Truck Bed Camper vs Van/Motorhome

[BeNimble]

So another question I don't quite understand...is a solid body Motorhome vs a Truck Camper.
Why have a camper that swings around rather significantly vs a solidly mounted full body like a van/suburban/motorhome ?
I've seen videos of the Fuso like trucks and the box is tilting rather significantly on any bumps or inclines compared to the cab itself.
This can't be good for keeping the contents intact, like a fridge, glasses, etc.

 

[Herbie]

The first and most obvious thing is the size of the box. You aren't going to find a steel-construction vehicle (Van) the size of the big fuso trucks.

The second thing is the flex you've observed. Your choices are that the box can move around on the frame, or the box itself would need to flex. A traditional North-American stick-framed cutaway RV would tear itself apart if subjected to the flex I can see even on just the thumbnail of your first video. It's not about keeping things inside the cabinets, it's about keeping the cabinets ATTACHED TO THE WALLS. Even "offroad" Class-C RVs like the Tigers have been known to peel the cabinets off the walls under serious use.

Managing that situation so that the vehicle can articulate without damaging the "box" is a non-trivial matter. Rigid mount, pivoting mount, flexible frames, etc. are all on the table and rigorously debated and discussed here.

 

[Porkchopexpress]

I'll throw in another observation, smaller/shorter pickup trucks for consumer use will frequently use boxed frames for rigidity and strength while longer commercial trucks almost always have c channel frames so they can flex under torsional forces associated with heavier loads over a longer wheelbase. So basically you can design more rigidity on a short wheelbase vehicle under lighter torsional stress like a class B van rv, but the longer heavy duty trucks need more flexibility to avoid failure.

 

[Peter_n_Margaret]

The 2 vids you posted show vehicles with fundamentally different suspension systems (the OKA coming down the Carson Track jump-up is mine).
The vast majority of trucks (the first vid) have short, relatively stiff springs. They often change them to parabolics to get better articulation. But the fundamental thing is that they MUST use flexible chassis rails in an effort to keep the wheels on the ground and hence the twist that you see between the back and the front.

My OKA has a stiff ladder chassis. It effectively does not twist. In order to get good suspension articulation it uses very LONG leaf springs.
The stiff chassis means the body (including the bed-over) can be joined directly to the cab. That is unusual for a vehicle of this size and weight capability and provides quite different design opportunities.

The side effect of the long springs is a vehicle that will handle corrugations and just ordinary "bad" roads in a manner that is far superior to most others.
EDIT: If you could increase the length of the springs on the first truck, it would be much better for the job it is being asked to do. Fact is, most of these vehicles are not designed for these applications as a first priority. The OKA was. The Unimog was too, but still uses a flexible chassis.
Cheers,
Peter
OKA196 motorhome

 

[Peter_n_Margaret]

Watch the vid of the first truck again, taking particular note of the wheel travel relative to the body work. It is quite small.
Now watch the wheel travel of the OKA. It is fundamentally different.
Here is another vid that shows it better.
Cheers,
Peter
OKA196 motorhome

 

[BeNimble]

Cool that was your truck in the video!! Excellent point about the suspension travel. And having independent suspension too would have a big impact.
So in my view, having a hard solid frame and box and let the suspension do the work is what I want to do with my build.
I understand the length increases the twisting leverage, but heck there are 40ft motorhomes.

My C700 is a cab and truck frame, but I am thinking of integrating the camper to the cab like a motorhome and more like the OKA.
It has a 12ft wheelbase, so relatively short as these things go. Get a trailer to go longer, it can flex all it wants and won't have people in it!

 

[rruff]

I understand the length increases the twisting leverage, but heck there are 40ft motorhomes.
My C700 is a cab and truck frame, but I am thinking of integrating the camper to the cab like a motorhome and more like the OKA.

No. This was explained in the post where you found those videos. You either have a stiff frame like the OKA or you don't. And I don't know of any other heavy duty trucks that have a stiff frame like the OKA... they are all made to flex.

If "C700" means an old Ford cabover, then it's made to flex. So you need a subframe and mounting system that allows for it, like the SLRV above. Possibly you could reinforce/reengineer the entire frame and camper to make it stiff, but that would be a challenging project.

40ft RVs will quickly self destruct on semi-challenging offroad terrain.

 

[Peter_n_Margaret]

The Toyota Coaster bus is an interesting deviation. I don't know how its suspension and body mounting works but they are a favourite vehicle for remote communities in Australia and are pretty much bullet proof. Thy are a popular conversion to 4WD, but they can't match the OKA
Cheers,
Peter
OKA196 motorhome

 

[BeNimble]

No.

No. I can build a truck anyway I want to,

 

[Peter_n_Margaret]

I suspect bus chassis are stiff to allow the bodies that they use, but busses are not designed to go off the black top and if they do they quickly run out of wheel articulation and that usually means that they "fail to proceed".
Suspension articulation has to be accommodated some how.
No reason why a different suspension with much more articulation can't be fitted to avoid a twisting chassis, but engineering changes like that can get expensive.
Cheers,
Peter
OKA196 motorhome

 

[javajoe79]

These large truck can't have really soft suspension that would allow a ton of flex because they are rated for quite a bit of weight. In general, they couldn't safely manage all that weight at highway speeds with soft suspension. You could always supplement soft springs with air bags for better handling on road but that adds cost and complexity that's unnecessary for most trucks like this, because they don't go off road. Reduce the spring rate = less chassis twist in a given situation. Ideally the suspension should always do all of the work in the flex department but that just isn't that simple with a big truck. Unless your box was a very strong design, rigidly attached to the frame and designed to withstand the twisting, you will have to let the chassis twist.
People have critiqued my build because the axles I am running have a lower weight rating than the stock axles in an NPR but that doesn't matter when there are zero plans of approaching that weight rating once the camper is complete. It flexes pretty good as is, rides MUCH nicer than a standard sprung NPR and handles pretty well on road.

 

[javajoe79]

No. I can build a truck anyway I want to,

Relax. He was just trying to help you understand. You're right though, you can build it however you want but unless you stop the chassis flex, it will just tear the box apart unless the box is designed as a structural element

 

[rruff]

No reason why a different suspension with much more articulation can't be fitted to avoid a twisting chassis, but engineering changes like that can get expensive.

More articulation in the suspension will definitely help ... but you also need to make the chassis+camper stiff and strong. Changing the suspension alone won't be enough.

Most of the flatbed+service bodies they put on pickups are just hard mounted. They are stiff enough to make a solid unit. I don't think it would be good to take them on challenging terrain though; not without more frame reinforcement.

 

[BeNimble]

One method to keep it rigid is just having the truck overbuilt, and that is what I get with the C700, it will flex at 26,000 lbs..but at 8,000 lbs?
And there are other ways to remove chassis flex, but in any case, one needs to build the house to account for just normal bouncing around.

 

[Joe917]

Our MB 917AF has a very flexy frame, and is about the same size as the C700. The box is a rigid mount, Mercedes approved. So the frame is held straight by the box. There is still quite a bit of movement between the cab and box. It would take a lot of engineering to design a one piece cab camper. The solid mount system makes a much nicer handling road, bad road vehicle but it will pick up a wheel on extreme terrain.