24 April 2016
24 April 2016, Comments Comments Off on The magic box
The magic box

If the foundations are the first key component to build a good house, the walls and roof are the second and the third, respectively. The combination of these elements is what closes the box, which must be closed properly. Compared to a house, a motorhome moves, travels, tackles different kind of roads, climates, countries, and extremely varied, often hard, weather conditions. Let alone the roadbed-generated stress and accidents. Security is 99% entrusted to the basic vehicle, the only item somehow designed and certified, after massive investments in design, development and real crash tests, to provide a “default” behavior in case of impact. In this unlucky event, the only task of the cabin body is to preserve passengers’ integrity and health while dispersing the kinetic energy generated during the impact by starting an instant programmed deformation process. After all, the box can break, if this is to save its contents. Just as it happens with a box and a house, the sandwich-panels body of a recreational vehicle takes the necessary rigidity and ability to adequately distribute stresses and kinetic energy, only when it becomes a single body, i.e. when its components are mutually interlocked. When the last piece of a giant puzzle made of thousands of pieces is placed in its own seat and, in doing so, connects, joins and fixes all parts of its three-dimensional perimeter: walls, tail, front on the outside; furniture, furnishing, environment on the inside. In a nutshell, the roof is the last carrier element of the body to be installed and the first to ensure solidity and strength to a heterogeneous set, which is potentially fragile and significantly sized as well.

Words and pictures Michel Vuillermoz

Let’s take a step backwards: before the body we have to consider the chassis, which must provide the new set with stable, safe, uniform and homogeneous behavior. No standard ideal chassis exists: every type of production has its ideal one. A structure that has to support considerable volumes, which are often very different from those imagined by manufacturer for the original van, with leverage effects that have major repercussions on static and dynamic setup. If the foundations are good, the role of the floor and of all that it will support (from the walls to the roof, from furniture to on board systems’ heaviest components), becomes easier. Even with the latest generation bodies, thickness, floor materials as well as construction and assembly types are the traditional ones: fiberglass sheets, phenolic plywood, solid wood, insulating foam, linoleum or PVC, polyurethane or extruded plastic materials. The vehicle walk-over flooring should merely support anything located above it and let itself be supported by what is below, i.e. the chassis. Therefore, the torsional rigidity of the original chassis and its presence thorughout the whole surface of the vehicle to support its body plays a crucial role, especially in the rear area that, with the increasing diffusion of large through garages, is often subject to dangerous overloads. The floor acts as an intermediary: it transfers to other items the loads it receives.

If we consider a recreational vehicle as a set of components, we can estimate the number of these in the order of thousands: it seems then logical that the combination possibilities between materials, assemblies and components are as diversified. Some products are manufactured with the side walls partially overlapping the floor, so as to physically discharge the weight in a linear and constant fashion onto the entire lower edge. In others the joint takes place with no overlapping, with the weight of the wall that is largely supported only by fastening screws laterally inserted during the assembly and with the use of wooden dowels to ensure the correct positioning of the various elements. The mounting systems vary depending on the brand, manufacturer, materials and, of course, technology, the selling price of the finished product and the experience gained over time. Among the vehicles included in the mainstream market, the French brands Challenger and Chausson have recently introduced a new building system for the IRP-named body (Insulation, Resistance & Protection) that combines the typical characteristics of the wooden skeleton with the polyurethane water resistance, used for the wall’s lower and the floor’s outer perimeters. As to wall and roof, the two Trigano VdL brands, which are among the most competitive in the mid-market, opt for a classic fixture, with wooden dowels that allow the discharge of a portion of the wall weight onto the floorboard and a correct alignment. The outer screws portion is adequately sealed with butyl adhesives, while it is noteworthy the wall outer edge that does not coincide with the deck’s but continues further downwards so as to create a sort of ‘gutter effect’ thus preventing a possible water rising that drips from top. A similar procedure, although without the latter measure, is implemented by the entry-level brand of the Erwin Hymer Group: Carado and Sunlight. We can spot some interesting variations also elsewhere: the Elnagh, McLouis and Mobilvetta vehicles, all featuring iTech body 4.0, have completely eliminated the wooden components inside the body by replacing them with special thermosetting resins, plastic conglomerate and, as regards the inner lining, a classic fiberglass sheet. The set, then, involves the inclusion of appropriate reinforcement discs embedded inside the structure to provide adequate pulling ability to the fixing screws. The joint between the floor and walls occurs both by sealing and screwing: in the absence of the classic support dowels, the screws entirely support the weight of the wall and, consequently, their use is quite frequent. Conversely, the alignment between the various elements, makes use of special supports that are temporarily assembled and fastened to the lower portion of the deck and then removed once the wall assembly is completed.

Moving up to a definitely higher technical level, the most prestigious German brands, true symbols of Made in Germany, have developed particularly interesting self-supporting bodies over the years, because, thanks to a double inner and outer aluminum sheet, very high density insulating materials and special assembly techniques, they do not require internal reinforcing skeletons. As it is logical to think, such a vehicle combines a number of major ‘pluses’, including the total elimination of thermal bridges, the highest thermo-mechanical resistance, an outer surface that remains perfectly smooth and car-like even after years of stress and usage and, of course, a total insensitivity to any problem related to water seepage. Prestigious vehicles, therefore, that often feature a significant tri-dimensional development of their side: length, surely, with much higher values compared to those of a normal caravan with a total mass of 3.5 tonnes, but also and especially height, by virtue of the double through floor that, on average, increases height by 20 to 40 cm to achieve the desired thickness. If, in fact, it is possible to identify the basic standards for the simplest vehicles within a range between a minimum of 27 to a maximum of about 35 mm (with 25 or 30 mm dedicated to insulation), by moving to a higher level often we reach values of 4.5 cm, with at least 40 mm left to the polyurethane foam or RTM used to best insulate the body.

Each vehicle has its own body. A prestigious body lowers the payload of a license B-vehicle and, conversely, a too light body does not offer adequate performance for a large vehicle’s needs, where insulation and aesthetics values counts more than 50 kg saving. The wall of a prestigious vehicle needs a suitable support. The wall is not positioned at the side of the floor, but partially above it, using a special enlargement (scalfatura), so as to better distribute the weight onto the entire bearing surface and offer the maximum strength and durability over the years. To this end, one of the the leading German brands, Niesmann + Bischoff, uses a special metal rail, connected to the perimeter of the floor and apt both to support the side and provide a solid anchor by offering proper mordant to the fastening screws, which are positioned every 35 / 40 cm proportionally to the vehicle length. Simpler but equally effective is the solution developed by one of the most successful brands, Morelo: the large Palace, Palace Liner, Palace Alkoven and Loft, but also the more compact Home, feature the wall directly resting on lower floor edge, whatever the level. In this way, the wall will always perpendicularly discharge its weight in any point of the vehicle (cockpit, living cell, rear through garage). The base chassis, often truck-derived, is further reinforced and prepared for installation by the addition of side and rear extensions, is not subject to torsion. The resulting benefit is real and easily understandable: a stable and robust system can ensure maximum silence when the van is in motion and, as it does not udergo twisting, it does not highlight the aesthetic imperfections that show the operating points and reveal the presence of any internal reinforcement stitches.
A simple, smart and convincing system is the one developed by a major player in the high-end market: Carthago. The Aulendorf factory, in fact, couples the lower floor to the wall by a preformed special trail which, running along the whole side of the vehicle, interlocks the two elements without the need for screws, through-holes, internal reinforcements or polyurethane strips. By doing so, the weight of the wall is entirely discharged onto the deck in a continuous, balanced, uniform and constant fashion along its entire perimeter within a stable set, which
is insensitive to water or other weather conditions.

Accordingly, the body assembly is a crucial differential: not only materials, thicknesses, components and coatings make a difference, but also and above all the assembling techniques adopted to ensure tightness and safety over time. It is imperative that a body, if subjected to a sudden pressure resulting from impact, does not collapse and open. Therefore, in addition to the floor / wall junction, the manufacturer will focus also on those between the side and rear walls as well as between the body and the roof. Just like for the lower portion of the vehicle, also for the wall / tail juction different solutions are available. An economic vehicle will tend to simplify this procedure as much as possible, opting for an angle overlap between the longitudinal and transversal portion, joining the components by means of screws, sealing and finishing with the protective angle irons. As it occurs with the side walls, even the tail will have to discharge perpendicularly its own weight: the surface is limited compared to the facade, but the presence of drop-down rear garages and the use of non-linear shapes require the utmost attention during the assembly process. Just as for the roof, the outer coating of the tail exceeds the underlying skeleton and also covers the thickness of the nearby side wall to be joined with the latter by screwing, stapling or gluing. Several manufacturers currently opt for preformed plastic carter, generally ABS-made, including optical units, bumpers and junctions / trunks with respect to roof and walls. The most sophisticated vehicles may feature a double tail wall, with the conventional sandwich paired with an external mould to ensure, at the same time, modern and car-like shapes along with insulation and robustness. Some manufacturers use an ABS product to cover the entire wall (see Laika with its Kreos motorhome and Rexosline), while more common are the fiberglass components, widely used by Niesmann + Bischoff, Morelo Reisemobile or Concorde. Interestingly, on Niesmann + Bischoff we note that even in the presence of an extremely stable and robust structure, the fastening points, involving screws and sealing, are particularly abundant, and a rear sub-wall of about 2,2×2,5 meters is fastened by no less than 40 different anchor points that use special metal rails and polyurethane reinforcements. Another very interesting solution, patented and used long since for its ability to combine style, strength and durability over time is the one adopted by Hymer for its Pual body. Featuring a very smart and peculiar concept that involves the injection of polyurethane foam, the body developed over the last 17 years, i.e. from the motorhome 1999 B-Klasse birth, stands out for total absence of angle iron joints between the roof, walls and the rear. As a result, it offers soft fittings, with clean and car-like shapes. On the models developed by the worldwide famous Bad Waldsee giant, the rear wall is connected to the adjacent ones through proper internal polyurethane profiles, embedded inside the sandwich and perfectly glued together: an extremely robust solution, which does not involve the use of screws ( and, therefore, of potential holes or entry points for the water), finished on the outer side by a special shock-absorbing rubber profile, with the outer sheet made of perfectly smooth curved aluminum running along its perimeter to be blended directly with that of the adjacent side walls. No termal bridges, and, therefore, no possibility of water ingress and highest resistance make this system one of the most convincing and enduring available today.

Having worked on the deck, then on the walls and the rear, we finally close the circle or, better, the magic box, with the roof. A crucial element that not only is required to offer resistance, thermal and acoustic insulation (consider the heavy rain impact), bearing capacity, strength, but is also one of the most important components of the entire structure. Objectively, the roof is the part which the weather conditions (rain, hail, snow, but also ultraviolet beams) impact on first. Besides, it often has the greatest number of openings (chimneys, portholes, panoramic windows …), is subject to major changes during the vehicle’s life, being the only usable surface for the installation of quite important add-ons (air conditioners, solar panels, satellite dishes, luggage racks…) and frequently has much more elaborated shapes compared to the classic sides. Let us think of the classic overcab, where the roof often consists of several elements that go make the entire curvature of the roof window, or the increasingly popular low-profiles with roof beds, on which the designers had to create the space apt to accomodate the pull down bed while maintaining the height of the living below: in these construction types, we hardly find a perfectly coplanar roof. An extra difficulty, then, that often adds to the need to allow the materials subject to very high temperatures to adequately expand (extreme cold in the winter, maybe even supporting the weight of the accumulated snow, or blazing heat in summer, with the sun heating the surface until it reaches a temperature far beyond the 50/60 Celsius degrees), and that is addressed with the most diverse technologies, materials, assembly systems and solutions. The only common denominator is the growing tendency to replace the classic, smooth aluminum coating with anti-hail fiberglass: this material, introduced in the Italian and French productions since the mid-nineties, in the past five years has started to be increasingly adopted also by the German manufacturers, although in different shapes varying according to the brand. Some offer it as standard on their whole production and some propose it as an option to be requested upon the order placement, but all, for better or worse, have understood the importance of providing a top surface that can withstand moderate impacts without undergoing aesthetic damage.
The glass fiber may consist of a sheet used to replace the classic aluminum, can be attached to the aluminum cover (Hymer) or may be a single block created ad hoc for each model or vehicle. One of the best examples is given by the French Fleurette, a medium-sized company based in Benet and, above all, the Rapid Group’s jewel in the crown. Specialized in fiberglass processing, Fleurette builds all the components of its vehicles and boasts a possibily unique know-how in the continental scenario. If, in fact, 99% of manufacturers build sandwich walls using, as outer coating, an aluminum foil or fiberglass roll supplied by an external specialist manufacturer, Fleurette directly coats the walls with resin: it implements the sandwich (skeleton and insulation) first, then lays the glass fiber, the resin and the gelcoat. The set becomes unique as it requires no glues or presses and ensures robustness and a higher gloss finish over time: the outer color, in fact, is decided directly during processing and not by a subsequent surface painting. Even more interesting, then, is the realization of the front, rear and roof, all made with in-house manufactured and assembled fiberglass monoblocks to compose a perfect joint, a sort of shell that ensures stability and robustness and that, as a shell surrounding the side walls, protects them by creating a stable and robust assembly.

As to the connection between walls and roof, four different schools of thought exist at least:
1. classic, which involves the use of angle irons and the related gutter effect aimed at safeguarding the side walls’ aesthetics,
2. the use of a clamshell cover, with no angle iron involved
3 the wall, curved in the upper part, directly joined with the roof
4.the roof that, bending, is directly blended with the wall.

The most classic and most widespread setup is obviously the one that makes use of the traditional external angle irons: once aligned, the walls and roof are joined in different ways (screwing, fitting, gluing …), adequately sealed or equipped with sealing gaskets and covered by cosmetic and not structural carters that, being able to take different shapes, are often used as actual gutters. Running along the whole side of the vehicle, they help the rain water runoff through the roof along the vehicle’s tail or front, while avoiding the annoying “cascade” effect on doors, windows and service doors and the classic black and unsightly humidity stains. From the aesthetic point of view, a structure of this type may be a little impressive, with edges more or less underlined by angle irons of a different kind (aluminum, ABS, fiberglass), with or without external screws and relevant cover profiles, and it is suitable for any type of vehicle, from small low-profiles to high end luxury motorhomes. In addition, the junction of flat surfaces allows a better coupling between the vehicles parts characterized by highly thick elements and, of course, an inner right angle that simplifies the furniture assembly. Obviously, there is a huge difference related to technology and holding ability over time between a joint implemented with wooden screwed straight edges, which are sealed and covered with aluminum or ABS angle iron with visible screws and rubber cover profile and the one that uses continuous metal rails, sealed from the inside and with no external visibile screw. As a result, the external angle iron of a maxi prestigious motorhome has quite different features from that of a cheap product.

The clamshell roof is an almost exclusive feature of French-produced vehicles: adopted over the years by Pilote, Levoyageur, Bavaria, Quick and Fleurette, it has some significant advantages but does not definitely simplify the assembly phases. The polyester outer shell is, in fact, only a giant protective casing that sits on a more conventional sandwich structure and connects the side walls by acting as a carrier element. The great aesthetic accuracy shown by this type of construction generally allows manufacturers to exploit the intrinsic fiberglass peculiarities: malleability, with the possibility to create smooth and harmonious shapes, also and especially in usually very linear parts, elasticity and resistance to stresses.

The use of a clamshell roof is not, however, the only option to build a particularly fluid roof-wall junction or inspired by car-like aesthetics: in fact, the solutions that replace the classic angle iron with a direct curvature of one of the two main actors are increasingly popular. Introduced in the late Nineties by Niesmann + Bischoff, almost simultaneously on the first Arto generation and the second Flair generation, the construction with the upper wall curved to be joined directly with the roof has become one of the strengths featured by the prestigious German brand’s motorhomes: combined with a self-supporting body with dual internal and external aluminum coating, this assembly method creates an anti-torsion box designed not to delegate anything to the interior’s furniture. The wall/roof junction is implemented through a very solid L-shapen rail, screwed from the inside to the wall first and then to the roof, which supports the latter. Thanks to the type of the materials used for the shell and the exceptional number of anchor points (on average, one every 15 cm), it creates a set able to withstand extremely high stresses. A similar process, with even closer anchor points (on average, one every 5 cm), is then used for the junction between resin monobloc front and roof. On the outside, the combination of the smooth aluminum walls and the anti-hail fiberglass roof is completed by a special rail that allows the anchorage of any veranda and that, of course, constitutes an actual gutter for the underlying side panel. Different but equally valid is the system developed and introduced by Carthago in 2003 for Chic models and still successfully used on the lines currently produced by the German brand, which is characterized by the elegant curvature of the upper wall. The curvature, obtained by milling the RTM insulator, the wall folding and the inclusion, on the inner side, of a stiffening aluminum profile, further combined with an increase of the insulation layer thickness, directly encounters the flat surface of the roof. The roof edges are joined by a special rail, which is rigidly coupled to the extreme edge of the wall and under which the cover fiberglass sheet is placed. Thus, exactly as occurs for the wall / floor junction, the body can be assembled by embedding, without the use of carrier screws. A similar conception of wall / roof assembly has been adopted for years by LMC for different vehicles including the BreezerV low-profiles, Cruiser and the low-profiles Explorer, as well as for the models derived from the latter: Buerstner Aviano MY 2016 and the recent Hymer Classic, while in France, Rapido has chosen this type of construction for the Distinction flagship series.

Among the first manufacturers to have conceived and developed a constructive concept of this type, Hymer unquestionably has the merit of having introduced a sophisticated, advanced, robust and beautiful type of construction both from the mechanical strength and thermal insulation point of view. The Pual structure, along with the significant stylistic evolution introduced since B-Class 1998, features, in fact, the presence of a roof-wall connection implemented without angle irons’ aid. In this case, however, it is the U-shape roof sloping toward the walls, with an aluminum / polyurethane / phenolic multilayer monoblock suitably bent on edges and strengthened, along the inner perimeter, by an inner Lambda-shaped plastic profile that, besides further strengthening the structure, provides a valuable tool for the on-board trunking or any accessories and components to be mounted on the roof. The wall- roof junction, designed in such a way that the latter can fully discharge its own weight perpendicularly to the previous one, occurs in a simple and natural way and without screws, as a special polyurethane internal profile replaces them. Divided into two portions (male and female), it is injected with the glue that, once dried up, becomes structural and completely insensitive to thermal shocks or weather conditions. The finish along the wall consists in a hook-shaped rubber profile that acts as a gutter that helps direct water runoff into the front or rear part of the vehicle, while preserving the sides.
A noteworthy variation to the assembly system proposed by Hymer is the new iTech4.0 body that equips the Mobilvetta products’ latest generation, includes, for the low profiles Kea P and new motorhome K-Yacht Teknoline, the clamshell roof of the vehicle connected with the upper part of the side walls thanks to a curved radius of 16 cm. The roof, in particular, consists of a 1.8 mm thick fiberglass outer shell, 27 mm styrofoam insulation (density = 35kg / m3) and the 3mm internal plastic laminate coating finished with faux leather with a total thickness of 32 mm. The junction with the wall is implemented through a suitable shaped, aluminium inner profile, insulated by a layer of extruded polystyrene and protected on the outside by the fiberglass shell that acts as external roof coating. This, in particular, joins with the fiberglass outer sheet on the side wall by using a second, C-shaped aluminum profile that is not tangent to the inner one in order to avoid any thermal bridge. The same solution, however, combined with higher roof walls thickness (50 mm) is currently adopted by the new Arca America, a motorhome that marks the return of the historic brand to the small vehicle high-end segment.