Anti-lock Device
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Would Aid Brake-torque Distribution
If the hydraulically braked wheels of one axle are prevented from skidding, extra braking force is transferred to the remaining wheels during the critical phase only THE recent symposium at the Institution of Mechanical Engineers on the subject of vehicle control has drawn attention to a number of useful developments—but seems also to have given rise in some quarters to a misunderstanding of the function of certain units, more specifically the behaviour of an anti-lock device. In particular, there is controversy regarding the function of such a device in relation to brake-torque distribution on heavy commercial vehicles, including articulated outfits, subject to large variations of loading and in the height of the centre of gravity of the load.
Before dealing in greater detail with the potential advantages of an anti-lock device, it is appropriate to review, briefly, the types of skidding caused by braking and the characteristics of variable ratio systems fitted to cars.
All the wheels of a normal fourwheeled vehicle are, under ordinary circumstances, solidly coupled together by virtue of the fact that the tyres are in non-skid contact with the road surface, and so long as locking of a wheel or wheels does not occur, the distribution of braking between front and rear axles can be arranged to suit average conditions with regard to lining wear. For example. if the front brake drums are 4 in. wide. the rear drums have a width of 6 in. and both are of the same diameter, it is good practice to provide a rear brake torque 50 per cent in excess of the front torque to promote even lining wear. As this arrangement fits in reasonably well with the space usually available for the drums and with the static load distribution, it is one that is commonly used by vehicle manufacturers.
Instability
If the vehicle is running on a dry road with a correctly distributed load and is not violently braked, this brake-torque distribution is also completely satisfactory in terms of brake performance. But heavy brake application if the vehicle is laden, or lighter application if the vehicle is unladen, will lead to instability, particularly on a wet or icy surface, because the rear wheels tend to lock and the braking force at the front axle. is insufficient to lock the front wheels.
As indicated by these comments, there is less likelihood of front-wheel locking than rear-wheel locking, a high rate of retardation resulting in a substantial proportion of the load being transferred from the rear to the front wheels. Locking of the rear wheels normally leads only to an unpredictable type of skid in which the rear wheels tend to swing round to the front, whilst front-wheel locking gives straight-line skidding.
• The danger associated with rear-wheel locking has created a demand for some form of variable-ratio brake-distribution device.
Of the two general types of device the first is exemplified by the hydraulic system employed by Citroen Cars Ltd. in which two brake valves are used connected by a balance beam, the point of application of the operating mechanism being regulated by the load on the rear suspension. The pressure on each brake valve varies according to load distribution between the front and rear axles and there is no reduction in the total braking force. This system is satisfactory for a reasonably heavy car but if it were applied to a truck with an axle load variation of 3 or 4 to 1 it would have obvious limitations.
Anti-locking System
Alternatively, a restriction device can be incorporated in the rear brake line which may take the form of a simple shut-off valve which limits the hydraulic pressure on the rear brakes to an arbitrary maximum, a reduction valve controlled by the weight on the 'vat' axle or a pendulum-controlled valve which reduces brake line pressure (or cuts off the fluid) at a certain rate of retardation. The theoretically ideal variable-ratio device would be sensitive to the coefficient of friction of the road surface. the transient coefficient of friction of the brake linings, the height of the centre of gravity and the centrifugal force acting on the vehicle. When the sensing unit had taken these factors into account and had proportioned the braking force uceordingly, the effect on brake torque distribution would, however, result in a highly disproportionate rate of wear between the front and rear brake linings. It would he ideal with regard to handling of the vehicle in emergency brake applications. but otherwise would have nothing to commend it.
It i of importance and related signifi
cance that an anti-locking system (such as the Lockheed Antilok device applied experimentally to the rear wheels of a number of car chassis) provides variableratio distribution of braking force only when it is needed. Under normal circumstances it does not interfere with brake-torque distribution as planned by the vehicle designer to give even wear of the front and rear brake linings with average loading, but it becomes operational at the first indication of wheel locking. During the critical phase, it reduces rear brake torque to an acceptable maximum and automatically increases the proportion of the total torque applied to the front brakes. If, for example, the designed front-rear brake torque ratio is 40-60. braking force will continue to be distributed according to this ratio under all conditions other than a circumstance tending to produce rearwheel locking. In a typical case, the device would automatically adjust the ratio to. say, 65-35 during emergency braking.
The Articulated Outfit The articulated outfit requires special consideration because braking is distributed to three or more axles and also individual locking of the rear wheels of the tractive unit is extremely critical with regard to jack-knifing. Although locking of the trailer wheels is less critical in terms of vehicle control, it represents a serious instability factor, and it is notable that an anti-lock device would enable maximum braking to be employed (possibly by a separate cab control) without danger of skidding. As with a rigid vehicle, front-wheel locking is " acceptable " because the wheels skid in a straight line, but the front wheels of virtually all tractive units built are underbraked. Could the danger of jack-knifing he eliminated by fitting the rear tractive unit axle and the trailer axle with an anti-locking device? To claim that it would be eliminated entirely might be regarded as unduly optimistic, but it would be a near certainty if designers of the vehicle braking system exploited the potential of the anti-locking device to the full. Unquestionably the overall braking efficiency of the outfit would he substantially increased.