Some questions for
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TRRL to answer.. •
AS WE REPORTED in last week's News Extra, the comparative tractive unit trials done by the Transport and Road Research Laboratory have caused a rumpus in the industry. Because some of the report's findings are controversial to say the least, in the absence of comment by the manufacturers, the .Engineering Editor here adds his own assessment to SM MT's brief statement last week.
THE OBJECTIVE of the comparative tests was to compare some of the safety and environmental aspects of maximumweight vehicles currently in use with others designed to operate at gross weights in excess of that permitted in the UK by the Construction and Use Regulations. Artics were used for the exercise with some of the tractive units being supplied by the manufacturers and some by rental fleets.
The loads were carried in two modified 6.1m (20ft) ISO steel containers which were loaded so that the c -of g was near the centre in order that a reasonably uniform and high load was achieved in all cases.
As far as the overall length was concerned, this was intended to be 15m (49ft 3m) for the 32-ton vehicles and 15.5m (50ft 10in) for the heavier ones but, because of the lack of an adjustable fifth wheel coupling and/or a movable kingpin, this length limit was sometimes exceeded. Three trailers were used — two from Crane Fruehauf (PSK 32 and PSK 44) and a tri-axle trailer from Hall Köln.
I would argue initially that the way TRRL got hold of the vehicles left a lot to be desired. If the purpose of the testing was to establish the performance of heavier vehicles with regard to braking, smoke, etc, then why not use new vehicles so that at least the mechanical condition be known and be comparable vehicle to vehicle?
If the reply to this is that the TRRL wanted to use "typical" vehicles in a general service condition, then why obtain some from manufacturers and some from commercial hirers?
• Braking performance
The mean overall deceleration was calculated from the speed and distance measured by a fifth wheel attached to the tractive unit. Some of the results can be seen in the accomPanying table. Some of these are not exactly earth-shattering in terms of stopping power — but perhaps this is not altogether surprising.
The report states that the brake systems were checked for leaks and correct wheel adjustment before the testing began. However, due to compatibility problems with twoand threeline systems, a 1-piece adaptor was used so that the two-line braking system of the tri-axle Hall Kan could be catered for. Yet the report admits that this, was not a satisfactory arrangement as there was a leakage in the system. Perhaps TRRL expected to get satisfactory braking with a leak in the system? With the exception of one 44-tonne combination, a tendency to, jack-knife when unladen occurred under some conditions of braking. This instability is reported to have occurred in many cases on a dry, high-coefficient surface. Two of the 17 combinations were completely unstable, jack-knifing when fully laden on the dry surface.
According to TRRL. the use of load-sensing valves did not always prevent jack-knifing. But were the valves operating correctly? The report admits that they were not checked for correct operation or adjustment prior to the test!
Tilt platform
To investigate the stability of
the various combinations, eac vehicle was chained to a ti platform with the wheels at th lower side being checked tl prevent slipping. The platforr was then tilted until the point c balance was reached. However doubts are cast on the result from this test because of thl peculiar proportions of many a the test combinations resultin from the lack of a suitable fifth wheel position.
The fuel productivity test' were confined to measurement' made on the test track, with on lap of the circuit bern approximately 7.5km (4.; miles) in length.
The vehicles were tested botl laden and unladen and the fue consumption was assumed ti be linear between these limits.
Previous TRRL test work had indicated that road test fuel consumptions increased by about 18.5 per cent compared to the track tests. The diagram shows these figures increased by 18.5 per cent and compared with some of our own CM road test figures.
I am left with the impression hat the TRRL is as mystified by the productivity figures as I am. The results indicate that an in:-.rease in weight would not esult in any substantial shift in Fuel productivity compared with 3 current 32-ton vehicle.
This goes against the 'indings of an earlier TRRL *eport and also against the CM :rial of four years ago with a i/olvo F88. Regular readers will -emember that this particular sehicle used 11 per cent more uel for a 28 per cent increase in )ayload capacity — the actual igures being 43.5 lit/100km 6 5mpg) for 17 tonnes (16.8 ons) payload and 48.7 lit/ 100km (5.8mpg) for 27.6 onnes (27.2 tons).
The earlier TRRL report Fuel Jtilisation of articulated /chides: effect of gross vehicle oveight came to the conclusion hat the use of vehicles in the 32 5 to 44-tonne gross weight -ange could produce significant rnprovements in fuel utilisation ompared with current vehicles. The potential saving was esmated to to be 7 to 10 per cent at 38 tonnes and 12 to 15 per cent or 44 tonnes.
I was rather disturbed to find hat the final published version )f the report differed in several ireas from the draft supplied )riginally to the SMMT. As a ournalist I am quite accustomed
to the -tidying upof an article from the use of English point of view — but the report goes well beyond that.
The final conclusion from the draft is omitted altogether. This stated: -In summary, some of the heavier lorries tested were inferior in terms of braking performance and slightly inferior in their roll-over stability.
In all other respects there was little to choose between the lighter and heavier vehicles. An advantage of the heavier vehicles, provided certain loading difficulties are avoided. is that a significantly greater payload per vehicle and driver can be carried without any excess fuel penalty.
It is impossible to establish from the results of these tests what impact heavier lorries would have on road safety and those aspects of the environment tested, but it seems probably that the adverse effects, if any, would be slight if their braking performance could be improved.
As I said earlier, the r-piece adaptor caused a leakage in the air system. Now any leakage in a braking system is a serious defect but even more so when you are trying to establish a comparative standard for braking performance. I wonder how serious the leak was? In the draft report, it was stated that "air leakage occurred,In the published report the word -slighthad been added.
I am not impressed by the report. I appreciate that the aim behind the test was to provide a direct comparison in various aspects of performance between the heavier weight vehicles and those currently allowed on the roads.
However the way in which the testing was carried out leaves a lot to be desired in my view.
The importance of having heavier vehicles in this country cannot be over-emphasised, whether we are talking productivity and economics or, more basically still, our ability to be competitive with the other countries of Europe.
Thus any tests which claim to provide independent and unbiased facts either to support or refute this claim on environmental grounds are to be welcomed. Hopefully Mr Rodgers et al will not place too much store in report No VSE 514.
If heavier vehicles are to be prohibited in the UK. then at least let it be on the results of tests which have been carried out in a professional manner and with a high degree of accuracy and fully reported. Will TRRL please supply the answers to these questions • If the sample of vehicles was too small for statistically significant inferences to be drawn, then why state — without qualification — that higher gross vehicle weights will increase the risk of roll-over accidents?
• Why was the final conclusion of the draft report omitted from the final version available to the general public?
• Why was the loadsensing equipment not checked before the braking tests were carried out?
• Why bother to carry out braking tests at all when it was known that there was an air leak in the system? And why was the word "slight" added to "air leakage" in the published report?
• Why were some of the vehicles obtained from the manufacturers and some from commercial hirers? Could it not be argued that the standard of preparation (or lack of it) would be different?
• If comments are to be made regarding vehicles' stability, why was the question of fifth-wheel position not taken into consideration?
I am left with the impression that the comparative testing was carried out in something of a hurry. If this was so, then it is unforgivable and irresponsible. In this context Shakespeare had this to say: ''Go wisely and slow for they stumble that run too fast."'