Stronger bodywork with grp?
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IT WOULD appear more operators than ever before are looking for vehicles with bodywork of greater durability, to enable them to outlast a number of chassis.
If building a one-piece van body, for instance, of grp-faced plywood, could be proved to have unquestionable advantages over one built of aluminium, the extent to which grp will replace aluminium will depend on the ratio of labour cost to the cost of materials.
Alcan Booth Industries Ltd, of Banbury, claims the current materials/labour cost ratio of a conventional aluminium body is 60/40, while for a grp/ply body, it is 75/25.
These proportions roughly correspond to those quoted by an old-established builder of each type of body, who agrees with the Alcan figures.
In the absence of a decisive operational advantage in favour of grp/ply, aluminium construction should more than hold its own with its new competitor.
Alcan offers a design service to customers, and late in 1975 announced the availability of special extrusions for non-insulated grp/ply panels. Demand for extrusions that could accommodate insulated panels is so limited, as to be non-indusive to their development.
While the company has something to lose overall from a more widespread adoption of grp/ ply one-piece bodies, the loss would be relatively small, despite the fact that smaller bodies of grp/ ply can be produced without the aid of aluminium extrusions.
When Alcan points out that sandwich construction has yet to prove that it has the life expectancy of a well made aluminium counterpart, the observation does not imply an unshared biased viewpoint. The same comment has been made by a number of bodybuilders, notably with regard to bodies for heavier vehicles.
In the case of non-hgv vehicles, the change to 7.5 tonnes (7.38 tons) as the upper limit has encouraged the development of longer-life bodies.
It has been said of the application of grp/ply that it is "too easy to do it badly," but this could also be said of any material.
Undoubtedly, the future reputation of sandwich construction will to some extent depend on how well the many smaller bodybuilders, new to the game, will seek to do it the best way.
Alcan said, as did a number of operators at the 1976 Commerical Motor Show, that weight and cost estimates by smaller builders of insulated and non-insulated sandwich bodies vary considerably. This is, presumably, because of differences in constructional methods and techniques.
Some bodybuilders at the Show said that the use of grp/ ply incurred a significant weight penalty, while others claimed it was negligible. No one said it had a cost advantage, although it has since been claimed by a number of bodybuilders that the cost of smaller bodies is comparable (to aluminium) or is "slightly less."
Comparing a well-built sandwich body with an aluminium type in the same category, Alcan contends that aluminium normally has an advantage costwise and weightwise, notably for insulated bodies. The bigger the body, the greater the advantage. Dealing with established merits of sandwich bodies, Alcan emphasises the interior cleanliness provided by grp /ply panels can be matched by aluminium, if it is coated with acrylic paint. This is used extensively by the makers of containers intended for the carriage of perishables.
Although grp/ply is said to be unsuitable for some container traffic, because the epoxy-resin ingredient has been known to exude from the panels during a long storage period in the sun, and contaminate the load, Alcan points out this would be an unlikely event in the case of a vehicle body.
The second major advantage of sandwich panels, namely that they are seamless, cannot in practice be matched by aluminium. But Alcan maintains that handling longer panels for bodies up to 12m (40ft) requires the use of high-cost factory equipment, which is only justified by a big turnover.
On the subject of repairs, the Alcan view is that damage to an aluminium panel, and its extent, can readily be seen. What may appear to be harmless scuffing of a sandwich panel may have caused extensive delamination which could spread over a large area, if it were not detected early on.
Local delamination of a stressed body side is particularly serious, as it may rapidly lead to disintegration of the whole panel.
Reverting to future trends in the relative costs of the two materials, predictions regarding comparative rates of increase would, at least, be inspired guesswork.
Alcan mentioned the worldwide shortage of timber a few years ago, as an indication of market uncertainties, and forecast that the price of grp/ply panels will keep pace with a rapid increase in the cost of aluminium.
A substantial differential could be the most important factor _determining the growth rate (positive or negative) of sandwich construction. And who is to say that labour costs will go on escalating at a lower rate? Paul Brockington