Total Composites flatbed build

andy_b

Member
Curious. When they say a bed is torsion free, what does exactly mean?

If have a flatbed on an F350 say, and building a camper on that, is it possible the bed itself is fixing this torsion problem?

Forgive me, just learning this stuff.

Thanks
JM
A fiberglass camper does not tolerate flex well. On the road, that isn't much of an issue, but becomes apparent offroad when the cabin attempts to twist but its material and design don't like doing so.

The goal of a dynamic mounting system is to limit the net amount of twist that is transferred to the cabin. Even with the most rigid bed and the most flexy suspension, some twist will make its way to the cabin if the cabin is fixed firmly to the bed. The goal of a dynamic mount is to allow the cabin to move (within some constraints) independently of the truck so that the cabin itself doesn't have to twist too much.
 

john61ct

Adventurer
The other approach being to avoid fibreglass, as with DIY foamie construction, choosing materials that themselves allow for a degree of flexing
 

Peter_n_Margaret

Adventurer
A fiberglass camper does not tolerate flex well. On the road, that isn't much of an issue, but becomes apparent offroad when the cabin attempts to twist but its material and design don't like doing so.
Definitely NOT my experience.
Our sandwich panel vehicle has been on the "road" now for 16 years and has experienced many km of serious stress without the slightest detriment.
The body is glued to the cab and 12 rubber blocks attached to the chassis. They have a maximum of 10mm of possible movement.


Cheers,
Peter
OKA196 motorhome
 

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rruff

Explorer
Definitely NOT my experience.
Great videos, Peter! Your whole vehicle appears to be very rigid. Heavy duty trucks usually have a flexible frame, but with a little internet searching I found this on OKA:
SubframeAluminium frame fixed to chassis. OKA has a rigid non flex chassis and 3/4 pint mount is not necessary.

So your rig is unique in more ways than one. I'm sure you've seen how flexy Fusos, NPRs, Mogs, Mans, etc and even heavy duty pickups are. The trend in the US is for trucks up to 1 ton to have a rigid frame these days, but that was not true in the past.

The reason for the flexy frame is to have a high payload (stiff springs) and still keep the wheels on the ground in "crossed up" situations. If you make the frame torsionally stiff, then it needs to be stronger (and usually heavier), and either have a more sophisticated suspension or just run on 3 wheels more often. Tradeoffs...

Here is a Man-based camper that appears to have a 3 point mount that pivots in the front. Pretty sure the box would be destroyed if it was rigidly mounted.


Curious. When they say a bed is torsion free, what does exactly mean?
If have a flatbed on an F350 say, and building a camper on that, is it possible the bed itself is fixing this torsion problem?
I think the F350s got stiff in 2017 ... or maybe later. The earlier models flexed quite a bit.

If the flatbed is very torsionally stiff, it's possible that it will save your camper, but I don't think this is a good solution for running loaded offroad... the frame or flatbed would be at risk. The stiffest member in the system must be designed to carry the bulk of the load when all your weight is on two wheels (corner to corner).

The torsion-free subframe is mounted so that it doesn't resist the twisting of the truck's frame; it pivots instead.
 

Peter_n_Margaret

Adventurer
The OKA is has a stiffer chassis than most, that is certain (it has very long leaf springs front and rear), but to suggest that it does not flex is stretching the feature.
What about this one?
A mate built this 14 years ago. It is a Defender chassis. The body is Sikaflexed to the top and rear of the cab and bolted and glued via multiple rubber blocks direct to the chassis. 100% successful in that time.
10-01-30 GM 003E.jpg

Cheers,
Peter
OKA196 motorhome
 

rruff

Explorer
What about this one? A mate built this 14 years ago. It is a Defender chassis.
I don't know for sure about Defenders, but body-on-frame SUVs tend to have stiff frames.

I'm still unsure how to mount mine. Originally I was planning to use cab mounts at the 8 bed mount locations, but these flex very little... and I just don't know if I want the camper to be the stiff load-carrying structure in flex situations.
 

andy_b

Member
What about this one?
A mate built this 14 years ago. It is a Defender chassis. The body is Sikaflexed to the top and rear of the cab and bolted and glued via multiple rubber
That actually looks like a Series 1 Discovery. As @rruff said, an SUV is pretty rigid. Specifically, a LR Disco was quite stiff - it had full-length box section frame rails and a very flexible coil spring suspension. I can see why it did well. Also, the cabin is relatively short. For example the camper on the MAN chassis above is likely longer than the than the entire vehicle your friend made. Looks cool!
 

socceronly

New member
A fiberglass camper does not tolerate flex well. On the road, that isn't much of an issue, but becomes apparent offroad when the cabin attempts to twist but its material and design don't like doing so.

The goal of a dynamic mounting system is to limit the net amount of twist that is transferred to the cabin. Even with the most rigid bed and the most flexy suspension, some twist will make its way to the cabin if the cabin is fixed firmly to the bed. The goal of a dynamic mount is to allow the cabin to move (within some constraints) independently of the truck so that the cabin itself doesn't have to twist too much.
Thanks!

So if one is buying a flatbed for a truck, say an F350 (any truck really) is that a feature of the flatbed in general, something you put the flatbed on (or neither)?

I am interested in building a camper on the flatbed, but this issue is clearly important so I am wondering if it is a normal thing in truck land or something specifically engineered into expedition vehicles.

JM
 

rruff

Explorer
So if one is buying a flatbed for a truck, say an F350 (any truck really) is that a feature of the flatbed in general, something you put the flatbed on (or neither)?
If it's a new Superduty, or a newer model of most 1/2-1 ton trucks, it has a stiff frame so you can hard mount it. Otherwise, it depends. Pretty sure most flatbed manufacturers have no clue about frame flex; they just solid mount them.

I ran across this photo and thought of this topic. The Tundra has a stiffer frame than an older model Ford, but it isn't as torsionally stiff as other new 1/2-1 ton trucks. It's boxed in the front, then heavy closed C back to the forward rear leaf mount, then light open C to the back. I think the flatbed shown is solid mounted to the frame, which is typical. You can see that the frame has a clockwise tilt relative to the cab in the front (look at the bumper), but right behind the cab it's counter-clockwise a good amount (look at the rack and front of the camper). But it looks like everything is quite stiff from there on back. It isn't the best angle, but I don't see the rear of the camper twisted counter-clockwise any more than the front. So it looks like the frame, flatbed, and camper is making a stiff unit... and all the twist is happening under the cab.

Is this an issue? I don't know. The frame is designed to be most flexible in the rear, so stiffening that part is going to put more stress on the frame and cab mounts in the span from the forward bed mounts to the front wheels. I can see the forward bed mounts getting quite a bit more stress than normal in this situation. But maybe there is plenty of margin and it isn't a problem? I know that when I drive offroad I encounter ditches and ravines that I have to traverse at an angle, and this sort of twisting happens a lot.

 

socceronly

New member
Not sure if this exists, but is there any kind of rating system for off road? Like OR1, OR2, OR3 ect... ?

I see so much time wasted with people talking past one another wrt what off road might mean.

The picture above is the kind of stuff I would be going on. Not climbing rock faces, which, is quite frankly amazing.
 

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rruff

Explorer
The picture above is the kind of stuff I would be going on.
With respect to frame twist, if you are teetering on 2 wheels (no weight on the 2 wheels in the ditch) then that's as bad as it gets. Nobody with a big camper is likely to do crazy trails, but getting twisted is very common. If you look at Peter's OKA videos above vs the MAN based camper, you can see how incredibly different vehicles can be in this regard. They both do the job, one with a very stiff frame and the other with a very flexible one + a pivoting mount.
 

Peter_n_Margaret

Adventurer
People often proudly show pics of their vehicle with a wheel (or 2) off the ground, but what it really means is that the capability of their suspension has been exceeded. A more capable vehicle will keep its wheels on the ground in the same situation, that, after all, is what it is supposed to do. How it does that varies from vehicle to vehicle.
Cheers,
Peter
OKA196 motorhome
 
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