FOUR-WHEEL BRAKES

Wanganui Chronicle, Volume 73, Issue 115, 17 May 1930, Page 12

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 are capable, the brakes have kept, pace with requirements and are now, on the average, better than ever before. It is not always realised that the deceleration obtainable from a car with a good fourwheel 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 the design of brakes has been neglected in favour of concentration upon the engine.

The chief fault of early four-wheel braking systems was undue complexity, it being common practice to employ two or three cross-shafts and a large number of links and levers in order to convey pedal pressure in an amplified form to the four-brake cams. Every bearing and pivot, however, contributes its share of friction, so that in such complicated systems, over 50 per cent, of the pedal pressure might easily be wasted in overcoming mechanical losses. Furthermore, early systems were made more complex by the use of two or three compensating devices, which have since been found to be largely unnecessary. Modem Improvements In a modern system there is quite commonly only one cross-shaft, connected direct to the pedal by means of a rod and lever. Further levers at the ends of this cross-shaft convey the effort through pull-rods to the brake-cam spindles, so that there 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 shaft is carried on a cross-member 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 alsq 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 the bonnet, in front of the pedal, or at the rear end of the 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 the central adjuster has reached the end of its range. Another scheme 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 co-efficient of friction they now provide much greater durability than heretofore. This, of course, naturally reduces the frequency with which brake adjustments are required. Emergency Controls An important point in which various braking systems differ materially consists of the 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 be used with safety when the car is travelling fast, or when descending a long hill. Another popular scheme is to couple the hand lever independently to separate rear-wheel shoes, placed side by side with those worked by the pedal. This provides a smoother brake than the transmission type. The third scheme consists of interconnecting the hand lever with the pedal -operated four-wheel brakes, a slotted yoke being used to enable the controls to operate independently. At first sight this may seem less safe than the other systems, but in reality it gives good results, especially when the hand level operates all four brakes. Risk of failure can be eliminated to all practical purposes if a steady bearing is incorporated at the centre of the crossshaft. In such a system any rod or lever, even the cross-shaft itself, can fracture without leaving the driver with less than two brakes under his control.

An important development this year is represented by the number of makers, who have adopted hydraulic brakes, mostly of the Lockhead 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 the pedal is depressed pressure is exerted upon the 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 be 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 ai\d noisy