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Black Marks for Heavies
Page 107
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Low Standard of Headlamp Aiming : Large Variations in Efficiency of Braking : Testing Vindicated
ASURVEY, made in 1952 by the Road Research Laboratory, of the aim and intensity of headlights on 800 vehicles, half of which were commercial, showed that the standard of aim on the latter was extremely poor, and generally, worse than that for cars.
Defective lamps, diffuse beams and beams aimed more than 3° from the correct position amounted to half the total. A tenth of the driving beams had deteriorated so badly or were found to be so incorrectly aimed that they bad intensities of less than 1,000 candelas, instead of at least 10,000 horizontally.
These facts were mentioned by Mr. G. Grime and Mr. R. D. Lister in a paper on the inspection of vehicles for roadworthiness presented to the automobile division of the Institution of Mechanical Engineers in London on Tuesday. In another survey of a random sample of commercial vehicles on the A45 route near Cambridge, braking performances had been measured.
Two points were particularly noteworthy. The first was the large scatter in the results, the worst 5 per cent. requiring three times greater distance to stop than the best 5 per cent. The second was the large lag in the operation of some brakes, particularly in the larger vehicles. The performance of the best 5 per cent. corresponded closely with that of new vehicles.
. Tests Raise Efficiency
There appeared to be little doubt that vehicle testing resulted in improvements in the standard of maintenance of the items inspected, although this might be more effective for some than others. In this country it was found, for instance, that taxicabs at their annual brake inspection had brakes in better condition than those of the average car, whilst observations of the prevalence of glare from headlamps in the United States had shown that it was significantly less in certain States where vehicle inspection was carried out than in others where none took place.
It was .generally agreed that tests of the brakes, headlights and steering of vehicles ought to be included in any scheme of inspection, and it seemed to be assumed—perhaps correctly—that these were the items which had the most pronounced influence on safety.
Results of tests at Hendon suggested that other items should be included. These were tyres, rear and stop lights, direction indicators, windscreen wipers, suspension systems and exhaust systems, whilst a general under-vehicle examination ought also to he made.
There was accident evidence for only two of these items—tyres and rear lights —which figured in police reports in proportions comparable with brakes, steering and headlights. A good case could, however, be made for all the others on the ground of common sense.
Equipment for determining alignment and checking the various wheel angles of the steering was available, but was more suitable for the garage or repair shop than for vehicle-inspection procedure. For a rapid check to determine excessive play caused by wear, a visual check by an "experienced observer was usually adopted.
Obligatory side and rear lights, stop lights and indicators were checked by visual inspection, no equipment being manufactured with which the intensity of rear lights could quickly be verified. An experimental arrangement had, however, been tried which would probably be satisfactory for inspection stations.
It was possible to check the aim of the driving beam of the headlamps of a vehicle with sufficient accuracy without the aid of specially designed equipment, but the operation could be made easier and more rapid by the use of testing equipment.
Standards to be used when testing was first introduced might have to be determined by the capacity of repairers to deal with the volume of adjustment and repair work caused by the test procedure and by the test equipment available.
Regarding play in the steering, a comparison of the Hendon results with those obtained in the second Slough inspection suggested that the Hendon examiners judged the backlash to be excessive when it exceeded 4 in. at the steering-wheel rim. In Pennsylvanian examinations, backlash had not to exceed 2* in. However, there was no evidence to relate either figure to the safety of the vehicle,and the best that could be done was to agree an arbitrary figure by consultation between vehicle experts.
The specification of a minimum rate of retardation alone, as with cars, was not entirely satisfactory for heavy commercial vehicles, in which there might be a lag in the operation of the brake. It would be more satisfactory to specify and test equivalent braking distances.
Load conditions also needed to be specified for medium-weight and heavy commercial vehicles, because there was so much difference between their laden ,and unladen weights. Hand brakes ought at least to hold a vehicle on the steepest incline likely to be encountered, and assuming this to be 1 in 4, gave a minimum retardation of 0.25g.
Standard for Headlamps The criterion for headlamps ought to be that with the most unfavourable combination of misaim and low intensity, the "seeing distance" should be above a specified minimum. There were strong reasons, however, for tightening up the standard of aim as far as possible and beyond anything that could be directly justified on the ground of safety.
These were, first, the substantial increase in the comfort of night driving because of the reduction in dazzle, and, second, the possibility of using beams of improved design, but which required more critical aiming. The border line between adequate and inadequate intensity for rear lights was not known and the logical approach was to specify an intensity which was known with some certainty from accident studies and experiment to be adequate, even if the standard might perhaps be higher than was strictly necessary.