rrp defends controversial
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:omparative trials results
E ENGINEERING Editor's allenge to the Vehicle indards and Engineering vision of the Department of ansport has not gone unswered.
in a recent (February 9) ue of CM, Graham Montmerie attacked the concluins drawn by comparative its at TRRL's proving 3und and in particular drew tention to the means wreby the DTp got hold of e vehicles and the way in vich braking tests were ne.
The Department has replied full to the criticisms, though .a h am Montgomerie mains unconvinced about )w hard the Department ed to get hold of vehicles )m the manufacturers. This evented one of the most 'portant tests (one from hich the most damning ridence was drawn) to be operly carried out, namely e braking test — as the Deirtment admits.
The Department says that !cause the vehicles belonged firms, it was not possible to fect a full strip-down of the .aking systems.
As for the load sensing pparatus, it should have no fect on the fully laden ehicle — but if it is maladisted, then it will play havoc ith any braking results. This, lys Graham, is what effecvely vitiates the conclusions f the report.
Not so, says the DTp. "So ✓ as the checking of load!ming equipment is conffned," writes the DTp's -ess Officer Freddie Taylor, t could be argued that before irrying out braking tests, the mplete system should be ripped and examined. Howrer, the vehicles did not bemg to the Department so it as not feasible to do this.
"It was therefore decided at only a visual inspection of le systems would be carried ut and the brake shoes adisted. This procedure also flowed some assessment to be lade of the brake efficiency nd other performance tandards likely to he achieved by vehicles in service (our italics). The condition of the load sensing equipment should not have contributed to the instability found during service braking with two fully laden vehicles."
In its own defence, the DTp stresses that it was essentially working within the realms of the possible, and that the intention of the exercise was to assess the performance of existing vehicles rather than to test the ultimate performance of particular, and perfectly set up, manufacturerprepared vehicles.
Mr Taylor argues that Graham Montgomerie has overlooked the tentative nature of the report's foreword, which states that the intention of the tests was to include "a reasonable cross-section of vehicles sufficient to provide some indication of the performance levels that might be expected on the road." He also stresses that the sample tested was only intended to "provide a useful basis for further evaluation and study".
As for Graham's comments about the weakness of the Tpiece adaptor used to ensure compatibility between tractive units and trailer, Mr Taylor suggests that the report has been incorrectly quoted: "The report rightly says that a Tpiece adaptor as used would not be satisfactory for use in service conditions because it permitted slight leakage through the service brake valve when the secondary brake was applied.
"Since the whole purpose of the secondary brake is for braking to be available in the event of the failure of the service system, an arrangement that fed air from the secondary line to the service line, which might be the cause of the failure, would clearly be unacceptable. The leakage of air caused by this arrangement was small and the word 'slight' was included in the final report to indicate that the air loss was not significant — with the secondary brake control fully applied and the engine idling, there was no reduction in pressure shown on the air gauge. The adaptor did not affect the service brake test results, and it is these which. are of first importance."
He also stresses that the results obtained are not so very dissimilar from those obtained in CM test results, averaging around 0.46g for their results as compared with around 0.5g for CM's averages — a discrepancy which is easily accounted for by the "used" nature of the vehicles under test.
Mr Taylor also makes light of Graham's criticisms on rollover stability and fifth-wheel position, arguing that he takes insufficient account of the factors which primarily control vehicle stability. These are, he argues, that "the roll behaviour of an articulated combination is controlled by the semi-trailer, which is supported on a triangular base of its wheels and coupling point.
"The length of the tractive unit and fifth-wheel position on the tractive do not change the semi-trailer geometry and will not therefore affect the roll stability of the semitrailer.
"If the semi-trailer kingpin position is moved, the geometry will be slightly altered and so the theoretical value of the maximum lateral acceleration required to overturn the combination will be changed. In the tests, only one tandem-axled semi-trailer was fitted with an adjustable kingpin and this was always positioned to bring the overall length of the combination as near as possible to the desired 15 or 15.5 metres.
"In practice, the suspension characteristics of the semitrailer and the tolerance at the fifth-wheel coupling would have a far greater effect than would any slight changes of the fifth-wheel position."
On the final point that Graham made, that the report added fuel to the fire of the "juggernut" brigade, the DTp's defence rests on what it describes as a "point of fact". Graham was unhappy about the report's unequivocal statement that higher gross vehicle weights would increase the risk of roll-over accidents.
"This is a statement of fact," declares Mr Taylor. "If the additional weight is to be carried without increasing the load platform area, then the centre of gravity of the load must rise. The only time this might be significant is in the carriage of very high-density loads (such as steel plates) where the increase in centre of gravity height is minimal.
"In most cases, the payload will increase at a greater rate than the unladen weight, even allowing for additional axles at heavier weights. An increase in gross weight would therefore result in an increase in the height of the resultant centre of gravity of the semi-trailer and its payload, which must result in the risk of roll-over being increased. The tests served only as an indication of the extent of the increase."
Mr Taylor goes on to mention the noise tests made at TRRL, and agrees that the test track "is not wholly suitable for accurate BS noise measurements." The report does state, he says, that although these tests were a useful comparative trial: "The individual results were not intended to verify whether the vehicles complied with existing regulations."
As a parting shot, Mr Taylor says he believes that the wording of the report — which Graham suggested may have been "cleaned up" at the stage of publication — does in fact represent "a reasonable account of the outcome of the trials."
It was not its intention to comment on the wider environmental implications of heavy lorries on the basis of the limited and specific brief of the trials, and that had never been its intention.
"The trials carried out were essentially limited in scope and afforded a basis for only limited and qualified conclusions, and this is made quite clear in the foreword."