ALL cars and motor cycles are required to have two independent brakes, and the driver is legally responsible for their efficient operation. Some motorists are not aware that the police have the right, at any time, to test the braking system of any vehicle, and to prosecute if it is found to be inefficient. Apart from this, the driver has a big responsibility to the rest of the community to keep them in good order for any emergency.
Types of Brakes
It is not possible here to describe more than a few representative designs of brake, but there are general principles which are common to most makes. The most popular systems are the Lockheed Hydraulic, Bendix or Bendix-Cowdrey, and Girling, or some combination of these. In all these systems provision for adjustment against wear is made at the shoe assemblies, not in the operating mechanism, and it is dangerous to jump to the conclusion, when the brakes need attention, that the operating rods or cables need adjustment, as was usually the case in the past. In general, it is true that unsatisfactory brake operation needs nothing more drastic than adjustment, since modern systems give efficient results throughout the life of the brake linings, but they need the right kind of adjustment. Another point to be borne in mind is the difference between what is termed a compensating system and a non-compensating system of .road wheel braking.
In a non-compensating system it is possible, by adjustment, to vary the braking effort of each individual wheel, but with a fully compensating system it is impossible to vary the braking of individual wheels. Thus, in a compensating system, should three wheels lock and the fourth fail to do so, it is not possible to rectify matters by adjustment. The trouble must be due to such causes as oil getting through on to the linings of the faulty brake, shoes seized on their pivots, or some other mechanical trouble at the shoes themselves.
We will review the Lockheed system first, as it will be evident that no adjustment can be carried out on the fluid conduits (there are no rods or cables); also the system is fully compensated, it being impossible to alter the braking at any one wheel in relation to any of the others. The only adjustment to correct lining wear is provided at the brake back plates on the brake shoes themselves, but this will affect the position of the shoes only, in relation to the drums, and not the braking effort. Briefly the system works in the following manner: When the driver depresses the brake pedal, the hydraulic piston is forced into the master cylinder which contains the brake fluid. The fluid in the master cylinder is delivered under pressure, along the pipes leading to the four wheel cylinders, situated one in each wheel drum, between each pair of brake shoes. These wheel cylinders embody double pistons, and each piston moves one brake shoe. The fluid, now under pressure from the master cylinder, enters the wheel cylinders and forces each pair of wheel pistons to extend and apply the brake shoes to the drums. When the brake pedal is released the fluid is forced back into the master cylinder by the retracting springs of the brake shoes, which force the pistons in the wheel cylinders together again. There are certain refinements provided in the form of seals to eliminate seepage, plugs for the replenishment of fluid and valves to ensure effective distribution of pressure in the conduits when the system is in operation. Care must be exercised to maintain the level of the fluid in the master cylinder at all times. If more than an occasional topping-up is required, the cause must be traced and rectified without delay, for a progressive loss of fluid will seriously affect the efficiency of the brakes. At all times there should be about½ in. free movement of the brake pedal before resistance is felt. This is to ensure that the piston in the master cylinder is fully retarded to its stop and uncovers the compensator vent.
If the stroke of the pedal is excessive, so that it nearly fouls the toe board before the brakes are fully applied, worn brake linings are indicated; a condition that can be rectified by re-adjustment of the brake shoes. The shoe adjusters in most common use consist of two snail cams on the brake back plate which when turned bring the shoes into contact with, or away from the drum. Jack up each wheel in turn, and with a suitable spanner rotate the hexagon heads of the snail cams; rotation away from the centre of the wheel brings the shoes nearer the drums, and rotation towards the wheel centre moves the shoes away from the drums. So, with the wheel spinning, rotate the adjusters away from the wheel centre until the wheel stops spinning and then slacken back just sufficiently for the wheel to spin freely again. Each shoe will need adjusting separately. Carry out the adjustment of each pair of shoes similarly at all wheels.
Caution: as the shoe adjustment is not the same in all the Lockheed designs, refer to your instruction book for details. It may consist of an internal adjuster located inside the brake drum, operated by a screwdriver through a slot cut in the back plate, or of a single hand nut protruding through the back plate surface. But whatever device may be provided on the particular vehicle you are concerned with, that will be the only adjustment in the system.
Should a feeling of springiness develop when the brake pedal is depressed, or should a conduit pipe have been disconnected or the fluid reservoir been allowed to run dry, air will have entered the system. In normal conditions this should not happen, but before the system will function properly the air will have to be expelled. This is called bleeding the system, and should be carried out as follows. Keep the supply reservoir full of fluid during the whole operation. Commence with one wheel and attach a drain tube (usually supplied with a suitable connection in the tool kit) to the bleeder valve at the top of the wheel cylinder on the brake back plate. Submerge the other end of the drain tube in some brake fluid in a clean glass jar. Slacken the bleeder screw one turn, and get an assistant to depress the brake pedal quickly, allowing it to return slowly and then a slight pause to refill master cylinder before again depressing. The pedal should be pumped in this way until all air bubbles cease to rise through the fluid in the jar. Ensure that the tube is kept submerged all the time, and until the bleeder valve is again screwed down on the cessation of the air bubbles. The operation should be carried out similarly at all wheels, and the brake fluid reservoir should be replenished from time ‘to time, as necessary, and be left full (I.e. at the proper level).
Only genuine Lockheed brake fluid should be used. Spurious fluid or oil will wreck all the composition cups and seals to the detriment of the whole system.
In this system cables are generally used to operate the brakes, but sometimes rods or hydraulic operation is utilized. When cables are used the system is definitely non-compensating, and when adjusting the system care must be taken to balance the braking effort at all four wheels. The shoe assembly, as, consists of two shoes forced open by an expander which is free to move circumferentially. The adjuster, provided to take up excessive movement of the shoes due to lining wear, is also free to float circumferentially. The only rigid fixture to the back plate is the anchor pin which prevents the shoes from turning with the drum. Consequently, as the shoes are not rigidly fixed to the back plate-in any way, when expanded they move out until contact is made with the drum over practically the whole surface of the linings. Furthermore the rotation of the drum tends to carry the shoes round with it, until the shoes pile up on the fixed anchor pin. This creates a self-wrapping or self-energizing action, the result of which may be increased, by gentle pressure of the foot on the pedal, into a powerful braking effort. This self-energizing effect, sometimes erroneously referred to as servo, should not be confused with systems employing separate servo assistance units. So much for a brief survey of the system. We will now proceed with the vital matter of keeping the system in efficient working order.
As wear occurs on the brake linings, it will be noticed that the stroke of the pedal increases until it nearly fouls the toe board before the brakes become effective. The brakes will also show a tendency to grab due to excessive movement of the shoes. Before this stage is reached, the brakes should be adjusted, and to do this it will be necessary to jack up the vehicle and have all four wheels clear of the ground. Normal wear can be taken up by means of the eccentric adjuster, this is only fitted to cars with brake drums over 5 in. diameter, or, on later models, by a spring centralizer, and a shoe adjuster. There are several different types of shoe adjusters according to make of vehicle, and three are shown in the illustrations*************** . Expand the shoes until a brake drag is felt when turning the wheel by hand. Then slacken off until the wheel spins freely. Proceed similarly, with all the wheels, and it is advisable to give the brake pedal several sharp jabs to settle the shoes, after which closer adjustment can often be made. As the system is non-compensating it will be necessary to equalize the braking effect at all wheels. Apply the handbrake; when cable operation is used the handbrake applies all four brakes, otherwise an assistant will have to apply the footbrake, until the wheels will only just turn by hand. Go round all the brakes and slacken off the adjusters on the brakes that are hardest until all are approximately equal. Generally speaking, if the operating cables have been regularly lubricated during maintenance, nothing more will be required to put the brakes in efficient working order again. The cables should never be altered to accomplish lining wear adjustment.
After continual use, and if for some reason the result of the foregoing adjustments is not satisfactory, it may be necessary to check over the whole system. Jack up all four wheels, and detach the four cables at the centre cross shaft by removing the fork end pins. While the cables are disconnected, make sure that they are free in the outer conduits, that the cross shaft works freely in its bearings, and that when the brakes are off the brake pedal and operating levers return against their stops. Now proceed to centralize the brake shoes in the drums, by slackening off the lock nut and turning the eccentric adjuster in the direction the wheel revolves when car is moving forward, until a slight brake drag is felt. Then slacken off the eccentric slightly until the wheel is just free, and then tighten the lock nut. Next, expand the shoes fully in the drums by screwing out the shoe adjusters. Do not strain the adjusters by over screwing. Now adjust the length of the cables, so that they are just long enough to pull tight, and insert the pins into their fork ends, and cross shaft levers. There should be no slack left in the cables. Split pin the fork pins and tighten the lock nut of the screwed end. Then slacken off all the shoe adjusters until the wheels spin freely. Finally balance the braking effort at all wheels in the manner already described.
In this system the shoe assembly is very similar to the Bendix, making use of the same advantage of self-wrapping effect of the floating shoes. But the cables are superseded by a fully compensated operating linkage whereby an all-square pull up is ensured even if shoe lining wear or adjustment is uneven. See also Figs. 6 and 7.
The only adjustment provided is to take up lining wear at the shoes themselves. The operating linkage should on no account be altered. When the pedal stroke becomes excessive, intimating that adjustment is called for, it is only necessary to use these adjusters on the brake back plate. There is no need to jack up the vehicle for this operation; just screw up the adjusters in a clockwise direction until tight, and then slacken back approximately six notches or three-quarters of a turn. Provided the operating linkage is moving freely and has not been damaged (this is easily done by careless jacking, binding or jamming the linkage) this should be all that is required. If the brakes do not pull up squarely, remember that as it is a fully compensated system it is no use trying to improve matters by using the shoe adjusters. Oil on the brake linings, or some such cause, is the probable trouble and must be rectified before looking farther afield. Check over the nuts securing the axles to the springs and tighten them up if slack; see that the linkage pivots at front and rear axles are moving freely, and lubricate if required. But do not alter the adjustment of the linkage rods.
This system is simplicity itself, in that it is foolproof against neglect or maladjustment. The operating mechanism is fully compensating and does not require maintenance in the form of lubrication. The design is such that a large increase in braking effort is effected at the shoe expanders themselves by means of which the operating linkage is relieved of strain.
The shoes are free to make contact with the drum over practically the whole of the lining surface. Lining wear is provided for by the adjuster on the brake back plate. This is the only adjustment in the system and the operating linkage must never be altered in this respect. When the pedal stroke intimates that adjustment is required, it is only necessary to rotate the adjuster clockwise until resistance of the shoes contacting the drums is felt. The adjusters are then slackened back one notch. This is the only adjustment provided or required, and recourse to jacking up of the vehicle is unnecessary. The handbrake must, of course, be in the OFF position when the adjustments are carried out. If the resultant braking effect is unsatisfactory check over the linkage for damage, oil on brake linings, slack bolts securing springs, etc., for it is of no use altering the shoe adjusters further, or the linkage, in an endeavour to correct uneven braking as the system is fully compensating. See also Figs. 8 and g.
Two entirely independent brakes, kept in efficient working order, are the rule for motor cycles. Generally, the practice is for a handbrake working on the front wheel operated by Bowden cables, and a footbrake operated by rod and lever on the rear wheel. The single expanding shoe with self-wrapping action, working in hub drums, is usually favoured. Adjustment is either by wing nuts or the usual Bowden adjuster, a simple and straightforward design that should present no difficulty.
Any excessive lost motion due to wear or lack of stability should be rectified without delay. Usually, all that is needed is adjustment, but here again it is correct adjustment that counts. Therefore a brief outline of the principles involved will not be amiss, as all designs incorporate the same basic principles. These are as follows: Ackermann angle, castor, camber, and toe-in or toe-out, brief descriptions of which are given below.
When a car is moving along a circular path it will be obvious that the outer wheel will describe a circle of larger radius than the inner wheel. Therefore the front wheels will not be parallel with each other except in the straight ahead position, otherwise the tyres will have to accommodate slip, and excessive wear will inevitably result. To obviate this difficulty recourse is made to linkage based on the Ackermann principle. The front wheels are coupled together by the track rod connected to their respective steering arms. An imaginary line drawn through the king pin and steering knuckle centres, will intersect on the centre-line of the car and rear axle. This linkage thus ensures that the axes of the front wheels intersect at the centre of the turning circle which the car is describing.
All drivers are aware of the tendency for a car automatically to travel in a straight-ahead path. This self-centring is brought about by incorporating the action of a castor wheel. Take a trolley wagon fitted with castor wheels and push it along; all the wheels will swivel round until they trail in line and the wagon will tend to travel in a straight line. This is accomplished on both motor cycles understood that the Ackermann angle will alter with the wheelbasc, castor, camber and toe-in or toe-out, according to design; and that they are all inter-related. Therefore it is impossible to give any general rules. Their correct maintenance and adjustment must be made according to the maker’s instruc- and cars by making the centre-line of the steering pin strike the ground at a point ahead of the point of contact of the wheel with the ground.
It will be noticed that on most cars the front wheels are not vertical but that they cant outwards, being wider apart at the top than at the bottom. This is done in an endeavour to get as near as possible to what is termed centre-point steering, and the resultant light steering.
Toe-in or Toe-Out
There is always a tendency for the front wheels to splay outwards. This is increased as the camber angle is increased due to the inclined axis of the wheel. To counteract this tendency and eliminate scruffing and ‘wear on the tyres, the front wheels are given toe-in. Even if the wheels are vertical without camber, as in some commercial vehicles, a certain amount of toe-in is necessary.
Where, however, the front wheels are used as driving wheels as in front-wheel-drive vehicles, this splaying tendency is largely counteracted and no toe-in may be required.
With some independent suspension systems the designers recommend toe-out. From the foregoing it will be clearlv tions, so consult your instruction manual. Also, refrain from driving the car over obstacles at speed, hitting the curb with the front wheels, and other practices likely to derange the steering mechanism. Regarding the steering box itself, there is a general tendency in design which seems to favour the use of large wearing areas and elimination of adjustments for wear. Non-adjustable steering knuckle joints have also become increasingly popular. Consequently, when excessive lost motion becomes noticeable, there is nothing to be done but to get the faulty part renewed immediately.
Alternatively, where the spring knuckles are adjustable, these should be adjusted to eliminate wear whenever it develops. This is usually .lone by removing a split pin and tightening up a screwed cup end. In this type of adjustment, screw in the cup as tightly as possible and then just slacken back sufficiently to prevent binding. Do not forget to replace the split pin, and to check all the joints similarly.
Wear can develop at the bearings and between the operating mechanism in the box. Provision may be provided at the head-stock, or at the box, to eliminate excessive end-float of the column. Wear between the screw element and drop arm may occur in the box itself, and this may be counteracted by the use of thin metal packing washers, or an eccentric bush or screw and lock nut. It is impossible to specify these in detail as the designs vary enormously. However, reference to the maker’s official instruction book should give the reader all the necessary details which will apply to his particular make of vehicle.