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I I.Meoh.E. SYMPOSIUM ON CONTROL OF VEHICLES
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Page 106
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Braking and Cornering
TEN papers were presented at a symposium on the control of vehicles during braking and cornering held in London on Tuesday by the Automobile Division of the Institution of Mechanical Engineers in collaboration with the Road Research Laboratory. Whilst many of them dealt largely with theory and mathematical evaluation of the behaviour of vehicles during braking and cornering, in most cases the theory was developed into practical applications.
The first two papers dealt with the loss of directional control in accidents. Both were by members of the Road Research Laboratory and were based on information obtained on " on-the-spot " studies carried out by the Laboratory on roads in an area around Langley, Bucks, two years of police reports of accidents on three , trunk roads in Bucks and from one year's record of accidents on MI motorway.
G. Grime dealt with car accidents, whilst
H. 1. H. Starks dealt with commercials.
About 700 accidents were investigated by the R.R.L. and of these 297 involved one or more commercial vehicles. Accidents on the trunk roads—A40, A5 and A4I3—in Bucks, for which police records were consulted, included 420 in wh:ch one or more commercial vehicles were involved, compared with 720 accidents in the same period involving one or more cars. In the motorway investigations, accidents reported by the police were examined and 147 involved commercials compared with 188 involving cars. Only those accidents involving loss of control were covered in the paper_ Striking differences between the accident stories relating to cars and those relating to commercial vehicles were apparent and, broadly speaking, loss of control occurred about half as frequently in accidents involving a commercial vehicle as it did in accidents involving a car, both on the trunk roads and on the motorway. Mr. Starks said it seemed reasonable to suggest that this was because of the lower general standard of braking performance of commercial vehicles and their lower speeds. Data OD the braking performance of new commercials indicated that the brakes were not always capable of locking the wheels on dry roads and to some extent this seemed to be confirmed by the accident stories in which skidding of commercial vehicles was not reported so often as it was for ears.
Head-to-tail Accidents
Certain features of accidents which might be regarded as associated with, or contributing to, loss of control were given. Nearly 30 per cent of the accidents involved a commercial vehicle striking the rear of another vehicle or itself being struck by another vehicle. As well as the obvious explanation that the striking vehicle might have an inadequate braking performance, other ' possible reasons for this were that drivers of commercial and other vehicles might not be allowing sufficient distance between themselves and that the vehicle struck might have inoperative stop lights. Headto-tail collisions occurred in 36 per cent of the accidents involving commercials on The motorway, in about 20 per cent of the accidents investigated by the R.R.L, and in about 5 per cent of the accidents on the Bucks trunk roads.
A noteworthy feature of the motorway accidents was the high proportion in which overturning occurred-63 per cent —and of accidents in which vehicle defects were a contributory factor-32 per cent. Of the latter, two-thirds were tyre failures.
The Importance of Grip Two important steps in the prevention of skidding were to make the adhesion between the tyre and the road as great as possible and to ensure that the available adhesion at the wheels was used to the best advantage. C. G. Giles, of the Road Research Laboratory, considered some of the contributions which could be made towards meeting these requirements by the characteristics of road surfaces, tyres and vehicles in his paper, "Factors Influencing the Friction Between Tyre and Road Under Wet Conditions."
The increased risk of skidding shown up in tests on wet roads arose from the lubricating effect of the water film. The faster the tyre moved over the road surface, the more difficult it was for this effect to be overcome and so skid resistance fell as speed was raised.
Tests made by the Road Research Laboratory showed that under wet conditions the use of tyres with treads of low-resilience rubber reduced braking distances, with locked wheels from speeds of up to 50 m.p.h., by as much as 15 per cent. On really smooth surfaces conventional tyres and the tyres with lowresilience treads gave very similar results.
In his paper, "The Dynamics of Vehicles During Braking ", Professor J. R. Ellis, director, Advanced School of Automobile Engineering, Cranficld, considered the effect of braking during cornering from a theoretical viewpoint.
Equations relating to a private car and art articulated vehicle were produced and the way in which these had been fed to a computer explained. A preliminary test had been carried out in which a vehicle was assumed to commence at a steady forward speed and be acted upon by a " step " (i.e., sudden) steering input. This confirmed that the vehicle was normally stable and, without braking. would proceed on a "steady-state" turn. A further test, adding braking, showed that the yawing and side-slip velocities changed continually and no true steadystate condition was reached.
Two front : rear ratios of braking were considered, 50 : 50 and 65 : 35. A braking force of 75 per cent with the 65 : 35 ratio reduced the lateral force available from the front wheels to such an extent that the vehicle could not turn. On the other hand the 50 : 50 ratio gave rise to more rapid and fluctuating yawing. Two eases of anti-locking " pulsed " braking systems were also considered; those with brakes on all wheels switched together and those with front and rear wheels 'switching alternately. The latter gave a satisfactory degree of stability combined with directional control, whereas the former gave rise to considerably increased yawing and side-slip velocities. It was felt that the completely random effect expected when each wheel had an independent sensing unit promised to be the most satisfactory system from the viewpoint of control.
Also confined almost entirely to theory was the paper presented by A. J. Harris, of the Road Research Laboratory, entitled " On the Analysis of the Possible Motions of a Vehicle ". A method of analysis was described and to demonstrate its application a simplified vehicle was studied. It was shown that if the rear wheels were locked and the velocity was below a certain value, the vehicle was stable but would turn completely round if given an angular velocity of sufficient magnitude. At higher velocities the vehicle was unstable and would always turn round, whereas if the front wheels only were locked it was always stable.
Rear-wheel Locking Dangers
In his paper, "Some Problems of Emergency Braking in Road Vehicles ", R. D. Lister, of the Road Research Laboratory, discussed the behaviour of a vehicle and its subsequent motion when a driver braked in an emergency in an attempt to avoid an accident. Although concerned mainly with private cars, the remarks could apply equally to light vans based on car chassis.
Mr. Lister pointed out that the greatest deviation from its normal path occurred when a vehicle's rear wheels locked before the front wheels, a condition frequently encountered in emergency braking. Although it was often said that the experienced driver could steer out of a rearwheel skid caused by locked rear wheels, laboratory tests and detailed analyses of accident reports did not support this view. On the other hand the inability to steer caused by the locking of the front wheels, assuming that only the front wheels were locked or that locking of these wheels occurred first, could be corrected by release or partial release of the brakes before full control was lost.
It was suggested that it might be worth while to reconsider the design of handbrakes so that they applied to the front wheels to allow for emergencies such as entire loss of braking on the footbrake.
There were, he said, three main ways in which loss of control might be prevented: (a) by appropriate design of the brake distribution; (b) by adopting a special method of applying the brakes, and (c) by the use of an anti-locking device to release the brakes on all or some of the wheels when locking was imminent. The practice of " pumping " the brakes—that is, momentarily releasing and reapplying the brakes as quickly as possible—not only enabled a straight -path to be maintained while braking rapidly, but also achieved shorter stopping distances than leaving the brakes hard on.
The landing-wheels of some aircraft were fitted with devices to prevent them from locking when the pilot applied the brakes. One of these was an inertiaoperated device which released the pressure in the braking system when the wheel slowed down at more than a predetermined rate, and a car fitted with this device was tested by the Road Research Laboratory. The investigations showed that during emergency braking the car did not lose directional stability, and the driver could retain some steering control. Later variations -of the device, combined with a modified braking system developed for use on road vehicles, gave an improved performance.
So far as was known the only car with a braking system which incorporated an anti-locking device controlling all the road wheels, apart from the experiments already referred to, was that being developed by the Harry Ferguson Research Co. Ltd. This car incorporated four-wheel drive, and a transmission and braking system in which a single antilocking unit could easily be incorporated. Mr. Lister went on to describe this system and said that the use of the antilocking device, together with the special transmission, resulted in considerable reductions in braking distances, the reduction • on a slippery surface amounting to 14 per cent (7 ft.), from an initial speed of 30 mph., rising to 35 per cent (38 ft.), from an initial speed of 55 m.p.h. This improvement on wet surfaces could be attributed to the use of the antilocking device, as tests in which the special transmission feature was used without the anti-locking device in operation showed no reduction in distances.
F. D. Hales, in his paper, The Lateral Stability of a Simplified Articulated Vehicle ", described the artic as a vehicle "which is economically sound, but unfortunately not completely vicefree under indifferent operating condi
tions ". Little work of a mainly theoretical nature on the behaviour of artics had been published, but with the application of techniques used in aircraft stability and control theory, it was now possible to look again at the stability and control of artics.
One of two simple cases considered 1351
was with angular motion of the tractive unit (the outfit following a circular path) when it was shown that the tractive unit was potentially statically unstable and that a static instability with increasing speed was possible. Static instability was explained as instability without divergent oscillatory motion. The occurrence of static instability at moderate speeds would depend on the difference between the sideways force coefficients of the tractiveunit front and rear tyres. By reducing sufficiently the rear tyre sideways force coefficient, static instability could always be produced. The loading of the tyres affected this coefficient and so far as they affected the loading. so did wheelbase and coupling position: but it was shown that moving the centre of gravity of the trailer loading forwards and backwards did not affect the incidence of instability and, apart from its effect on the weight disttibution, neither did the coupling position.
Limiting Speed for Artics
If the static instability was only likely to occur at speeds greater.than the normal operating speed of the vehicle, then its occurrence would be of academic interest only. But this was not the case and instability at speeds well within the operating speed range was easily found. It could be argued, said Mr. Hales, that trained personnel who had good coordinating ability were able to control satisfactorily systems which were unstable. But the driver of an arne could spare only part of his attention in keeping his vehicle behaving properly and so was unlikely to be able to control an unstable vehicle. This implied that those artics which tended to become statically unstable must be considered as potentially dangerous since it could not be assumed that skill on the .driver's part would "transform them into docile, well-behaved vehicles ". The static instability was due to the potentially unstable motion of the tractive unit about its attachment point, together with any unstable braking.force contributions. The instability that occurred-could become severe, and under these conditions the possibility of a driver controlling such a vehicle could not be seriously considered.
Wheel locking was not an essential part of the phenomenon of jack-knifing and, therefore, devices which prevented locking would not, in general, solve the jackknifing problem. The deterioration, with increasing speed, in the stability of an artic which had a tendency to jack-knife, was very marked and, therefore, a maximum speed limit for such vehicles was an obvious way of reducing the adverse handling associated with high speeds and, clearly, increased speed limits should follow improved handling characteristics and not the development of more powerful engines.
Jack-knifing in the simplified artic considered need never occur as it was only necessary to meet the requirements detailed in the paper for the vehicle to be stable. The changes in these requirements resulting from the removal of some of the assumptions in the analysis were
changes in detail rather than principle and therefore it might be concluded that the tendency to jack-knife need not be present in articulated vehicles.
In the introduction to his paper, "The Distribution _of Braking on R-oad Vehicles ", J. A. Rouse, research manager of Girling Ltd., stated that any measure which raised the level of deceleration before skidding occurred would make an overall contribution to general safety. Only when the braking distribution was identical to the instantaneous load distribution would the maximum value of this limit be achieved, and as the load distribution varied due to cornering and deceleration 'forces and also to changes in static loading, a variable braking distribution was required if this ideal performance was to be approached. _In considering the braking performance that it was possible to achieve on some typical vehicles—including a 2-ton van, a 7-ton lorry and an 18-total-gross artic—two criteria were examined. These were the maximum deceleration it was possible to achieve without any wheel locking under various loading and road adhesion conditions, and the variation in pedal etfcirt required to produce a given deceleration because of changes in vehicle loading. The use of three -types of valves -produced to improve the utilization of available adhesion and attempts to match the braking and load distributions were discussed. Of these, the one with the major -application for commercial vehicles was the load-conscious valve, which altered the braking distribution according to the load as measured by the deflection of the road springs or the pressure in an air suspension chamber, The author said it was doubtful if any device of this kind had yet been developed which would give a perfect response, and so its main benefit was that -of taking account of the static weight changes, not of the weight changes due to load transfer during braking.
Achieving Good Adhesion
Considering vacuum-assisted hydraulic brakes, by suitably matching the valve characteristic to the spring load/deflection characteristic, it should be possible to achievea very high adhesion utilization at all load distributions on all road surfaces, when such a valve was fitted to each axle. It was shown that it was unlikely that a single valve operating off one axle would provide a really worthwhile improvement in the adhesion utilization of commercial vehicles.
When apportioning valves were fitted to each axle, good adhesion utilization and a satisfactory pedal effort would be obtained but larger brakes would have to be fitted to ensure that severe fade and excessive wear were not encountered. Taking a 7-ton lorry as an example, results showed that a 30 per cent increase in drum and shoe width on the front brakes and a 10 per cent increase on the rear brakes would be required.
The remaining two papers, "Analysis of Steady-State Turning Behaviour of an Automobile" and "The Skid Resistant Properties of Butyl Tyres "• were concerned particularly with cars.