Development Of Braking Systems
In these safety conscious days and car manufacturers touting their passive and active safety systems credentials, it remains that the greatest accident preventative ever developed are your car's brakes. How many times in the past few months have your brakes - slammed on in an emergency - saved your car from severe damage and you from injury or death? But when you stamp on that middle pedal do you know what is happening? Usually when you think of a brake system, an assortment of parts including the foot pedal, hydraulic cylinders and the disc/ drum units themselves, come to mind.
But these are only the basic means by which stopping is performed. Braking in its entirety is largely influenced by other factors including the tyres and suspension. The finest brakes in the world are no better than the worst when the tyres are not gripping the road. And the tyres' adhesion is often de-pendent on the suspension operating properly. Braking is also affected by weight distribution and transfer which, in turn, are subject to the deceleration rate, centre of gravity height, and wheelbase length.
One point sometimes not understood is the brakes are the opposite of the engine in purpose and operation. The engine provides go, the brakes provide stop. Elementary. But where the engine converts heat into energy, the brakes convert energy into heat. The energy produced (via heat) in the engine serves to drive the vehicle. And it is the brakes conversion and dissipation of that energy (via heat) that slows the vehicle. An engine operates efficiently only when its temperature is held at a particular level.
If that temperature is exceeded the engine overheats and mechanical damage must follow. To avoid this, the engine incorporates a
cooling system which removes excess heat and transfers it to the air. Brakes also dissipate their heat into the air. But if heat is produced faster than the brake can shed it, overheating occurs and may be accompanied by a phenomenon known as fade. When this happens the driver must apply heavier pressure than normal to the pedal for a given amount of braking. Extra pedal travel must also result.
Understanding Brake Fade
With extreme fade, the driver may exert all the pressure he can muster, and the pedal may be near the floor, but the brakes operate poorly until they have a chance to cool down. Fade is generally due to the heat being so great that friction between the lining and disc/drum is reduced. It may also be caused by Incredible nose-dive and all wheels locked-up combine to provide very little stopping power. A disc brake is not necessarily superior to a drum brake for normal braking. Main advantage of the disc is that it has greater heat capacity (more resistance to fade) than a drum giving comparable stopping power.
The reasons for this are that the disc is better cooled, expands towards the linings, and has no self-assisting characteristics. The shoes of drum brakes are usually either leading or trailing, according to the way in which they are designed. A leading shoe has self-assistance because it is automatically pulled harder against the drum by the drum's rotation. Thus, less pedal pressure is needed to obtain a given application pressure between the lining and drum.
A trailing shoe lacks self-assistance, meaning relatively heavier pedal pressure is needed to obtain a given application pressure. One benefit of non-assistance is that trailing shoes, and disc systems, are less susceptible to fade. Also, the braking action is more progressive and has better feel, even if a separate servo or booster unit is  required to prevent pedal pressures being unduly high. Worse in in this respect are drum systems which have not only two leading or self-servo shoes but also include a booster. The driver must develop a very fine touch on the pedal to avoid excessive applications.
The Servo-Assisted System
A servo-assisted system does nothing to improve the effectiveness of the brakes or braking. Its ] purpose is simply to multiply pedal pressure and, like power steering, reduce driver effort. Although disc brakes have high resistance to fade, they are not immune to the condition. Given unsuitable pads, for example, the coefficient of friction between lining and disc may deteriorate as very high temperatures are attained, and fade follows. Front brakes have to work hardest because weight is transferred forward during deceleration.
The extent of the weight transfer depends on three factors - the deceleration rate involved, the height of the centre of gravity, and length of the wheelbase. The brakes must be chosen to allow for the difference in weight distribution. Imagine we have a vehicle with exactly the same brakes front and rear, and 50/50 weight distribution. If the brakes are applied gently, so that negligible weight transfer occurs, it would be feasible to have the brakes at each end sharing their duties almost equally.
But with the brakes applied hard, weight distribution may change to 80/20 front/rear meaning the front tyres are carrying four times more weight, and have four times as much grip, as the rear. Now, if effort was still distributed equally between the front and rear brakes under this condition, the pressure applied to the rear units would clearly be grossly excessive. Friction within the brakes would exceed that available between the rear tyres and road, so the rear wheels would lock. Locked wheels don't stop the car better than slowly turning ones.
A wheel achieves maximum retardation when brake friction is just slightly less than the grip existing between tyre and road; so the wheel is rolling very slowly and has almost, but not quite, stopped. Also, a rolling tyre naturally travels in the general direction it is pointing. But a sliding tyre is ready, willing and able to travel in any direction with equal ease.
Consequently, when the rear brakes lock the tail accelerates. Unless the car is travelling perfectly straight and the rear wheels are not jostled sideways, the tail will swing out to one side and may actually overtake the slower-travelling front-end. Should the front wheels lock, the car tends to continue in 9 straight line. The front-end accelerates, towing the rear-end where the rolling wheels prefer travelling straight. The front wheels when locked provide no steering regardless of where they are pointing.
In earlier days car makers faced a difficult problem with changing weight distribution because the ratio of braking between front and rear wheels was fixed by the choice of brake size and type at each. But weight distribution and the required braking effort are not fixed. This conflict of interests was most marked on front wheel drive models as they carried the greatest proportion of weight on the front wheels. In this respect, front-engine, rear wheel drive cars have an advantage of more equitable weight distribution. Rear-engined models are better again since weight is biased towards the tail to begin with, and the addition of passengers and luggage has least effect on overall distribution.
The Maxaret Anti-Lock Brake System
It was largely the increasing popularity of front wheel drive that spurred the development of variable-ratio braking. Mounted on the chassis, some such devices had a shuttle or valve which was controlled by a spring or inertia to prevent hydraulic pressure to the rear brakes exceeding a pre-determined limit during hard braking. Others had the valve controlled by a link which connected to the suspension so pressure limitation varied according to the height the tail rises above the ground.
The more the tail rose, as during heavy braking, the more the valve restricted the supply to the rear brakes. Regardless of how it was achieved, and how effective it may be, pressure limitation was not completely satisfactory. It did nothing to prevent the wheels locking on slippery surfaces or under panic-stop conditions. For that there was only one solution - a true anti-lock system. Until the early 1970s the only such system available was the Maxaret, a mechanical unit developed by Dunlop. It was complicated and expensive and the four-wheel-drive
Jensen FF was the only production car to employ it. But the advent of electronic devices was the breakthrough which made anti-lock brakes a widespread reality.
