Captured spring cabin mounting help needed

[Coachgeo]

Pretty sure Jack was saying to go with spring mount and not to use the rubber donut mounting system.

The rubber donut system creates a ridgid coupling between box and frame, so if the frame flexes it will twist the box unless the box is strong enough to resist the twist, in which case it strengthens the frame.

Fine for an on-road truck, not good for off-road....

Thank you....... but let me repeat myself, As the title of thread suggest...... this whole thread is about idea of modifying the front one or two, or even three of the five OEM chassis mount brackets on each side of the truck to utilize captured springs system. Never mentioned; nor have any desire, to utilize existing front mounts in OEM fashion.

 

[dwh]

Don't blame me, blame Jack. I was responding to this question:

which photo are you refering too and why bad?

But as to the rest, I believe normal practice is as according to Toner; the only ridgid mount is the last one at the rear.

 

[dlh62c]

Spring mounts allow the body to lift away from the frame as it twists. A twisting frame driving a rigid body is hard on the frame. A twisting frame driving a non rigid body is hard on the body. Window and door seals start to leak, cabinet frames and doors start to split and plumbing fittings start to leak.

The body needs guidance as it settles back down. Your spring mount design should prevent any shear forces on the spring bolts. For example, as you move forward, the body will shift back, as you stop, the body will shift forward. You need to contain side to side movement on the frame as well.

Pivoting and spring mounts have been discussed before.

http://forum.expeditionportal.com/threads/25494-pivoting-frames-and-mounting-campers

 

[Coachgeo]

Spring mounts allow the body to lift away from the frame as it twists. A twisting frame driving a rigid body is hard on the frame. A twisting frame driving a non rigid body is hard on the body. Window and door seals start to leak, cabinet frames and doors start to split and plumbing fittings start to leak.

The body needs guidance as it settles back down. Your spring mount design should prevent any shear forces on the spring bolts. For example, as you move forward, the body will shift back, as you stop, the body will shift forward. You need to contain side to side movement on the frame as well.

Pivoting and spring mounts have been discussed before.

http://forum.expeditionportal.com/threads/25494-pivoting-frames-and-mounting-campers

Thank you.... desire to do pivoting frame,thus this thread, is based on same points you mention above about tearing things up. Also, very aware of the long long long long long thread on pivoting frames that goes on forever resulting in mass confusion lol. Wise moderator would split that thread up into several sections. Each one on the different way to go about it (Mog type, captive spring front, captive spring rear etc.) Hence the start of this new thread about this specific rig and plan to go Captive Srping set up. No need to convulute the one above further than it has been already.

Thank you for pointing out possible motions and where they would come from that could put shear into play on the captive spring boltss. As mentioned only the front few on each side would be captive...... rest at the rear would be the solid mount. Are you suggesting that more than the rear two or three solid mounts is needed to prevent unwanted movement?

 

[Ramdough]

I think the general consensus is that if you are using springs, only bolt in one location at the rear. The rest are springs with a guide at the very front ones.


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[pappawheely]

I have some questions about the spring method of mounting. Does the box bounce up and down on a rough road? Do you need to cushion between the frame and the box? Would a cab shock cure that problem? Joaquin, do you have some type of anti-friction material on your guide plates?

 

[Ramdough]

No more or less then everything else does. :sombrero: The valve springs that I use are quite stiff, thus the movement in unnoticeable.



I have 1" oak strips and 1/4" strips of UHMW plastic. I don't have any anti-friction material on my guide plates.

Is the oak actually necessary, or could you get by with UHMW only. The reason I ask, is integrating wood into a system exposed to the underside of a truck sounds like it may lead to rot, swelling, holding moisture against steel, etc....

Thanks


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[Ramdough]

Sorry I wasn't able to reply sooner. I posted then took off to a dinner party that went late into the evening.

This is the picture I was commenting about:

That mounting system without captive guides over the frame is a accident waiting to happen. I have seen some really slick sprung mounts that were also captive guides on military trucks but have not been able to find a picture of them.



The box on Casa is held on by 8 of these in addition to 4 captive guides made from 3/8" plate:

View attachment 395411

As I have stated several times in the past... 3 & 4 point mounts are necessary for vehicles like Unimogs (not the U500's) because their frames are designed to be extremely flexible, while their driveline is designed to be extremely rigid. But on "regular" trucks it is a waste of space, weight, and money. Not to mention potentially dangerous (Remember the guy with his over built Fuso down is South America).

Coachgeo is building up an LMTV. I know the LMTV/FMTV is not your favorite vehicle to start with, but based on just that platform, I would assume something like what you did or the other spring option with guide plates would be your recommendation. Is that true?

Did you have any direct bolt connection or shear plate to prevent the box from walking forward/backward?


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[Coachgeo]

.... The bed on my prototype LMTV was directly bolted to the frame. For just a bed, i don't see why not. For a box that you have put quite a bit of time and money into... I'd go for springs and guide plates....

The M1079; which has a permanently mounted box, has that box mounted solidly in back just like the beds but it does in front use captured spring arrangment. Almost exact captured spring set up as the one picture I used earlier. Im not sure if there are guides used. I'll ask around.


If I use the brackets that came with the Ambulance (both pictured below and both from Freightliner truck setup) and modify them for captured spring use..... then I won't have any contact at all with the chassis. This is its OEM arrangement on the Freightliner.

You can see in this photo that I built the "stops" right into my rear bumper......

As to guides..... even if they are not used on the setup used on the M1079....... I probably will do them anyway on mine at sides front and rear. Speaking of rear... Joquan..... why ones in the rear? Is that not where the box is solid mounted already or is that not enough clamping force.

Questions remain on how to measure the length of springs and bolts to use, as well as how stiff of springs? Any suggestions? Also with there being 5 mount locations... how can I test to see if I should spring capture on front 4 of the 5... or... front 3 of the 5? any suggestions... or should I just assume I need to do spring capture all all but the back most one on each side?

 

[Ramdough]

Now that is an interesting idea. Use the old mounts and just add springs and longer bolts. You could possibly use the bushings as well, just need a big fat washer plate, then the spring, then another washer and nut. All you would have to add are some guide plates.

I haven't thought this all of the way out.....may need more springs. But the idea is interesting.




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[Coachgeo]

Did you see the original Freightliner set up before it was dismantled ?
Is is possible long bolts protruded thru those rubber bushed pads with springs underneath ?

I wish... but no springs in OEM set up.

 

[biotect]

Hi Coachgeo,

Many thanks for soliciting my input. I am a designer, and not a self-builder nor an engineer. Looking the thread over, seems like your project has already attracted input from some of the very best minds on the forum.

Here is an excerpt from something that I posted in the TerraLiner thread, that perhaps might be of use to you:


*******************************


Hi Safas,


....In the meantime, I would strongly recommend reading the following book by Steven Wigglesworth, that appeared appeared just a few months ago (December, 2016): Build Your Overland Camper --see https://www.amazon.com/Overland-Camper-manual-Haynes-Manuals/dp/1785210769 . Perhaps Wigglesworth's book has already been recommended and reviewed elsewhere on EXPO?

The book is terrifically well-written, well-illustrated, and jam-packed with excellent information. And it is endorsed by no less an authority than Stephen Stewart -- see http://www.xor.org.uk .

Particularly noteworthy are Wigglesworth's comments and his chapter about pivoting subframes. Wigglesworth is quite clear that one possible solution might be no pivoting subframe at all; but rather, a super-rigid chassis frame that does not twist, or that twists very little. Indeed, Wigglesworth's first "case study" is the conversion of a security truck into an overland camper, where the cargo box that was to hold the money was constructed so robustly, and placed directly on top of a chassis frame strengthened to be very rigid, such that no pivoting subframe was necessary.....


*******************************


To which grizzlyj replied:

This one I think? http://forum.expeditionportal.com/threads/82997-Jim-the-Truck-a-mercedes-1823

.....I have not quite finished Wigglesworth's book, about half way through, but I had previously read Ulrich Dolde's. So far I think Ulrich's is more helpful, and well worth a look although targeting the same people in a similar way. The English version has been available on DVD for a while, and is now in print too.

https://www.selfbuildmotorhome.com/

To which I replied:

....I have Dolde's book and CD in English, which I ordered via Amazon.de, but haven't started reading it yet -- see https://www.amazon.de/gp/aw/d/39818...SY340_QL65&dpPl=1&dpID=51gTN7lIGQL&ref=plSrch and https://www.amazon.de/gp/aw/d/39814...qid=1489091259&sr=1-10&pi=AC_SX236_SY340_QL65 .

Hope that helps!!

All best wishes,


Biotect

 

[Coachgeo]

The box on Casa is secured down my only the 8 sprung U-bolts. The 4 captive guides keep the box aligned with the frame, and the stops on the rear bumper keep the box from sliding off the back. The box has "stops" to keep it from pinching the cab as well.

Quick question... Have you downloaded the Freightliner body builders guide? If it were me, I'd just follow what they have to say (ie: just replicate what the ambulance builders did), and call it a day! ...

Ahhhhhhh..... thought those ubuilts with springs were your guides..... missunderstood. Now understand...... they are your captured spring mounts. Very clever. hmmmmmm.

As to to freightliner..... I'm not remounting on a Freightliner in pictures. Only showed them to reference how the mounts were used in OEM configuration. Am remounting the box on an X military FMTV. Luckly the chassis dimensions are same width wise. Maybe the Freightliner build guide does mention off road mounts though? Like you I've no plan to stay on pavement and will get in situations way more twisted up than a street Ambulance would experience or be designed for.

 

[Coachgeo]

Thank you to al who responded thus far. Anyone got suggestions on:

. ways to determine strenght and hieght of springs?

Also curious about springs above and below each mount (one on box and one on Chassis) like below picture. Have seen some pics of builds where they used combination like pictured and some where it is only on bottom of bolt also pictured. So question is when can you go with a spring on just the below the chassis mount. Asking cause in my case one above on the box mount side looks to be reall hard to make happen so if I can go without.... would make the project much less complex.

 

[Ramdough]

Thank you to al who responded thus far. Anyone got suggestions on:

. ways to determine strenght and hieght of springs?

Also curious about springs above and below each mount (one on box and one on Chassis) like below picture. Have seen some pics of builds where they used combination like pictured and some where it is only on bottom of bolt. So question is when can you go with a spring on just the below the chassis mount. Asking cause in my case one above on the box mount side looks to be reall hard to make happen so if I can go without.... would make the project much less complex.

When I have seen one spring above and one below the springs have been shorter. When there is only one spring, they appear to be longer. Based on this, I would choose a longer one on the bottom.

Can you look an expansible van truck to see what springs they used?


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