Engineers analyse vehicles, roads
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and tyres as safety factors by Paul Brockington
• "Vehicle and road design for safety" was the subject of a symposium jointly sponsored by the Institution of Mechanical Engineers, Automobile Division, and the Advanced School of Automobile Engineering, which was held on Wednesday and Thursday of last week at Cranfield.
The effects of noise and vibration on a driver's efficiency were dealt with in detail by Mr. J. M. Dickson-Simpson, Transport Press Services, in his paper "Safety and commercial vehicle design". Evidence was accumulating, said Mr. Dickson-Simpson, that quite serious physical disorders could result from protracted exposure to excessive vibration. Some reflex senses were diminished and visual perception was impaired by vibration after a time.
Measures that could be used to reduce the noise of a diesel without undue cost included: the more extensive use of aluminium-alloy (thick wall) castings; locating the cold-air , inlet in a remote non-resonant area and designing the air filter to function as an induction silencer. Others included shrouding the fuel injection pump; employing bonded two-piece or plastics moulded rocker covers, side plates and sump; staggering the fan blades; having two-stage exhaust silencing; discharging the exhaust gases rearwards and using a large-diameter tail pipe; installing the engine on mountings of suitable frequency based on a rigid structure. Potentially, the greatest contribution to engine silencing was, however, appropriate location of the unit combined with efficient sounddamping. The engine could with advantage be installed under the seat pans.
After emphasizing that a proper sounddamping job involved sound-proofing the complete floor of the cab, Mr. DicksonSimpson pointed out that undamped areas and small apertures could transmit an amount of noise disproportionate to their size. The noise level could be doubled by a 2mm gap.
Attention should be given to large panels to prevent attenuation of engine noise through resonance. Resonance of the transmission could be particularly fatiguing; this could be produced by torsional oscillation of the engine/gearbox drive and of the propeller shaft. The latter could be countered by incorporating a torsionalvibration damper in the line. More use could be made of torsion-bar input drives to damp clutch and gearbox cyclic reverberations.
Much needed improvements in suspension systems did not necessarily involve the use f novel or expensive spring layouts, Mr.
ickson-Simpson said. Driver comfort could be influenced by the rear suspension as well as the front. Improving roll stability was a "pressing issue". The coupling of an articulated outfit had a nodding frequency; the high brake-torque reactions of modern vehicles should be given more urgent consideration. Suspension stresses should be spread as widely as possible.
It was not, seemingly, immediately necessary to "depart from the attractive inherent simplicity" of the leaf spring. Reducing internal friction could be as rewarding as lowering the spring rate. This could be achieved by employing a two or three-leaf tapered spring with a reverse-camber bottom leaf combined (possibly) with a slippered tail and a profiled abutment bracket to give progressive shortening of the spring with deflection. It would probably be necessary to employ radius rods to absorb braketorque reaction.
Air suspension fulfilled the requirements of commercial vehicle suspension more comprehensively than any other type, Mr. Dickson-Simpson claimed. The eyentual introduction of independent front suspension was surely inevitable.
When power-assisted steering became virtually universal it could enable high-geared steering to be employed. Assistance should not be applied to the steering for 30deg. of wheel movement on either side of the straight-ahead position. It should increase progressively at a greater angle, but the manual effort should also increase.
Mr. Dickson-Simpson repeated his plea for improved roll stability after stating the need for a better understanding of aerodynamic effects. The problem of handling a container-carrying vehicle in a high-cross wind on an exposed motorway could be critical.
Dealing with vision, Mr. Dickson-Simpson said that improving vision to the nearside by using an extra small mirror was to be preferred to the addition of an extra window, which could be distracting to the driver when the vehicle was in motion. The only practical advantage of a one-piece windscreen was that it provided an overlapping wiper area. Internal reflections at night could be reduced by better concealment of instrument lighting.
The ultimate had not been reached in getting the best distribution of braking force. There had to be proper emphasis on front-wheel braking, and it was noticeable that there was only one make of British vehicle that had the same size of brakes all round. The favourable characteristics of the anti-locking brake might be obtained by reducing or eliminating the braking of one rear wheel. Demonstrations by the Road Research Laboratory had shown the beneficial effect on straight-line stability that this practice could provide.
The need to combat roadstone polishing, dealt with in a paper by Miss B. E. Sabey of the Road Research Laboratory, was also mentioned in a paper "Tyres and the design of vehicles and roads for safety" presented by Mr. V. E. Gough and Mr. B. J. Allbert of the Dunlop Rubber Co. Ltd. A coarse macro-texture layer with a harsh micro-texture aggregate that was sufficiently friable to maintain its harshness and to resist polishing during use was cited by the authors as a desirable surface for high-speed roads.
Dealing with the stability of semi-trailers of the four-wheel-in-line type when manoeuvred on a dry high-friction road surface, the authors emphasized that the trailer could become unstable if the limit of surface friction were approached. If the outfit entered a curve at too high a speed, the outer tyre would carry most of the load normally carried by the four tyres because the spring shackles acted as the pivot points of the oscillating axle when its normal rotational movement was exceeded.
Semi-trailers with such axles had been known to overturn on account of this phenomenon and should be regarded as unsafe when driven hard if a high tyre-toground friction was provided by the road surface.
The outstanding safety problems that remained to be solved in the construction of roads was combating spray, stated Mr.
W. T. F. Austin of Freeman Fox and Partners, and Mr. H. E. C. Nash of the Plessey Co. Ltd., in their paper "Thoughts on vehicle and road design for safety". Although the problem had been the subject of research, new road surfaces did not appear to be satisfactory in this respect.
After stating that a thorough investigation of the spray problem was needed, Mr. Dickson-Simpson said that this included study of the air swirl pattern round the cab. Air-spoilers could also assist in controlling spray—the object being to keep it from the side and rear cab windows, mirrors and particularly the rear lights. Mirrors were likely to be kept much clearer when mounted on forward-projecting arms. In the discussion on Mr. DicksonSimpson's paper, Mr. G. H. Tidbury, senior lecturer of the Advanced School of Automobile Engineering, supported Mr. Dickson-Simpson's contention that an 'improvement in the roll stability of heavy vehicles was an urgent necessity. "When a vehicle is driven in and out of traffic, through an S-bend or round a roundabout," said Mr. Tidbury, "and the cyclic changes in direction correspond to its natural roll frequency, the vehicle is very liable to overturn. There have been relatively frequent cases of articulated outfits overturning caused by this type of resonance."
Overturning could be prevented by increasing roll stiffness, said Mr. Tidbury. The problem was being investigated by the director of research of British Railways in connection with the operation of BR road vehicles. Commenting on the liability of articulated outfits with an oscillating axle trailer to overturn if driven at speed round a curve on a dry surface, Mr. Dick sonSimpson pointed out that the oscillating axle system offered a high resistance to roll "up to the tilt point". The system had practical advantages and the tilting tendency could be minimized.
Replying to a question, Mr. DicksonSimpson said that the exhaust brake was "not worth bothering about" and that talk about power-to-weight ratios had been largely "emotional". Other types of retarder were heavy and costly and the fact that they had to act on the driving wheels was not conducive to safety, particularly in the case of an artic; their use was only justified if the vehicle operated over routes with long descents. Increasing the power-to-weight ratio to 6 bhp /ton or even 8 bhp /ton would do little to raise the ratio of traffic flow on steep hills. There was no need for legislation stipulating a minimum ratio. Ratios were progressively increasing as a result of normal development.
Conclusions regarding retarders, generally similar to those mentioned by Mr. Dickson-Simpson, had been reached by the Ministry of Transport, said Mr. J. W. Furness, superintending engineer of the MoT. Some vehicles had power-to-weight ratios as low as 2.95 to 1; legislating for a minimum ratio of 6 to I could be beneficial.
Prof. F. P. Malschaert, senior research engineer of Ghent University, Belgium, asked whether the 13-ton axle would pose any braking or tyre problems. The width of tyres on a heavy vehicle reduced the spring base to 3ft, whereas the body had a width of 8ft.
In reply, Mr. Dickson-Simpson said that the braking of a 13-ton axle should not be a difficult problem, partly because a lot of weight was transferred forward during highg braking. Tyres with the necessary heat capacity could be produced. Dunlop lowprofile tyres would reduce weight and provide greater clearance.
Many speakers mentioned the merits of an anti-locking brake system. Mr. G. Jones of the British Motor Corporation pointed out that, although use of the system was beneficial at higher speeds, it increased stopping distances at lower speeds. On an icy road it would be no substitute for ice tyres.
The first speaker to mention vehicl maintenance (late in the discussions), Mr. A. V. Williams, principal of the College o Aeronautical and Automobile Engineerin said that it was all-important with regard t safety, particularly in the case of com mercial vehicles. All the improvements i vehicle design that had been mentioned the symposium could be negatived by d fects arising from poor maintenance. M Furness replied that the great problem implementing the testing scheme was t train a sufficient number of the right peopl for the job. A good repairer did not alway make a good inspector.
Dealing with the problem of maintainin clean headlights when continuously spraye with muddy water, Mr. K. J. B. Teesdale, the Ford Motor Co. Ltd., mentioned th the Stockholm branch of Philips Teleindu tri had developed various experiment cleaning systems. [These include a rotatin brush type; a rotating shield, bombarded b rubber balls; a directional air-stream sy tem; a controlled air-stream type; and honeycomb protecting shield. The la named prevents settling of the dirt. —Ed.]
An air of defeatism prevailed during discussion of safety factors involving driv responses. Mr. D. J. Lyons, director of t Road Research Laboratory, observed th "drivers drive to a constant risk" by exploi ing improvements in vehicle and road desig in the form of faster travel. The majority speakers agreed with Mr. Lyons, but the were exceptions. For example, Mr. Gou (of the Dunlop Company) said that only few drivers took full advantage of improv ments. The many continued to drive " before" in greater safety.