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Sympathy and Understanding

A basic understanding of what goes on under the bonnet as you drive will give you a greater appreciation of your car, and may even help you to drive it more sensitively. After all, if you treat your car properly it will serve you better. A car is an elaborate machine designed to transport you and your passengers; you, as the operator, can control it more effectively if you know how it works. It is beyond the brief of this book to explore the engineering of a car in great detail, but there are many publications on the subject suitable for everyone from interested layman to qualified engineer. If you count yourself as one of the former, an elementary description of how a car works might be helpful.

Understanding how a car works

The internal combustion engine works by burning petrol or diesel fuel with a mixture of air to create an expansion of gas which drives the pistons . These pistons drive a rotating shaft, called a crankshaft , which is connected to the gearbox by a clutch . The clutch can be released so that the engine can run while the car is standing still. A Wankel rotary engine effectively has its rotating shaft profiled to create a rotor which is driven by the expanding gases.

The gearbox contains combinations of gearwheels which give the various ratios as they are selected by the driver. A gear box is necessary so that the engine can run within its comfortable speed range (normally about 2000rpm to 6000rpm) regardless of road speed. You start in first gear and change up as speed increases. Power is taken to the driven wheels (which may be at either the front or back, or both on a four-wheel drive car) by means of articulated shafts.

The car is suspended on its wheels by pivoted arms which allow the movement necessary over the undulations of the road surface. The springs, either of the coil or leaf type, insulate the car and its occupants from bumps and hollows in the road surface, and shock absorbers prevent the springs from continuing to oscillate after they are compressed and released again. Springs and shock absorbers also act to keep the wheels and tyres firmly in contact with the road.

The brakes consist of special high-friction pads which are pressed against each side of a disc or the inner face of a drum to slow down the rotation of the wheel. The driver's movement on the brake pedal increases the pressure in the hydraulic actuating system so that the brakes are made to grip the discs or drums with great force .

People with a good understanding of a car's engineering will hopefully forgive this most basic summary, but it may help some readers who have always preferred not to worry about what goes on under the bonnet.

Sympathy with your car

A good driver never abuses his engine by asking it to pull above or below its normal operating range. Large engines (perhaps with six, eight or even twelve cylinders ) can run quite happily down to virtual tickover speed, and highly tuned engines can rev higher than the average, but between 2000rpm and 6000rpm is the approximate range for a typical four-cylinder engine. A good driver does not attempt to engage top gear much below 2000rpm, nor does he rev his engine constantly to its limit. He uses the gearbox so that his engine is always running comfortably, which means using only part throttle once he has reached a reasonable cruising speed. It is quite wrong to drive unnecessarily quickly and be forced to jam on the brakes at every obstacle. You should always drive at a 'happy balance' speed, and err on the slow side rather than the fast.

The clutch is invariably the most abused mechanical part of a car, and for many drivers it is the first major component to fail as the miles clock up. As the device responsible for transmitting the engine's power to the gearbox, the clutch comprises two facing discs with a mechanism which brings them together to engage drive or pulls them apart to release it. A lining of friction material provides the grip, so it is easy to see how slipping the clutch causes this lining to wear out more quickly. A clutch can last for 100,000 miles or more if it is treated with respect, yet a driver with no mechanical sympathy can ruin one within 5000 miles.

A driver who uses the clutch pedal as a rest for his left foot (and there are plenty!) will have to pay for many new clutches throughout his motoring life, yet may never realise that he is at fault. He might think that the slight pressure of a foot resting lightly on the pedal causes no harm, but it is just enough to let the clutch slip a little all the time, which wears it out quickly. Some lazy drivers also use the clutch to hold the car on a hill, balancing the clutch's biting point against engine revs so that the car remains stationary. It is far safer, and in the long run cheaper, to use the handbrake, as you were taught to do for the Government driving test.

Another fault which can develop, without a driver realising it, is to use the clutch pedal to control the speed of the car. He might dip the clutch to lose a little speed when negotiating a corner, then feed it in gently for the next straight stretch of road. The thinking seems to be that this tactic avoids the need to change down a gear, but the effect is to wear out the clutch and leave the driver in the wrong gear for the conditions.

Slipping the clutch occurs when it is released too gently, but it is also possible to use it too sharply. Frequent jack rabbit starts can cause all sorts of expensive damage to your car. They strain the entire transmission system as well as the engine (causing gearbox breakage in extreme cases), wear out the tyres and damage the universal joints in the drive shafts , this last fault being recognised by a loud 'clonk' every time you change gear.

The need for mechanical sympathy also extends to the gearbox. Any graunching always results in wear to gearwheel teeth, or even broken teeth, so take care to disengage the clutch fully before changing gear and push the gear lever fully home before releasing the clutch again. Trying to change gear in too much of a hurry is the cause; the worst graunching occurs when a driver attempts to select reverse gear too quickly after rolling to a standstill, or even an instant before the car has stopped.

Every component on your car benefits from careful treatment, including the brakes. Leaving your braking to the last moment is not only risky, but also wears out the pads and tyres more quickly. New tyres amount to a significant proportion of the cost of running a car, so remember that mechanical sympathy - an essential part of advanced driving - extends to these too.


  • If you do not know how a car works, acquire at least a basic mechanical understanding so that you are better equipped to treat it properly.
  • Drive with mechanical sympathy: do not extend the engine below or above its reasonable operating range; treat the clutch with respect; and avoid causing a graunching sound in the gearbox by changing gear too hastily.
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