FOUR-WHEEL BRAKES.

New Zealand Herald, Volume LXVII, Issue 20554, 3 May 1930, Page 12 (Supplement)

FOUR-WHEEL BRAKES.

THE MODERN TENDENCY.

IMPROVEMENTS IN DESIGN.

For some years four-wheel brakes have been accepted as a standard feature of modern cars, but this does not mean that their design has stagnated. On the contrary, braking systems have been consistently improved, so that despite the advances that have occurred year by year in the acceleration and maximum speed >of which cars aro capable, the brakes have kept pace with requirements and aro now,, on the average, better than ever before. 'lt is not always realised that tho deceleratiqji obtainable from a car with a good four-wheel braking system at any speed in the normal range is about six times as great as its acceleration capacity. Consequently it is hardly true to say that tho' design of brakes has beon neglected in favour of concentration upon tho engine. Tho chief fault of early four-wheel braking systems was undue complexity, it boing common practice to employ two or three cross-shafts and a largo number of links and levers in order to convey pedal pressure in an amplified form to tho four-brake cams. Every bearing and pivot, however, contributes its share of friction, so that in such complicated systems, over 50 per cent, of tho pedal pressure might easily be wasted in overcoming mechanical losses. Furthermore, early systems were made more complex by tho use of two or three compensating devices, which have,, since been found to bo largely unnecessary. Modern Improvements.

In & modern system thore is quite commonly only one cross-shaft, connected direct to the pedal by means of a rod and lover. Further levers at the ends of this cross-shaft convoy the effort through pull-rods to the brake-cam spindles, so that thero is only a small number of joints requiring lubrication and producing friction. The mounting of the cross-shaft itself has also been given great attention, because if this part of the system be neglected any distortion of the frame may cause the bearings to bind on the shaft. Consequently, in many cars the ihaft is carried on a cross-mem-ber or on the gearbox, free from the frame. In cases where the cross-shaft is carried by the frame, spherical or ball bearings are employed, which are not affected by frame distortion. Brake adjustments have also been simplified, so that nowadays on a large number of cars ordinary brake maintenance consists simply of giving half a turn now and then to a nut on the pedal pull-rod, placed either under tho bonnet, in front of tho pedal, or at the rear end of tho rod beneath the floorboards. In addition, however, it is usually necessary to provide independent adjusters on the brakes themselves, which can be taken up. at long intervals when, tho central adjuster has reached the end of its range. Another schemo which has recently become quite popular, consists of locating a means of adjustment on each brake, individually between the shoe pivots, leaving the cams and operating gear unaffected. The adjuster moves the pivots apart to compensate for wear. There has also been a marked improvement in brake-lining materials. So that without loss in the coefficient of friction they now provide much greater durability than heretofore. This, of courso, naturally reduces the frequency with which.brake adjustments are required. Emergency Controls.

An important point in which various braking systems differ materially consists of tho way in which provision is made for emergency control by means of the hand' lever. In many cases a transmission brake is employed for this purpose, but such brakes, whilst providing a secure hold for parking purposes, are usually both too small and too fierce to bo used with safety when the car is travelling fast, or when descending a long hill. Another popular scheme is to couple the hand levor independently to separate rear-wheel shoes, placed side by side with those worked by the pedal. This provides a smoother brake than tho transmission type. The third scheme consists of interconnecting the hand lover with the pedaloperated four-wheel brakes, a slotted yoke being used to enable tho controls to operate independently. At; first sight this may seem less safe thun the other systems, but in reality it gives good results, especially when the hand lever operates all four brakes. Risk of failure can be eliminated to all practical purposes if a steady hearing is incorporated at the centre of the cross-shaft. In such a system any rod or lever, even tho crossshaft itself, can fracture without leaving the driver with less than two brakes under his control.

An important development this year is represented by tho number of makers, who have adopted hydraulic brakes, mostly of tho Lockheed type. In this system a master cylinder contains a piston operated by the pedal, and this is connected by pipe-lines to cylinders and plungers on the brakes themselves. Consequently, when tho pedal is depressed pressure is exerted upon tho liquid, which forces the plungers and brake-shoes outwards. The two outstanding advantages of this system are automatic compensation, it being obvious that identical pressures must bo exerted on each pair of shoes, and lack of the losses which ordinarily occur through friction.' In addition there are no mechanical joints and bearings to require maintenance work or to become loose and noisy.