The Road Toad v.1
(curved back version)
The Road Toad
(final design)
(straight back version)
The Road Toad
(interior)
all content & images
©2003-4 Rik Keller
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The Road Toad trailer
Basic trailer specs:
● main body length: 10'
● total tip-to-tip length: 14' 6"
● main body width: 60"
● top of body from ground (fully loaded): ~62"
● interior cabin width: 57 7/8"
● maximum interior cabin height: ~46"
● sleeping area length: 80"
● footwell: 30"x51" by 8" deep (7" interior depth)
● ground clearance (from bottom of footwell) (fully loaded): 6.25"
● tongue box length: 18"
● tongue box width: 48"
● tongue box height: 24"
● anticipated Gross Vehicle Weight (GVW): ~950 lbs.
Design features:
● fully insulated (roof, sides, floor, galley, bulkhead)
● aluminum exterior skin
● Finland Birch interior with all-natural oil finish
● full-height rear galley hatch with standard height countertop
● foldout child's bed across the width of trailer above the foot of the bed
● front tongue box has exterior access for bottom half and interior access for top half
● 7" deep footwell in middle of sleeping compartment and 4-piece mattress cushions allow interior seating both to the front and rear
● the footwell covers become an interior and an exterior table
Design considerations:
● the trailer body panels are constructed as a "sandwich": 4mm (5/32") Finland Birch on the outsides, with 3/4"x1.5" poplar framing, and 3/4" (for floor and sides) or 1.5" (for roof, front wall, and hatch) Styrofoam in the middle. Total panel thicknesses are 1 1/16" & 1 13/16", respectively.
● the sandwich is glued together with Titebond II wood glue and System Three marine epoxy thickened with plastic mini-fibers. The plywood skins take up all of the compressive and tensile forces while the foam core takes up all of the horizontal shear force. Even given the relatively low shear strength of the foam, the maximum load for such a sandwich panel based on a given deflection is still governed far more by the bending force than the shear force. In other words, the structure will deflect more than the allowable maximum if overly loaded based on the lack of bending resistance of the skins before the shear strength of the foam comes into play. The bond between the core and the skin does have to be good to prevent localized buckling of the skin.
● the sandwich ends up being a very strong, relatively lightweight, structural insulated panel. The side panels, for example have a lot of poplar framing, but still weigh only 1.8 lbs./sq. ft., which is a bit more than 1/2" (nominal, actually 15/32") plywood at 1.5 lbs./sq.ft., equivalent to 5/8" (nominal, actually 19/32") plywood at 1.8 lbs./sq. ft., and less than 3/4" (nominal, actually 23/32") plywood) at 2.2 lbs./sq. ft. However, the side panels have about 3.3 times the stiffness of 3/4" plywood, and almost 12 times the stiffness of 1/2" plywood.
● the side panels have an R-value of approximately 4.1. This is more than 4.5 times the R-value of 3/4" plywood (R-0.9). The roof panels have an R-value of approximately 7.9.
Materials considerations:
● Poplar is a fast-growing, mostly sustainably-harvested domestic hardwood. It has a high strength to weight ratio.
● Finland Birch plywood is made in Finland from sustainably harvested birch trees. It is very strong. Much stronger than standard 1/8" plywood (that typically has paper-thin veneers and poor quality core material) or 1/8" "Luan" plywood (also called "Phillippine mahagony" even though it does not contain any mahogany). Luan does contain wood from any number of species from old growth Asian rainforests.
● The 4mm (5/32") Finland Birch plywood is from MacBeath Hardwoods in Berkeley. It has three plys, all veneer, with the face plys close to the interior ply thickness. It is constructed with exterior-grade glues.
● Exterior-grade plywood is good for two reasons: the ability to stand up to moisture that cycles through the wood, and also because of reduced formaldehyde out-gassing. "Interior grade" plywoods are made using adhesives that contain urea-formaldehyde (UF) resins. Exterior grade plywoods use the dark-colored phenol-formaldehyde (PF) resin. Although formaldehyde is present in both types of resins, the PF resin generally emits formaldehyde at considerably lower rates than the UF resin. Ironically enough, interior-grade plywoods are much worse for indoor air quality than exterior-grade plywoods.
● Dow Styrofoam is extruded expanded polystyrene (XPS). It is different than what most people call "styrofoam," which is molded expanded polystyrene (EPS) or "beadboard", an open cell, crumbly foam. Styrofoam is a structural, rigid, closed-cell foam board. I used the 3/4" Dow Styrofoam "Tongue and Groove" for the floor and sides. It has a compressive strength of 25 p.s.i. and an R-value (thermal resistance) of 3.75 [Btu x in/ft^2 x hr. x °F max]. Styrofoam has an R-value of 5 per inch thickness, compared to wood which has an R-value of about 1.25 per inch.
● Styrofoam does not have any recycled content, although it can be recycled itself. Chlorofluorocarbons, or CFCs, which deplete the ozone layer and are also greenhouse gases, were used until the early 1990s as blowing agents in extruded polystyrene and other insulating foams. Among building insulation materials, extruded polystyrene (XPS) led the shift to less damaging hydrochloro-fluorocarbons (HCFCs). The current blowing agent in Styrofoam is HCFC-142b, which is scheduled to be phased out by 2020. HCFCs are only 5% to 11% as damaging to ozone as CFCs because they do not last as long in the atmosphere, but they are almost as damaging for the period of time when they are there.