SAFETY DEVICES.

Auckland Star, Volume LXVII, Issue 297, 15 December 1936, Page 18

SAFETY DEVICES.

NOTABLE ADVANCES. PERFORMANCES ON ROAD. IMPROVED BRAKE SYSTEMS. With the evolution of the modern motor car lias come a series of contributions toward greater safety in performances on the road. One of the most notable advances lias been with

tyres; there is little resemblance between the tyres of to-day and those of the knobby tread that' characterised early vehicles. The modern low-pressure type gives a greater area in contact with the road, has greater gripping power and makes for increased stability. The low centre of gravity of the car also gives greater freedom for cornering.

A n <111) or important safety-first improvement is tlie use of shutter-proof "lass for windscreens, lamps and in some oases windows. The chassis of cars lias been considerably strengthened, steering control is definitely better, and in other details prescnt-dav models have been improved almost out of recognition. foremost of all have been the developments in braking systems, and tlie introduction of fonr-wheel broking revolutionised design. Some braking systems can now give up to HO per cent efTicienev.,

i The value of the innovation in brakl ing is illustrated by the fact that, [where as with the earlier two-wheel braking operation it took 43.3 ft, for a car travelling at a. speed of 20 miles an hour to stop, with four-wheel brakes it takes 22.2 ft. By scientific study and treatment, linings have been improved out of all sight. The average careful driver, provided he understands what 1 operation of the brakes can actually | achieve, and provided he pays regular I attention to the maintenance of the ij braking system, need have no fear of an accident.

The motorist need not worry himself with a mass of mathematical calculations; all he need remember is that the distance travelled by a car per second is approximately one and a half times the number of miles being registered by the speedometer. Thus a car travelling at 20 miles an hour would cover .'iOft in one second. If the idriver remembers this and adjusts bis speed accordingly when passing intersections or places where vision is obscured' he will be perfectly safe. He must also remember, of course, that the average time for mental reaction of motorists is one second, that is. one second will elapse, before the average driver can transfer his foot from the accelerator and apply the brakes. Pulling Up Distances. Brake performances depend upon the percentage of efficiency of the braking system, the condition of the tyres and the degree of cohesion of friction between the tyre and road surfaces. Statistics relating to distances that will be travelled before a ear can be stopped have been carefully prepared by overseas bodies and have been confirmed by the Bureau of Standards, Washington. The table shows that the average car travelling at a speed of 30 miles an hour will require 104.3 ft before it can be brought to a halt. Tn the second needed for mental reaction it will travel 44ft (on the basis of time and a half), and after the brakes are applied it will travel 00.3 ft, giving the total of 104.3 ft. There is a separate statistical calculation for the time taken for cars to be halted, and in this illustration an efficiency of 50 per cent has been taken as a basis.' The operation of the brakes, of course, merely stops the rotation of the wheels, and the ear will skid forward according to the resistance of the tyres and the type of road surface. All this sounds perhaps a little confusing, but it serves to drive home the fact that if a driver has common sense and is in possession of a .modern vehicle embracing the latest ideas in safety-first devices and construction he runs little chance of an accident.

The art of gentle and progressive braking is most quickly acquired if the driver can picture himself, or herself, to be a part of the braking system. No matter how cleverly the brakes are designed their ultimate effect is largely dependent upon the way in which the driver chooses to load the brake pedal. This loading process is carried out by straightening the right leg, the body being braced against the back of the seat in order to take the reaction. The reason why a person of modest physique can exert such a high pedal pressure in an emergency is found in the fact that the knee works like a toggle lever. Quite moderate muscular effort, exerted in straightening the leg. will produce a big load upon the brake pedal.

Any ordinary person can quite comfortably exert 1501b in this way. provided that the driving seat is properly adjusted in relation to the brake pedal. This adjustment has also a vital bearing on the progressive control of the brakes, it being difficult to graduate the leverage of the leg if the distance from the seat to the pedal is either too great or too short. Preferably, matters should be so arranged that when the pedal is fully depressed the leg is nearly, but not quite, straightened out.

Art of Braking. This important preliminary having been settled, the next point to consider is the way in which tlie pedal can be best operated. It is a great mistake even in an emergency, suddenly to apply full load. Precipitate braking is much more likely to lock one of the wheels than is progressive braking, and as soon as a tyre substitutes sliding for rolling it loses all sense of direction. Even on a dry road the best results are obtained by keeping the wheels rolling; on a slippery surface a. locked wheel may. of course, have disastrous efleets. Special care should also be exercised if the road is rough or rippled, because wheel bounce then results in loss of adhesion.

When the brakes are properly adjusted quite a small movement of the pedal should suffice to take up any slack in the system and to move the brake shoes outward into contact with the rotating drums. Anyone who gives a little thought to this matter, and is willing to practise, will find it quite easy to "feel" the point at which shoes and drums come into contact. The combined process of shifting the foot from the accelerator, and then bringing the brake shoes up to contact point, can be carried out just as rapidly as conditions demand. From this point onwards the driver's aim should be to apply a progressively increasing load to the brake pedal. In other words, the shoes should be squeezed against the drums with increasing force; on no account should they be thrown on with a jerk.

Only by making a habit of applying the brakes in this way is it possible to avoid the use of panic pressure in an emergency. The driver who has seriously cultivated sensitive braking will find himself able to slop his car on a slippery road quite safely under conditions which would otherwise have resulted in sliding and skidding.

Another important point is to accustom oneself to the way in which the brakes respond to various pedal pressures. Some systems are nicely proportioned. so that doubling the pressure produces double the retardation. Tn others, the driver has to accustom himself to the fact that his brakes are not proportionate. For example, it may be that 801b on the pedal will afford quite a normal rate of retardation, whereas a force of 1001b is sufficient to lock the rear wheels. There are big differences in pedal pressure between one ear and another, so that special caution is advisable if one is driving a friend's car with which one is not familiar. Necessary Adjustments. Finally, a few words will not be out of place regarding brake maintenance. There must, of course, be a working clearance between the linings and drums (so that the car may run freely), and this, together with any play or stretch in the operating gear, necessitates an inch or so of pedal movement before the linings make contact. As wear occurs this initial movement increases proportionately, so that only by regular adjustment is it possible to retain the normal range of pedal movement.

Adjustments are also necessary from time to time in order to secure equal braking from the four wheels. If it, is found difficult to obtain this result it may lie that oil has leaked into one of the drums. When adjustments are being made the hand brake should on no account be neglected. It is essential that it should work properly when the car is parked on a gradient and. furthermore, it should provide a practical alternative to the pedal should failure occur in the operating gear.

Sensitive brake control is not possible, even to the expert, if there is an excessive amount of friction in the gear between the pedal and the brake shoes. For this reason it is unwise to neglect the lubrication of minor parts, such as brake-cam spindles, cross-shaft bearings. ete. In hydraulic systems the pressure in a fluid replaces the action of cables, rods and levers. An essential point of maintenance is regular inspection of the reservoir and its replenishment with the right fluid. Sometimes brake shoes show a tendency to oral) and lock the wheel, no matter how careful the driver has been in the progressive application of pedal pressure. As a rule this trouble is best tackled at a properly equipped service depot. Grab can be caused by any one of many derangements. Amongst these are scored drums, badly worn linings, the use of unsuitable linings and looseness in the bolts which secure the axles to the spring anchorplates.