should be able to help, at least point you to the correct source.
 

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Ramdough

Adventurer
A local heavy truck suspension shop should be able to help, at least point you to the correct source.
So I found pivots that would work for the front and rear. I have yet to find any that are split like the side ones the German company’s always use.

Anyone know what those are called? Where to get them?

Thanks!


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Victorian

Approved Vendor : Total Composites
So I found pivots that would work for the front and rear. I have yet to find any that are split like the side ones the German company’s always use.

Anyone know what those are called? Where to get them?

Thanks!


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those are oem parts off the shelf in Germany. I was lucky to get my hands on the parts number a while ago 😎 before you ask, I can’t pass on that info. Sorry.
 
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