Cab Over Camper Design Question

[revel]

Hey all. I'm a long time reader and am designing a slide in camper most likely to be constructed of nida-core of, maybe, a foamie. I see all of these cab over designs without framing and cannot figure out how the cab over is supporting the weight of two adults let alone the weight of a queen mattress.

Can anyone school me on some lightweight construction methods that make for a strong cab over? Thanks!

 

[Victorian]

Your concerns are very valid. When we sell Cab Over campers we watch for the tensile strength of our FRP Skin. Some are better than others.
As soon as you are adding off roading, you need to calculate the cantilever in a way more serious matter. We recommend additional supports like the ones in the link below. https://expeditionupfitter.com/coll...d-aluminum-cab-over-braces-set-left-and-right

You may wonder why others don't require these braces. When we design our campers, thermal efficiency is most important. This can only be achieved by avoiding any internal structure (Aluminum tubes, wood etc). That only leaves us with the FRP skin and the braces.

Hope this info helps.

 

[revel]

Your concerns are very valid. When we sell Cab Over campers we watch for the tensile strength of our FRP Skin. Some are better than others.
As soon as you are adding off roading, you need to calculate the cantilever in a way more serious matter. We recommend additional supports like the ones in the link below. https://expeditionupfitter.com/coll...d-aluminum-cab-over-braces-set-left-and-right

Very interesting. But there's nothing on that page that indicated the strength of the brace or intended/actual loads. How much can they take?

 

[Victorian]

Very interesting. But there's nothing on that page that indicated the strength of the brace or intended/actual loads. How much can they take?

There are no numbers attached to this design. We know they are more than enough for our campers. After all, you are not loading them up with with 5000LB. Just to be clear, our campers DON'T NEED those braces if driven on the road. Everything is self supporting. But we like to play it safe and recommend the braces. Just imagine: Someone decided to convert the cab over bed into storage and does load all his tools and water in that space... The last thing we want to see is one of ours failing somewhere in Africa or South America and ruining someones dream trip.

 

[revel]

There are no numbers attached to this design. We know they are more than enough for our campers. After all, you are not loading them up with with 5000LB. Just to be clear, our campers DON'T NEED those braces if driven on the road. Everything is self supporting. But we like to play it safe and recommend the braces. Just imagine: Someone decided to convert the cab over bed into storage and does load all his tools and water in that space... The last thing we want to see is one of ours failing somewhere in Africa or South America and ruining someones dream trip.

OK. But what's the point without ratings or numbers? Necessary or not with your campers it seems tremendously odd to be offering additional support without an indication of how much the support can actually support.

 

[Victorian]

OK. But what's the point without ratings or numbers? Necessary or not with your campers it seems tremendously odd to be offering additional support without an indication of how much the support can actually support.

Sorry I did not have the answers you expected, was just trying to give you an idea and being helpful.
These braces are designed for our product. We do not have to publish or argue about the data if our engineers have signed off on them for our system. As mentioned above, you can avoid these type of braces by embedding internal beams in the sidewall.

 

[revel]

Sorry I did not have the answers you expected, was just trying to give you an idea and being helpful.
These braces are designed for our product. We do not have to publish or argue about the data if our engineers have signed off on them for our system. As mentioned above, you can avoid these type of braces by embedding internal beams in the sidewall.

No need to be sorry at all. Just trying to get a useful handle on such ideas from a design perspective.

 

[NeverEnough]

Think of the cabover over sidewalls as beams with a cantilevered load (the weight of the floor, roof, and whatever sits inside). The taller the "web" of a beam is, the stronger it is, and the further the "beam" extends back into the body of your box, the more load it can support. My big camper is made from plascore, similar to Nidacore. The cabover sidewall panels extend 90" back into the camper body, and are 1.75" thick and 36" tall. The panels were bonded and mechanically fastened into the body. The cabover floor extends 64" over the cab and also uses 1.75" panel, which, when bonded to the sidewalls, provides the torsional rigidity needed for the "web" of the beams- so the cabover is a basically a torsion box hanging off two very tall beams. You can go through my build thread to see the cabover design. It's been bomber for 9 years of use, and I don't get any support contribution from the roof, since it hinges open. If I was to build another cabover, I'd do it the same way since it's stood the test of time.

 

[rruff]

Hey all. I'm a long time reader and am designing a slide in camper most likely to be constructed of nida-core of, maybe, a foamie. I see all of these cab over designs without framing and cannot figure out how the cab over is supporting the weight of two adults let alone the weight of a queen mattress.

I'm curious what you mean by "foamie"... if it's the XPS with PMF covering I'd strongly advise against that. Nidacore has poor thermal performance, but it's structurally good. You might check out CarbonCore, they have clone that is much cheaper. BTW, I'm building a camper out of PVC foam and hand laid fiberglass.

If your skins and joints and core are substantial, you really have a monocoque structure that is very strong. The cabover is more like a short box beam (very rigid and strong) than a cantilevered panel with weight piled on it.

The only place where you need to be careful is where the bottom of the cabover meets the rest of the camper. When loaded that panel will be pushing against a vertical wall that doesn't have lateral support so you'll get some high stress in that area. If you beef up that spot you should be fine.

On mine the floor of the cabover extends into the rest of the camper 20", and will be well attached to the side walls, so the load is already spread out quite a bit. I'll also have a generous radius and extra layers where that panel meets the front wall.

 

[tanuki.himself]

My build is 2mm FRP skins over 30mm styrofoam, factory made - i've put my faith in the manufacturer's claim that it will be strong enough on the cantilever without additional strengthening or brackets, and so far its working ok for me to sit right in the forward corner of the cantilever working on the interior fitting. I'm 125 kilos and TBH i was more worried about the empty shell tipping forward rather than cracking the cantilever. I can't say how it will hold up to years of driving, but i have built it in such a way that if I ever wanted to I could put additional fibreglass angles/beams/channels on the inside and down to the subframe to take some of the load off the skins.

My only concern with using nidacore or the equivalent is that it is not as strong across the cell structure - foams are more homgenous

 

[rruff]

My only concern with using nidacore or the equivalent is that it is not as strong across the cell structure - foams are more homgenous

Here is a similar product: https://www.carbon-core.com/pdfs/pds-plastic-honeycomb.pdf
Compared to PVC foam that is a little lighter. https://www.carbon-core.com/pdfs/pds-carbonfoam-pvc.pdf

The plastic honeycomb has much better compression strength, much worse tensile strength, and shear is a little worse. I think it should be fine.

Do you know what styrofoam was used in your panels? It tends to be much weaker.

 

[tanuki.himself]

Here is a similar product: https://www.carbon-core.com/pdfs/pds-plastic-honeycomb.pdf
Compared to PVC foam that is a little lighter. https://www.carbon-core.com/pdfs/pds-carbonfoam-pvc.pdf

The plastic honeycomb has much better compression strength, much worse tensile strength, and shear is a little worse. I think it should be fine.

Do you know what styrofoam was used in your panels? It tends to be much weaker.

all the told me was:

"We use predominantly RTM STYROFOAM core foam insulation which has an excellent ‘k’ factor, tensile and compressive strength, as well as being closed cell and impervious to moisture. "

They also put Tancast at the cut edges to give a stronger cleaner cut for glueing and anywhere i requested for mounting points - its amazingly tough stuff for a lightweight foam and takes a woodscrew very well. I think they may have run a length of it up the junction of the cabover.

below styrofoam is grey, tancast is yellow - these are offcuts that have been sitting outside for a while, hence the mould

 

[rruff]

"We use predominantly RTM STYROFOAM core foam insulation which has an excellent ‘k’ factor, tensile and compressive strength, as well as being closed cell and impervious to moisture. "

That's 60psi foam, so pretty stout for XPS. Plastic honeycomb would be stronger though. But like you said, it just needs to be strong enough.

It kills me how many options you have for custom panels in Oz vs the US...

 

[revel]

I'm curious what you mean by "foamie"... if it's the XPS with PMF covering I'd strongly advise against that. Nidacore has poor thermal performance, but it's structurally good. You might check out CarbonCore, they have clone that is much cheaper. BTW, I'm building a camper out of PVC foam and hand laid fiberglass.

If your skins and joints and core are substantial, you really have a monocoque structure that is very strong. The cabover is more like a short box beam (very rigid and strong) than a cantilevered panel with weight piled on it.

The only place where you need to be careful is where the bottom of the cabover meets the rest of the camper. When loaded that panel will be pushing against a vertical wall that doesn't have lateral support so you'll get some high stress in that area. If you beef up that spot you should be fine.

On mine the floor of the cabover extends into the rest of the camper 20", and will be well attached to the side walls, so the load is already spread out quite a bit. I'll also have a generous radius and extra layers where that panel meets the front wall.

Which one is yours?

 

[rruff]

Which one is yours?

No thread yet... if you mean the foam, it's from Carbon Core.