Structural differences between PVC & coachbuilt MHs

[TheMobileWanderer]

Hi Funsters,



I've just been reading several threads generally, via google (i.e. not here), about PVC & coachbuilt vehicles regarding the difference in their structures.



Some people say that a PVC vehicle is a PVC construction with insulation foam (PET), sandwiched between two sheets of PVC.

I thought in this context, PVC simply means 'panel van conversion'.



Anyhow, the structure of the PVC was explained but not that of the coachbuilt variety.



So, given that I am planning the build of a MH, with the best possible structure my question is:-



QUESTION: what is the normal structure of the roof, walls and floor of a coachbuilt motorhome?



As for my build; I have considered GRP outside with 40mm - 50mm foam sandwiched between that GRP and an internal (BS standard) marine plywood.

I have set a brief so I can try to follow it.

In case it is of interest to others here it is.



When finished:-

1. The outside should have the appearance of any other high-end motorhome.

2. The roof should not sag, regardless of any reasonable weight on it.

That could be two maintenance operatives in addition to the panels and other normal MH equipment, to be seen on roofs.

The roof should have additional insulation (over the amount in the walls), due to the fact that heat rises.

The roof should be able to cope with various ceiling trims being screwed bolted or stuck to it.

The overall thickness must be within the tolerances of skylights and other roof fittings' maximums.



3. The walls should have enough insulation to resist heat transfer both, to maintain heat and to repel hot summer sun.

The walls should be of sufficient structural integrity, to accommodate the weight of wall cupboards and other items screwed, bolted or sikaflexed to it.

The walls should be no thicker than the maximum set out in the windows and habitation doors' specification.



4. The floor should be robust to prevent road debris from damaging it.

It should also have sufficient insulation properties and a suitable surface to cope with wear and tear.

It should also be able to facilitate/support the installation of hatches for extra storage and underfloor access.



So, the materials I am considering are:- (numbered, relatively, to the points above)

1. GRP or other composite furface with or without gel-coat.



2. The roof to comprise three layers:-

a. 50mm PET sandwiched between GRP.

b. Alloy 'joists', running across the roof, bolted through that 50mm foam, into the alloy frame that runs along the side of the roof.

(That may require sleeving on the bolts, to prevent the crushing of 50mm foam board).

c. A further 50mm foam insulation sheeting, sculpted around the joists, such that there's minimal gap between both sheets of foam.

d. 15mm marine plywood (to BS standard) bolted through that inner 50mm foam insulation board into the joists. This also may require sleeving.





3. The walls shall comprise two layers

a. 50mm PET sandwiched between GRP

b. 18mm marine Plywood (to BS standard)



4. The floor shall comprise several layers:-

a. An aluminium base layer

b. 50mm PET sheeting

c. further PET foam sheeting of 20mm(to accommodate channelling for underfloor heating),

d. 18mm marine plywood to BS standard.

(the 20mm insulation foam will enable mistakes to be made, without wrecking the main 50mm under-floor insulation).

(The marine playwood at 18mm should be adequate to support hatches for underfloor access).

 

[funflair]

If you are going coachbuilt which by your description I think you are it would be worth having a look at how the likes of MORELO or Concorde build their box, the walls and roof are 1.4mm alloy bonded to a closed cell PU or EPX foam core, have look at this thread and watch a couple of the videos at the start.

How they build a MORELO

I think this is interesting even if you don't speak German, which of course I don't. Martin

www.motorhomefun.co.uk

 

[Kannon Fodda]

PVC at least in the context of Motorhomefun means a conversion if a panel van. Typically a courier's / builders van like the Ducato, Transit. The metal outer shell of the vehicle is largely intact, apart from perhaps holes cut for added rooflights or living area windows. The habitation stuff such as bed, toilet and cooker is all fitted out inside c including wall and ceilings affixed to the original metal frame.

PVC can also be used to describe a plastic material.

What is being considered here is more a coachbuilt. The original rear end cargo area of the vehicle sides and roof are removed leaving the main chassis to the rear and the drivers cab. A new enclosure is created of side and roof, plus flooring, and frame. Internal fittings are added. The sides and roof will be composite panels, perhaps with reinforcement framing. Composite of various plastics, wood, insulation, metal sheeting.

For a converter creating a quality composite panel that has sufficient structural integrity must be a challenge.

 

[pappajohn]

Coachbuilt is built from the floor up on a naked van chassis and cab.
A pvc IS a regular van with furnishings built in.

 

[stevewagner]

Your build sounds like it will be very heavy.

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[Lenny HB]

QUESTION: what is the normal structure of the roof, walls and floor of a coachbuilt motorhome?

The walls roof and are sandwich of foam between a either an outer aluminium or GRP panel with the inner wall of ply or aluminium with varying types of insulation and they are bonded in a vacum press.
The insulation varies with budget vans using polystyrene better ones use varying grades of closed cell foams PU, EXT etc.

The floor can be ply top & bottom bonded to the insulation most of the better vans use GRP top & bottom.

 

[Bertilak]

You don't mention your length, base vehicle or target weight. If 8m, I can see >1/2 a tonne of plywood there (which if marine, will also be expensive) . What are your influences? (Is this an ex-army expedition look?)
IMHO A super duper spec will be heavy which will reduce progress and driving pleasure. Everything is compromise!

 

[LithiumConvert]

The best of the coachbuilts have spent decades researching and developing how to do it.

One thing that the German ones, at least, have decided is that wood in the construction is an absolute no-no.

 

[TheMobileWanderer]

Thanks, people.
The base vehicle will be Mercedes or Ducato, platform chassis or cab chassis. I haven't yet identified the difference(s) between those two types of chassis.

The 'box' will be 6m - 6.5m in length and 2.4m wide externally, so that will be the finished vehicle's width, excl mirrors.

Twin axles and air suspension. Think Swift Kon-Tiki 894 or Coachman 545.
(Different internal layout).

It may be heavy which is why I'm seeking out weights of the various boards and sheet components just as much as their prices.

Pleased to read how others are built. One of my concerns and reasons for the question was because I wouldn't want the weight of cupboards, for example, screwed into the walls, to pull the surface away from the rest of the wall materials, effectively to de-laminate.

My aim to use marine plywood was to ensure minimal damage even if there were ever to be any leaks. If ordinary and possibly thinner plywood would be suitable, if painted then I'd look at those.

Tmw

 

[funflair]

Thanks, people.
The base vehicle will be Mercedes or Ducato, platform chassis or cab chassis. I haven't yet identified the difference(s) between those two types of chassis.

The 'box' will be 6m - 6.5m in length and 2.4m wide externally, so that will be the finished vehicle's width, excl mirrors.

Twin axles and air suspension. Think Swift Kon-Tiki 894 or Coachman 545.
(Different internal layout).

It may be heavy which is why I'm seeking out weights of the various boards and sheet components just as much as their prices.

Pleased to read how others are built. One of my concerns and reasons for the question was because I wouldn't want the weight of cupboards, for example, screwed into the walls, to pull the surface away from the rest of the wall materials, effectively to de-laminate.

My aim to use marine plywood was to ensure minimal damage even if there were ever to be any leaks. If ordinary and possibly thinner plywood would be suitable, if painted then I'd look at those.

Tmw

A chassis cab would be a chassis with the cab attached where a platform chassis would be a rolling chassis with all the controls dashboard etc but not cab so you would need to build all the way and including the windscreen, this would be an A class.

If I was looking at this type of project and it might depend on your particular skill set but I would be tempted to look at a old/quality coachbuilt box from say Phoenix or a Niesmann Bischoff Clou, this could be swapped onto a newer chassis and you could refit and modernise the interior.

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[peterc10]

Do you have a C1 licence? If not you will be building something that you would not be allowed to drive, because it would weigh more than 3500kg, which is the weight limit for a car licence.

 

[TheMobileWanderer]

Yes, I do have the necessary licence. The only limitations are budget and talent.

 

[jezport]

I prefer GRP exterior panel over Ali as they dont dent with the slightest touch and are stronger. Having a Frankia previously this faired well and bodywork wise was immaculate at 10 yeasrs old when we sold it. I now have a PVC which I feel is safer then a coachbuilt in an accident and stronger structrurally.

 

[dunnah01]

I think you'll struggle to get the weight of your proposed construction low enough to work with the Ducato/Sprinter type chassis You may need to start compromising on the thicknesses to add lightness.

 

[Bertilak]

Your ambition is impressive.
However to do this well is a "once in a lifetime" endeavour, and to me this doesn't square well with your apparent lack of experience of motorhomes (and your particular needs in one), and the community adage about "it takes three vans to find the one one really need".

Together with a budget limit I would recommend you buy the most similar van you can afford, even if it's 20 years old, and try it. Perhaps sell it and try another. Perhaps rent. Ask round at rallies. - In Manufacturing this is called "benchmarking" the market.

Then, if you think you can do better, and that it would be worth the grief for the marginal gain, you can build you own, and get it right first time.
[I have seen my father fail at this, spending 18 months building a trailer tent with masses of clever features, but ultimately too heavy, odd and unwieldy to use to be his "forever" trailer. In his case he couldn't even get a buyer, and broke it up.]
Good luck: I don't want to knock you, just nudge you to get it right for you.

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[PP Bear]

I think nearly all the coachbuild videos I’ve watched, have the manufacturers building the cupboards and kitchens etc outside the coach and then move the module into the frame, keeping the weight on the floor area and not hanging from the walls.

 

[Ernesto]

Your ambition is impressive.
However to do this well is a "once in a lifetime" endeavour, and to me this doesn't square well with your apparent lack of experience of motorhomes (and your particular needs in one), and the community adage about "it takes three vans to find the one one really need".

Together with a budget limit I would recommend you buy the most similar van you can afford, even if it's 20 years old, and try it. Perhaps sell it and try another. Perhaps rent. Ask round at rallies. - In Manufacturing this is called "benchmarking" the market.

Then, if you think you can do better, and that it would be worth the grief for the marginal gain, you can build you own, and get it right first time.
[I have seen my father fail at this, spending 18 months building a trailer tent with masses of clever features, but ultimately too heavy, odd and unwieldy to use to be his "forever" trailer. In his case he couldn't even get a buyer, and broke it up.]
Good luck: I don't want to knock you, just nudge you to get it right for you.

This is good advice.
The ambition is impressive but a spec list that includes Aircon, double oven and mirowave, two beds, grey and black tanks …all fitted on a 6.5m Ducato-type platform, feels unrealistic.

 

[Pausim]

Some people say that a PVC vehicle is a PVC construction with insulation foam (PET), sandwiched between two sheets of PVC.

I thought in this context, PVC simply means 'panel van conversion'.

PVC has meant PolyVinylChloride (a cheap thermo plastic) for so long that it is a lost cause to use it on Google for anything else. Most people associate it with windows, doors and gutters or the sticky back plastic loved on Blue Peter in the 60s.

 

[TheMobileWanderer]

I think you'll struggle to get the weight of your proposed construction low enough to work with the Ducato/Sprinter type chassis You may need to start compromising on the thicknesses to add lightness.

Thanks. I accept that possibility. I wonder, however, whether a weight plate upgrade with air suspension and twin rear axles might make it possible. At least I should need to add less 'lightness' if my spec is too heavy.

I think nearly all the coachbuild videos I’ve watched, have the manufacturers building the cupboards and kitchens etc outside the coach and then move the module into the frame, keeping the weight on the floor area and not hanging from the walls.

Thanks; Yes, I'd be doing that too. Well outside build first and then re-assembly once I've got it's components through the habitation door. My concern was about repeated lateral stresses such as cornering, rather than vertical stresses from gravity and vehicle movement.

This is good advice.
The ambition is impressive but a spec list that includes Aircon, double oven and mirowave, two beds, grey and black tanks …all fitted on a 6.5m Ducato-type platform, feels unrealistic.

Thanks; Yes, the double oven is an indulgence. It would be the first thing to go.
Given that there is the GVW increase available when having the addition of air suspension and a third axle, it may all be feasible but, I wonder, is there a second different axle I should consider? I'm exploring Al-Ko but finding difficulty in getting a company to fit one. I've also found 'Masters', who seem to do an Al-Ko modification so I'll explore them.

Thanks all. Very helpful

 

[oldiesontour]

It appears from some of the materials you list good structural engineering is being substituted with heavy,bulky,dated materials that are from ideal,.In my opinion you are likely to end up with a massively expensive,heavy,,(to be polite) not very attractive unsaleable over time and over budget project... sorry just my opinion

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[TheMobileWanderer]

It appears from some of the materials you list good structural engineering is being substituted with heavy,bulky,dated materials that are from ideal,.In my opinion you are likely to end up with a massively expensive,heavy,,(to be polite) not very attractive unsaleable over time and over budget project... sorry just my opinion

Thanks.
Are you referring to the 18mm plywood or something else?

I've read from the beginning of my research that 6mm is best however, with the units floor-mounted and secured to the walls, I'm not convinced that 6mm is thick enough to secure them to at the top. My aim is to have them detachable and refittable to the same mounts (or same mounting plates) in the future, to enable upgrades and layout changes if desired by someone.

I'm yet to be certain that simple grp-sandwiched foam insulation is robust enough on its own to secure things to at all or without, at least, causing delamination.

 

[oldiesontour]

Thanks.
Are you referring to the 18mm plywood or something else?

I've read from the beginning of my research that 6mm is best however, with the units floor-mounted and secured to the walls, I'm not convinced that 6mm is thick enough to secure them to at the top. My aim is to have them detachable and refittable to the same mounts (or same mounting plates) in the future, to enable upgrades and layout changes if desired by someone.

I'm yet to be certain that simple grp-sandwiched foam insulation is robust enough on its own to secure things to at all or without, at least, causing delamination.

Many manufacturers use as low 3mm inner and outer skin for floors with very little support,walls are often 3mm ply , careful design for load spreading is the key,you would be far better to buy a ready built and once you've used it refit to suit your needs or go with a panel van to convert IF it could suit your needs.(Xlwb ,twin rear wheel ?)

 

[pappajohn]

I'm yet to be certain that simple grp-sandwiched foam insulation is robust enough on its own to secure things to at all or without, at least, causing delamination.

Seems to work very well for professional coachbuilt builders.
Why add a lot of unneccerry weight when 6mm ply is perfectly adequate.

 

[TheMobileWanderer]

Thanks both.

Many manufacturers use as low 3mm inner and outer skin for floors with very little support,walls are often 3mm ply , careful design for load spreading is the key,you would be far better to buy a ready built and once you've used it refit to suit your needs or go with a panel van to convert IF it could suit your needs.(Xlwb ,twin rear wheel ?)

I expect 'load-spreading' was the missing link in my knowledge. I was thinking along the lines of screwing into the internal face, to mount an, em, mounting panel, to which I could secure the top of the wall units. Such screws would pull on one place and I'm not sure that would be the best way. However, 'load-spreading'; that may be the necessary consideration. If I were to Sikaflex (or other strong glue) the mounting plate, its 'pull' would be spread across area glued. I could then secure the units to those plates. They would be hidden and wouldn't cause the wall units to be set out from the wall - just like bracketry behind domestic wall units doesn't set them out from the wall. Also, unlike a domestic wall unit, it wouldn't be the weight of the units being borne by these fixing 'mounts'. That would be done by the floor. These mounts are only to hold the top securely from wobbling about.

Also, the floor thickness: that was an error in my spec. That will be 6mm ply, with PIR foam beneath. When typing, I had visualised the old floor I am familiar with - 3/4inch ply.

I had thought about buying an existing one and re-fitting. Not sure that, for me, buying a motorhome at 15k+ and ripping its insides out to discard them, would make sense, when I can get a brand new 'box', peak and cowls and used base vehicle and install the brand new up-to-the-minute electrics for less.

Seems to work very well for professional coachbuilt builders.
Why add a lot of unneccerry weight when 6mm ply is perfectly adequate.

Agreed. I'm not trying to add weight. Robustness and, basically, brilliant build quality is the actual aim.

 

[pappajohn]

Ply weight.....
6mm approx 8kg per 2.4mtr x 1.2mtr sheet
18mm approx 22kg per 2.4mtr x 1.2mtr sheet.
14kg per sheet is a LOT of unnecessary weight.

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[funflair]

Thanks. I accept that possibility. I wonder, however, whether a weight plate upgrade with air suspension and twin rear axles might make it possible. At least I should need to add less 'lightness' if my spec is too heavy.

Thanks; Yes, I'd be doing that too. Well outside build first and then re-assembly once I've got it's components through the habitation door. My concern was about repeated lateral stresses such as cornering, rather than vertical stresses from gravity and vehicle movement.

Thanks; Yes, the double oven is an indulgence. It would be the first thing to go.
Given that there is the GVW increase available when having the addition of air suspension and a third axle, it may all be feasible but, I wonder, is there a second different axle I should consider? I'm exploring Al-Ko but finding difficulty in getting a company to fit one. I've also found 'Masters', who seem to do an Al-Ko modification so I'll explore them.

Thanks all. Very helpful

If you want weight capacity go straight to a chassis that can handle up to 7500kg, really that would mean the IVECO daily with air suspension, or if you are happy over 7500kg use a truck chassis.

 

[TheMobileWanderer]

Well I could do that but I've read that there are other things to consider with 7.5t+.
For example, putting stickers on every side outside, to show where the potential blind spots are. I'm reluctant to do that.

 

[Figaro]

Can I ask, have you had a motorhome before ?
It seems a huge task to build one from scratch including the body with no experience, even some of the recognized converters can't get it right with years of experience.
Would you not be better buying an older quality motorhome that needs updating giving you the opportunity to to rebuild it as you wish without having to consider structural integrity, weight and safety issues ?

 

[Minxy]

Well I could do that but I've read that there are other things to consider with 7.5t+.
For example, putting stickers on every side outside, to show where the potential blind spots are. I'm reluctant to do that.

If you mean angle mortes stickers in France then they are required for anything over 3500 kg so you'll still need them.

 

[TheMobileWanderer]

Oh, bum. That's the washing machine deleted from the spec!

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