Getting away from steel
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World volume-produced trucks are currently based on ladder-form chassis frames, with steel longitudinal& Foden in the UK, like its Paccar parent in North America. fits weight-saving aluminium crossmembers and supporting bracketry. And in the 1960s some US manufacturers, led by Freightliner when it was a trucking company building its own vehicles, offered all-aluminium frames.
But the cost-benefit ratio proved questionable. Aluminium could be made to provide the strength, hut if eventual fatigue failures of the kind which plagued aircraft structures for many years were to be avoided, unacceptably time-consuming quality control measures were called for, focusing on details such as corner curves and potential stress corrosion points.
For the Americans, the cost of aluminium was, and still is, less of a barrier to its adoption. With steel cost as a baseline, aluminium is about three times as expensive in Europe as in the US. And the availability of progressively higher grade—and correspondingly expensive—steels has tilted the weight/cost balance further away from aluminium.
Similar costs have deterred European truck builders from making more extensive use of sheet aluminium for cab panels, although the material's corrosion resistance adds to its appeal in exterior cab applications. Foden continues to skin its cab doors in aluminium: for weight reasons it moved away from composite material, glassreinforced plastics (GRP), about 15 years ago.
But chief engineer John Gerrard says advances made in plastics technology through the 1980s could well force a reevaluation of non-metallic material for those door skins. The moulding of GRP and the way its glass fibres are woven before they are encased in resin are being looked at "more scientifically", says Gerrard.
ERF engineering director Martin Harper agrees that plastics continue to improve and that they point the way forward, Since the 1970s ERF's special form of cab construction has used a steel spaceframe "skeleton" to support an outer semi-loadhearing shell of hot-pressed SMC (sheet moulding compound) which is stronger in relation to its thickness than GRP.
ERF's limited production volumes make steel cab tooling costs prohibitive but the company makes a virtue of a necessity by highlighting the low manufacturing energy input for SMC construction—some 40% less than for steel. This brings environmental as well as cost benefits, which can be further enhanced if mouldings are colour impregnated to avoid later solvent-based painting.
On the new-generation EC cab, a higher (American class A) standard of outer surface finish matching that of ERF's continental competitors has been achieved. This is due in part to the use of more costly dies from the American company Autodie (which supplied the SMC moulds to General Motors for its GMC A-van plastic body panels).
The SMC mouldings are produced at Winsford, Cheshire by the Dutch-owned Autopress Composites, whose processes have been refined a) to get a more even curing temperature across the whole area of each moulding and b) to regulate more precisely the critical closing speed of the press.
All the SMC material in the ERF cab, unlike cold-set GRP, is recyclable, though Harper points out that it must be uncontaminated—in particular free of paint—to be reconstituted into new automotive standard SMC.
Steel skeletoned
A cab could be made entirely from SMC, says Martin Harper, but it would be likely to be heavier than ERF's steel skeletoned hybrid, because of the need for increased moulding thickness and metal or pre-moulded inserts to give substantial local reinforcement.
At the other end of Sanclbach Foden engineers have undertaken extensive work on alternatives to glass in resin-based mouldings. They include carbon-fibre and Du Pont's Kevlar. John Gerrard and his team have led the way on the experiments, ahead of their Paccar colleagues in the US, whose use of plastics in cab construction tends to be confined to roof panels and bonnets.
Kevlar is good for impact strength, making a security vehicle bullet-proof for example, but its cost cannot so far be justified in a standard cab, says Gerrard. Carbon fibre is also ruled out currently on cost.
Closed-mould plastics technology, which makes use of a vacuum to draw the resin more efficiently into the mould, produces a denser and stronger structure. It involves a resin transfer process (RTM) which first came to prominence in the bodyshell of the Lotus Elan car. An RIM moulding is seen by Foden's Gerrard as a halfway house between GRP and SMC, mouldable at room temperature into load-bearing sections, so that a cab floor for example can be fully selfsupporting, but with high strength-inducing density and close to SMC standard of surface finish.
An innovation on today's Foden cabs points towards the elimination of metal parts: the use of "pultrusions". These are fibrereinforced composite extrusions, typically tubular in section, which are cut off to the length required and moulded into the cab shell at strategic points in place of heavier pressed-steel box or Z-sections.
More use of composites in cabs is inevitable, and not just for the low-volume producers of Sandbach. The move is driven not only by plastics technology, but by the steeply escalating costs of heavily mechanised steel tooling of the kind installed by Iveco at its Brescia plant. The champions of GRP stress its ability to absorb accident impacts through progressive delamination, to give a "crumpleability" quotient as good as that of an equivalent steel pressing.
But what of non-metallic materials for truck chassis where structural strength and rigidity are everything? To use the word "frame" here begs a contentious question, because to commercial vehicle engineers it implies some kind of fabricated ladder construction. And designers agree that for the versatility of moulded composites to be fully exploited, direct substitution of "will fit" components in an alternative material brings minimal benefits.
No one knows this better than Jim Mason, former technical director of ENASA, before the Spanish manufacturer was taken over by Iveco in 1990. Mason, who now undertakes contract design work for the South Coastbased consultancy LAD, was responsible in Madrid for a succession of Pegaso concept vehicle projects under the name Solo. This sought to look again at chassis design by exploiting a host of new technologies drawn from the aerospace and satellite electronics worlds.
Mason admits that any attempt to get away from a steel ladder frame tends to be immediately inhibited by the practicalities of needing to accommodate conventional springs and axles. Provision for different wheelbases is also a major obstacle, which is one reason Mason's team focused its attention on a twoaxled artic tractor, with a universally acceptable frame length. by Mason as a "bathtub". The Iveco acquisition put a stop to the Solo project, which might otherwise now be undergoing trials.
Computer-aided design and engineering methods were used to optimise the shape and, more critically, the thickness and disposition of fibres in what was, by any vehicle industry standards, a huge integrated moulding. The parts of the structure required to absorb the usual bending and shear loads normally taken by steel channel-section sidemembers were varied in depth and thickness in precise proportion to those loadings. It ensured, as near as possible, uniform stress levels throughout, eliminating dead weight and failure risk points.
Spring travel
It made possible the adoption of independent Macpherson strut-type front air suspension, whose mountings were part of the total "bathtub", contributing to its rigidity, not least in torsion, where extra spring travel is required to compensate for the frame twist which on most vehicles helps in negotiating uneven ground. The tractor's engine could then sit much lower than usual in a big Usection bay at the front of the moulding. The GRP structure on either side of and below the engine served as effective noise dampening. The low-slung engine, lowered the vehicle's centre of gravity, helping roadholding and safety, at the same time as making cross-cab access easier.
Fuel was carried aircraft style in what amounted to internal tanks, making use of the inevitable voids within a big torsionally stiff moulding.
Air reservoirs, because of the pressures involved, are less readily integrated into an all-in-one GRP chassis. They need to be
cylindrical metal reireff pressure vessels,
located by Pegaso in midships side "recesses". At the rear a composite chassis can be tapered in depth, again to optimise strength and cost with weight and can, if required, incorporate the coupling lead ramps as a further useful weight saver.
Mason estimates that maximising the use of composites in the chassis and cab of a tractor unit could reduce its weight by as much as 25%. On the debit side is a rise in what he calls "process cost", which diminish little with increasing production volumes. Moulding techniques do not lend themselves readily to automation, so labour cost is high.
The exception which perhaps proves the rule is composite leaf spring construction. GKN Composites shut up shop at Telford two years ago because it only had one main customer, namely Leyland Daf Vans for the 200 range.
The numbers of springs being produced in what was a highly automated (and secret) plant was insufficient to justify the venture continuing.
Getting away from steel is also a preoccupation with engine and driveline producers. GKN supplies carbon-fibre propshafts to Renault for its Espace range, where the material's stiffness does away with the need for centre bearing support.
Composites such as ABS and SMC are now commonplace for cab components that save weight and will not corrode or suffer stone chips. Their structural strength is apparent from the number of truck builders now fitting plastic bumpers and wing valances. As buyers become ever more payload hungry and industry becomes more energy conscious, the use of non-metallic materials is surely unstoppable, although at present there is a technologically "quiet" phase which the European recession has imposed on many truck manufacturers.
• by Alan Bunting