How high trailers increase roll-over risk
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Safety conference hears of critical factors in artic stability
• Roll-over of an articulated vehicle will occur at only 15 mph on a 6011 radius bend when the centre of gravity of the semi-trailer is at the not unusual height of 911 from the ground. The remarkable sensitivity of the artic's stability to trailer cg height was pointed out by D. W. G. Miller and N. F. Barter in a paper given at an IMechE conference on "Vehicle safety legislation — its engineering and social implications" held at the Cranfield Institute of Technology this week.
The paper, entitled "Roll-over of articulated vehicles", described theoretical and practical test work carried out by MIRA staff and drew several definite conclusions on the roll-over problem.
, The most important single factor governing the roll-over limit proved to be the height of the semi-trailer cg. The effective tyre track, particularly of the semi-trailer, is also very important but it was pointed out there is little scope for increasing this dimension within, the allowable overall vehicle width. It was not clear from the work done whether the wide-single type of tyre gives a wider effective track than conventional twin tyres.
An increase in roll stiffness will increase the roll-over limit and this can be achieved by increasing the stiffness of suspension and tyres, and the torsional stiffness of the fifth wheel. The point was made that an increase of only one of these stiffnesses may not, in general, be beneficial, and suitable balance of stiffness throughout the vehicle is very important. Road conditions, it was said, also contributed to the roll-over problem and part of the solution would seem to lie in the modification of, for example, the layout of roundabouts where a number of roll-over accidents have occurred. The easing of the entry and exit to a roundabout and a reduction in the size of the centre island would allow a greater turning radius, thus reducing the danger of roll-over.
Lack of warning The authors also thought that truck drivers themselves might not appreciate just how slowly laden vehicles should be driven round bends and that a campaign of driver education in this direction might be beneficial. During the tests the speeds which drivers estimated as being quite safe were often, in fact, very close to the vehicle's overturning limit and drivers who had been involved in roll-overs were typically surprised at the lack of warning they had had.
Most of the tilt tests carried out were done with the vehicle in the dead-ahead position but roll-over often occurs at low speed where the angle of articulation between tractive unit and semi-trailer is high and the authors point out that at high articulation angles the roll restraint provided by the fifth wheel is small (indeed it is probably zero at 90 deg of articulation).. The effect of this is that for a trailer cg height of 811 and a fifth-wheel height of 411 a considerable overturning couple is exerted by the tractive unit if it is
suddenly accelerated. A tilt test carried out at an articulation angle of 35 deg gave almost the same result as one with zero articulation, so reducing roll restraint with articulation does not seem to present a problem in normal driving situations.
The work presented dealt mainly with static tests and the authors suggest that further work should cover dynamic tests both at constant and varying speeds for although the work done was a good guide to the roll limit these dynamic effects probably lower it further.
Comparing theory and practice
J. R. Ellis, the Director of the School of Automotive Studies at Cranfield, and C. G. Shapeley in their paper "Analysis and testing of the articulated semi-trailer vehicle" describe a road-going model of an articulated vehicle which can be used to check mathematical predications of artic performance. The model, which is based on a Ford Transit, has known parameters such as mass, movements of inertia, spring brakes and tyre characteristics and thus can be used to check theoretical results arrived at from a study of a mathematical model.
The authors have now shown that the results of the road-going model and the mathematical model do agree and the way is now clear for work to proceed on more complex handling manoeuvres such as braking or accelerating in a turn.
The big advantage of using mathematical model is that once it is proven, as this one is, many vehicle configurations and loadings can be quickly studied with the aid of a computer and potentially dangerous situations which are beyond the skill of a test driver can be fully explored very easily without risk.
. In a paper entitled "Safety regulations for public service vehicles" N. Danskin gave in some detail the multitude of conflicting regulations and requirements of. various countries. Public service vehicles had to be designed with a view to accident avoidance, occupant protection and increasingly for a growing population of elderly people, said Mr Danskin. In 1971 over lflim people in the UK, or more than 20 per cent of the population, were over the age of ,66 and many of these were the travelling public. While much information existed on healthy individuals (mainly derived for car design) little relevant data were available on this elderly group.
Bus seats cramped The divergence of regulations was wide and some did not adequately reflect the true need. For example, Mr Danskin pointed out that the minimum seat spacing legally permissible in the UK did not comfortably accommodate a large proportion of the travelling public, for in a recent experiment where senior citizens were given the choice between minimally spaced seats and the proposed EEC spaced seats, 85 per cent preferred the wider spacing.
Mr Danskin strongly attacked the legal jargon in which so many regulations are couched, even the recently formulated EEC regulations he thought were defined subjectively, and difficult to understand and enforce. Entry into the EEC had compounded the problem, for legislation made under the auspices of the Council of Ministers was identified as directives, decisions orders, statutes or recommendations. The language used in the requirements themselves was often the source of endless confusion, said Mr Danskin, and was frequently misleading and ambiguous.
What is needed, said Mr Danskin, were consistently justified requirements formulated in conjunction with industry and based upon adequate research data, accident statistics and cost/benefit studies.
In the UK, new designs of psv had to take into account and include the C and U regulations, the Certificate of Fitness regulations and the many non-legal Codes of Practice. Mr Danskin approved of the code of practice as it appeared to be a simple method of introducing a minimum level of design or performance without the difficulties of processing a regulation through Parliament.
With the problems currently facing psv manufacturers in having to satisfy both domestic and international legislation the variation in requirements left only a narrow band within which to manoeuvre and thus few manufacturers would welcome the additional burden of particular operator requirements.
The unloved lorry K. J. B. Teesdale, talking on the "Social effects of safety legislation", was mainly concerned with the public attitude to the private car but in a survey of 815 adults at 53 random sample points throughout Great Britain it came to light that considerably fewer people regard the lorry as a benefit to society, than they did the car and conversely that more people regarded it as causing more harm to the society than the car.
The accompanying list of the results shows that the lorry is considered only slightly more beneficial to the community than alcohol and much less beneficial than television!