Increases in the maximum length and weight of trucks permitted under federal law in the United States are in prospect. A number of individual states have, for some time, sanctioned longer and heavier tractor-trailer combinations operating within state boundaries, but the Federal Highway Administration (FHWA) has now embarked on a study ‘to assess whether larger trucks affect highway safety, pavement wear and modal shifts’.
Doubts, particularly in the public mind, are regularly raised about what might be termed the dynamic safety of articulated and drawbar rigs. Under emergency braking conditions or on low-adhesion – most obviously snow- or ice-covered – road surfaces, the risk of instability, potentially leading to an uncontrollable ‘jack-knife’ situation, remains an ongoing concern. In practice, the advent of ABS anti-lockup air brake systems and the subsequent development of electronically-signalled (rather than pneumatic pulse initiated) braking over recent decades has reduced the incidence of jack-knifing dramatically.
Double or even triple rigs, which are by definition exceptionally long, but nevertheless compliant with US truck dimension limits, are seen to pose specific safety problems
But worries about uncontrolled coupling articulation understandably remain, especially in relation to the different configurations of roadtrain that comprise a drawing vehicle hauling more than one trailer. Those so-called double or even triple rigs, which are by definition exceptionally long, but nevertheless compliant with US truck dimension limits, are seen to pose specific safety problems, in addition to the challenge they present to faster vehicles wanting to overtake them.
The FHWA study, being undertaken by government and university researchers, will look at six heavy truck configurations, including rigs grossing 36.3 tonnes with two 33ft (10m) trailers and others grossing up to 58.5 tonnes hauling three 28ft (8.5m) trailers. Potential extra damage to road pavements and bridges resulting from increased all-up weights will also be included in the FHMA study, as well as broader effects on freight transport modes – by implication long-haul movement of bulk cargoes being possibly transferred from rail to road.
For truck manufacturers, higher tractor-trailer gross weights are set to accelerate the now widely identified trend towards more powerful engines, with diesels of around 15 litre capacity, developing 500hp or more, usurping the market for those 12 and 13 litre units which have accounted for the majority of top-weight class 8 sales through the last decade.
A recent project undertaken by Volvo Trucks in Sweden, aimed at improving the safety of tractor-trailer rigs in Scandinavian winter conditions, particularly on downhill gradients, would seem to point the way
On the aforementioned roadtrain stability issues, there will have to be closer liaison between truck manufacturers and the technologically less-sophisticated trailer producers, especially on brake systems and controls. A recent project undertaken by Volvo Trucks in Sweden, aimed at improving the safety of tractor-trailer rigs in Scandinavian winter conditions, particularly on downhill gradients, would seem to point the way.
‘Stretch Brake’ is a new system, intended as an add-on to the electronic stability programme (ESP) now incorporated on a number of European commercial vehicles. It operates only at speeds below 40kph. As the rig approaches a downgrade, especially where tyre adhesion might be reduced by water, snow or ice, the driver manually activates the ‘Stretch Brake’. When he then takes his foot off the accelerator, the brakes on the trailer are applied in pulsated mode, effectively to ‘pull the rig straight’, correcting even the smallest tendency to slide out of line.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
Alan Bunting has a background in engineering, and has been writing on commercial vehicle and powertrain related topics since the 1960s. He has been an Automotive World contributor since 1996.
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