Disculpe, aún no hemos traducido esta página al español. Estamos trabajando en ello.
Si prefiere, nuestro traductor, y solicitarle que lo realice antes.
Proper use of the brakes is an integral part of advanced driving and involves rather more than simply pressing the pedal when you want to slow down or stop. Before looking at the subtleties of braking technique in detail, it is first necessary to deal with the brakes themselves.
Types of brakes
Most cars in regular use have disc brakes at the front and drum brakes at the rear; more expensive cars with higher performance have disc brakes on all four wheels. Although many people believe that disc brakes offer better stopping power, they are not intrinsically more powerful than the old-fashioned drum type. It is their resistance to fade which makes the disc brake superior; this explains why they are almost universally fitted at the front, since weight transfer when braking means that the front wheels provide as much as 70 per cent of the stopping effort.
A brake of any type heats up when it is used, and if used frequently and heavily it will heat up to the point where braking power diminishes — 'fades' — or even disappears altogether. It is possible to drive for a lifetime without ever experiencing fade but it can arise when you most need the brakes, such as through a series of hairpin bends on the descent from a mountain pass. You can detect brake fade from the extra pressure which begins to be needed on the pedal to achieve the same stopping power; in these circumstances it is best to stop to allow the brakes to cool down, for all braking response can be lost if the overheated surfaces are punished any further. Drum brakes fade more readily than discs because they disperse heat less efficiently.
Either type of brake, disc or drum, is capable of locking a wheel if you hit the pedal hard enough, even on a dry road. The grip of the tyres, not the brakes themselves, determines how effectively a car stops in an emergency. A set of four modern tyres, each with a contact area the size of a man's shoe sole, does a remarkable job in keeping a car weighing a ton or two on the road.
Even the best tyres can be pushed beyond their limit. They will begin to lose their grip, lock up and start skidding if the brake pedal is pushed too hard, particularly on a wet road. A car with locked wheels cannot stop at anywhere near the best possible rate, and may even feel as if it is gaining speed. Since the most powerful braking occurs just before the wheels lock up, it is worth developing a feel for this moment in your own car, although the public road is not the place to practise. A skid pan or disused airfield are suitable places, but in the absence of these your local road safety officer may have a suggestion. Practise emergency stops at progressively higher speeds, as this experience will be invaluable should a real emergency occur.
Advanced drivers should be familiar with the dabbing technique — 'cadence' braking — which keeps the wheels rolling for optimum braking in an emergency stop. This is how it works. When the driver feels one or more wheels begin to lock, he momentarily releases the pressure on the brake pedal to allow the locked wheels to rotate again, then re-applies pressure for maximum stopping power. The process may need to be repeated several times before the car comes to rest. With practice, this on-off technique can be refined to the point where you can keep the wheels almost continuously gripping at that threshold of lock-up where the car stops most efficiently. An advanced driver should never demand so much of his brakes in normal use that cadence braking is necessary, but in an emergency on a slippery surface its use could avoid an accident.
An increasing number of new cars, including some relatively ordinary models, are fitted with an Automatic Braking System (ABS), otherwise known as anti-lock brakes. In effect, these do the cadence braking for you, although more quickly and efficiently than any driver could manage. Sensors and pressure-limiting valves, controlled by computer, do the work of the human brain and foot, their operation being felt as a series of pulses through the brake pedal. This technical advance is a valuable safety aid, although no driver of a car with ABS should ever be lulled into a false sense of security, or abuse the system by relying on it to make quicker progress when conditions are poor. An advanced driver ought to be able to drive for years without ever using his car's ABS.
The real experts in the technique of cadence braking are rally drivers who time their hard pushes on the brake pedal to coincide with the spring frequency of the front suspension, thereby taking advantage of the 'nose-dive' characteristic inherent in most cars. Braking imposes extra load on the front of the car, pushing it down on its springs and increasing the weight on the front tyres. With the front tyres doing most of the work under braking, their grip is usefully improved. When the brake is released the front of the car lifts momentarily and then bounces down as the springs compress again. At this moment the pedal is pushed hard again, the front tyres grip better because of the increased load and the wheels are less likely to lock. With practice, really skilled drivers can time their pedal movements to coincide perfectly with the car's nose-dive action. This technique is highly specialised, suitable only after considerable private practice, and appropriate on the road only in dire emergency.
The essential thing is to remember that the brakes stop the wheels, but that the tyres stop the car. You should always know you car's braking capabilities and learn to recognise its limits of adhesion on all kinds of road surface.
Just as important is an understanding of the distance needed to stop a car from any speed. As the frequency of nose-to-tail accidents on busy motorways shows, many motorists seem to have little idea of how much room is needed to stop a car, even in good conditions. The old rule of thumb for stopping distance, 'one car's length for every 10mph', in fact represents only thinking distance, which is only part of the picture. A mere 90 feet at 60mph, which this misconception suggests, would be adequate if the car in front also slows down at a normal rate, but just occasionally it stops a good deal quicker if it hits a vehicle ahead. In emergencies you need at least twice as much space between you to pull up.
The distance required to stop increases in indirect proportion to speed: double your speed from 30mph to 60mph and you will need four times the braking distance. The advanced motorist will learn to judge safe braking distances automatically, but there is a useful formula to remember if in doubt: square the speed and divide by 20 to get the distance in feet. For 60mph, therefore, 60 x 60 = 3600 ± 20 = 180 feet — in other words, exactly double what the old rule suggests. And this formula is appropriate to a good car on a dry road surface. Braking distance increases dramatically in the wet, or even after a light shower on the slippery film of oil, dust and rubber which coats roads in summer. As for snow and ice, the figures rise so alarmingly that someone idiotic enough to be travelling at 60mph could take a third of a mile to stop.
To these calculations must be added the thinking distance. Even people with the sharpest reactions need time for their first sight of a hazard ahead to produce an order from the brain to the foot, which must then move from the accelerator pedal to the brake and start applying pressure. Someone who can do all this in half a second has superb reactions, yet in this time a car travelling at 30mph moves 22 feet, and one travelling at 60mph moves 44 feet. For most drivers, with slower reactions, thinking distance at 60mph is nearer 60-80 feet. This is a significant figure to be added to actual braking distance.
Avoiding fierce braking
The importance of travelling at a safe distance behind the vehicle in front must always be stressed, but for advanced drivers this becomes second nature. As a result, anyone who has acquired the skills of advanced motoring will seldom find it necessary to brake fiercely. Unnecessarily heavy braking is uncomfortable for passengers, wears out brake pads and tyres more quickly and can alarm other drivers.
Good braking procedure is simple: you should apply the brakes smoothly and progressively for about two-thirds to three-quarters of the distance in which you wish to stop, easing up on the pressure for the last third to a quarter. Gentler braking for the last section leaves a margin if you have miscalculated or need to stop sooner than you expect, perhaps if the man ahead pulls up short of the Stop line.
A driver who keeps his brakes on quite hard until the car stops makes life uncomfortable for his passengers, even though he may not notice the jolt to standstill himself. Advanced drivers come to a stop smoothly and gently by slackening the pressure on the brake pedal for the last 10mph or so, and then for the last few feet easing back still further so that the car rolls to a halt under the lightest touch of the pedal. Any good chauffeur knows that practice can make the moment of stopping imperceptible. You can discover whether you have achieved such a fine touch with the brakes by glancing out of the corner of your eye to see whether your passenger's head nods forward when the car stops.
Brake in a straight line
Braking through a corner is a cardinal sin which most drivers do much of the time. Only the excellent handling qualities of modern cars allow drivers to get away with this continually without incident. Except when moving slowly, braking should always be carried out with the car travelling in a straight line. Sometimes braking in a bend may seem unavoidable, but more often than not you are guilty of poor anticipation if you need to do it.
It is easy to cause a skid by braking on a bend when driving at all quickly, particularly in the wet. This occurs because centrifugal force makes the body roll towards the outside of a corner, imposing more load on the outer tyres and removing weight from the inner tyres, which thereby become more prone to locking up and precipitating a skid. In extreme circumstances, braking on a corner can exceed the limit of a tyre's adhesion. If 80 per cent of a tyre's adhesive ability is being used to maintain course round a corner and the driver suddenly asks for another 40 per cent by braking suddenly, the tyre will be unable to cope. The result will be a skid. There used to be a time when drivers were all too aware of the limitations of a car's handling, but modern suspension and tyre design enable cars to travel round corners very much more quickly. When sudden braking finds today's higher road-holding limits, the result can be frightening, or worse still can end in a crash.
The almost universal adoption of dual-circuit braking systems by car manufacturers means that complete brake failure is very rare nowadays, but it can still occur. If the cause is a slow leak of hydraulic fluid you may have some warning from the pedal, which will travel further and may feel spongy. Pumping hard on the pedal to bring more fluid from the reservoir into the system can produce a temporary improvement, but the cause must be rectified before you lose the brakes altogether.
The most alarming kind of brake failure is when there is no warning, just the awful realisation that the pedal produces no response. You must do what you can with the hand-brake (which has a separate mechanical, not hydraulic, linkage) and use the engine to help slow down the car by dropping through the gears as quickly as possible without revving up between each down-change. With luck and skill, you may be able to steer out of trouble. Few drivers ever experience this frightening occurrence, but should it happen and you keep your wits about you the hand-brake and gears might get you out of trouble.
Although disc brakes have reduced the fade problem, their performance can suffer because they are exposed to the elements. If water builds up between disc and pad on a long motorway drive through a rainstorm, there can be a momentary lack of response when eventually you apply the brakes. It is wise to dab the brakes occasionally to keep them clean if you drive many miles in torrential rain without using them, but only when no cars are behind.
A few drivers with automatic transmission in their cars sometimes use the left foot to operate brakes, but this really is not sensible. Your early training as a driver makes right-foot braking an almost instinctive action, and in an emergency you could find your feet confused. You may lock up the brakes with both feet on the pedal, or even press on the accelerator with the right foot at the same time as using your left foot on the brake.
Racing drivers use the 'heel and toe' technique pivoting the right foot so that the heel presses the throttle at the same time as the ball of the foot operates the brake to achieve clean and swift changes down through the gears as they approach a corner on the track. Some drivers put this into practice on the road, but there is little point. The fractions of a second saved on the track mean nothing on the public highway, and it is always possible that you may not brake properly while trying to use two pedals with one foot. It may seem clever to 'heel and toe', but it has little relevance in everyday driving. In any case, the pedals in most cars are not ideally arranged for this technique.
Finally, before we leave the subject of braking, keep an eye on the other drivers around you. Be prepared for the driver in front to pull up sharply without any obvious reason by allowing even more braking distance in case he miscalculates. Look out, too, for the crumpled old banger looming up in your interior mirror, and allow for the fact that his brakes might not be as good as yours. And try to give extra warning to a driver who 'rides' your back bumper by braking earlier than usual, starting with a light touch on the brake pedal to bring on your brake lights. Leave yourself more braking distance than usual so that your own gentle braking can be used to give the thoughtless driver behind more stopping distance.
Familiarise yourself with your car's braking ability, and practise cadence braking to avoid locking the wheels in an emergency.
Always be aware of the braking distance you need at any speed, and allow for thinking distance too in the gap you leave for the vehicle in front.
Avoid fierce braking. Brake smoothly and progressively over the first two-thirds or so of your braking period, then release the pressure gradually so that you come to a stop gently.
Except at low speed, try to brake in a straight line, since sudden braking on a corner can cause a skid.
Allow for other drivers around you in your use of the brakes.
Traducido por Juan Pita
Juan es una máquina de traducir. En todo momento parece estar estimulado por algún tipo de bebida que prepara en un recipiente portátil y que requiere de dos litros de agua hirviendo.
¿Encontró útil esta información?
Ahora puede obtener por completo Cómo Funciona un Auto, en un hermoso libro electrónico en PDF.
Más de 250 páginas ilustradas
Imprima las páginas que necesite y llévelas al garaje.
Apoye el sitio
Por tan sólo $5. Gran relación calidad precio.
(Sólo disponible en inglés)
Descargue el PDF
El tamaño del PDF es de 80 MB. ¡En alta resolución!