Dealing With a Garage and Buying Car Parts

How can I get the garage to do a good job on my car? You have to act in such a way that you don’t give them any leeway. Always give then definite instructions, and make a fuss if they don’t keep their promises. Always insist on a report-back before they tackle any expensive repairs. […]

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Tuning and Modifying Your Car

Is it worthwhile taking my car for a dynamometer tune-up? The answer to this question, like so many others, depends to a large extent on who’s doing it. There’s no doubt that a good tune-up, combined with a before-and-after dyno-test, will normally benefit your engine. The problem is that the chassis-dynos used by these shops […]

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What Car Tyres Should I Buy

What is the meaning of the speed symbol on a tyre? This is the manufacturers guarantee that the tyre, in good condition, will not self-destruct when the maximum speed, shown on a speed rating chart, is maintained for a reasonable length of time. This does not mean, for example, that if your car is fitted […]

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Tyre Maintenance

How often should tyre pressures be checked? Options differ, but we feel that once a week, or whenever you fill up – whichever is soonest – is sensible. This duty should not be neglected, because slow leaks are a major cause of tyre failures. At the same time, make sure the valve caps are in […]

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Understanding Suspension and Shock Absorbers

What is the best way to test shock absorbers? The motor trade tests shock absorbers in a number of different ways. Pushing the car down at each corner and looking at the resulting bounce is useless, because it cannot be done fast enough to give meaningful results. Some of the so-called test machines are not […]

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Steering, Alignment and Wheel Balance

Why is it so difficult to find a shop that can align a car’s wheels properly? There are several reasons. Modern cars are more sensitive to wheel alignment than older models, due to the almost universal use of independent suspension plus the fact that in many cases the rear wheels also need to be aligned. […]

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Diesel Engine Combustion

Diesel engines have been around since February 17, 1894. On this day Rudolf Diesel first succeeded in getting his initial prototype to run for a minute at 154 r/min to deliver 13 kW. Since then the diesel’s favourable fuel consumption has made it the engine of choice for all forms of heavy transport. Diesel engines […]

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Clutch and Drivetrain Issues

Why do front wheel driven cars use expensive CV joints, while most rear wheel driven cars are fitted with relatively inexpensive flexible joints? Cross-and-yoke or Hooke joints transmit movement at varying velocities whose values depend on the angle of rotation as well as the angle the two shafts make with each other. This is not […]

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Gasoline or Petrol Engine Combustion

Engines can in general be divided into two groups. Steam engines are classified as external combustion engines because they rely on a fire outside the engine to supply the high pressure fluid that is needed to develop a worthwhile torque. These engines generally have such a high average pressure on the pistons that they develop […]

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Advice on Car Brakes

I’ve just installed new brake pads, and they squeal every time I touch the brakes. The pad supplier maintains that the noise will go away once the pads have bedded in. Is this likely to happen? It’s very likely that you’ve bought cheap pads, or an unknown brand. Squeal is caused by a vibration resulting […]

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How is Speed Affected by Altitude

How is the maximum speed affected by altitude? To answer this question we will firstly calculate the effect of a drop in air density from coastal levels to a typical value experienced in Gauteng on the power needed to maintain a certain speed (see How to Calculate Top Speed for the formula).  Secondly we’ll calculate the […]

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Electrical Devices in Cars

The battery in my first car, which I bought 40 years ago, lasted six years. Many modern batteries don’t last more than two or three years. Why? Some of the main causes of battery failure are lack of maintenance, overload, vibration and heat. A lower percentage of motorists take a mechanical interest in their cars […]

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How to Calculate Top Speed

There is a relationship between the speed a car can maintain and the power transferred from the driven wheels to the road. The formula is: kW = (0,5ρ.Cd.A.V3)/1000 ……………………………(1)  where ρ = air density in kg/m3 Cd = drag coefficient, a unitless number whose value depends on the smoothness of the shape and not its […]

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Specific Fuel Consumption Issues

Can I save fuel by changing my driving habits? Drive sensibly, avoiding both full-throttle acceleration and slow pottering in a high gear. Every time you move the throttle pedal fast the engine is fed with a richer mixture than it will get at a steady speed, whereas a very small throttle opening will cause the […]

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Understanding Distance, Speed, Acceleration, and Time Using Graphs

The relationship between distance (s), speed (v), acceleration (a) and time (t) is easy to understand if we employ a speed versus time graph, such as the one shown below. On such a graph the shaded area represents the value obtained when the units on the T-axis is multiplied by the units on the V-axis. […]

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Engine Oils and Lubrication

Most modern oils are so good that I don’t see the need to use the special oil that the owner’s handbook recommends. Modern oils are a vast improvement over what was available a short ten years ago, so that for many motorists any good quality oil in the 20W/40 range will do. However, there are […]

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Change of Attitude in a Motorcar

For most performance calculations the change of car’s attitude while braking or accelerating can be ignored. This means that the point of application of the forces can now be shifted to the centre of gravity, as shown in figure 3. Here we’ve shown only the forces that play a role during straight line motion. Mathematically […]

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Catalytic Converters

I would like to replace the catalytic converter in my new car with a silencer. How should I go about it? The converter should not simply be removed and replaced by a normal silencer, because there is usually a lambda sensor in front of the converter that will now start to give false readings to […]

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Forces Acting on a Moving Car

All the forces that act on a rear-wheel-driven car while moving forward is shown below. The contact forces between the tyres and the road are shown per axle ie the forces on the two front wheels and the two rear wheels are shown as one vector. These are: 1. The car’s weight, which is a […]

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Car Fuel Issues

Why should I use unleaded fuel in a car that hasn’t got a catalytic converter? Modern engines contain coatings and sometimes even exotic materials that are easily damaged by the lead in the fuel. Furthermore, unleaded fuel forms a lesser amount of harmful products of combustion than leaded fuel. This means that spark plugs and […]

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Determining the Static Weight Distribution of a Car

We will use the same sketch as for Blog 5. At CAR we determine the static down force for each axle (of the same size but in the opposite direction as R1 and R2 shown above) by putting a scale underneath each wheel. This down force can also be calculated if we know the position […]

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Engine Cooling

My engine overheats in town driving, but is fine on the open road. What should I look for? It’s likely that the radiator fan is not working properly. Its purpose is to suck extra air past the radiator when the car is going slowly. If it is driven from the engine, the belt may be […]

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How to Calculate Center of Gravity

This blog illustrates how the position of the centre of gravity of a car can be calculated. Fig (1) shows the forces on a stationary car. The earth’s gravitational pull (mg) acts through the centre of gravity and the reaction (remember: to every action there is an equal and opposite reaction) acts through the contact […]

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Diesel Engine Myths and Truths

Diesel engines use up to 35 per cent less fuel than petrol engines, hence if economy is important it makes sense to buy a diesel. Not necessarily. It depends on how many kilometres you cover per annum. Diesels are usually more expensive to buy and service, and in this country the diesel fuel price is […]

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Forces and Vectors in Motorcars

This week I’m sending you an interesting curiosity that illustrates how vectors can be used to prove a result that would otherwise need some careful calculation. Imagine a car whose driving wheels have been jacked-up and the engine/transmission employed to spin the wheels. If the wheels rotate anticlockwise the speed of the outer fibres at […]

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Petrol Engine Myths and Truths

In winter my neighbour starts his car five minutes before driving off, lets it idle on the choke, and then goes back into the house. How can I convince him that it is not the correct thing to do? Most modern owner’s handbooks state that you should drive off slowly, immediately after starting. This has […]

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Forces and Vectors 2

In normal speech, weight and mass are treated as being equal, but in engineering these words refer to different concepts. The mass of a body is a number that measures it’s inertia (reluctance to be accelerated). The weight of a body is the force with which the earth’s gravitational pull attracts it. This follows from […]

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Introduction to Car FAQ

We’re presenting a collection of frequently asked questions, with their answers, for the second time. Most of these questions are new, but we’ve also included some from the previous booklet that are asked very often. They’re taken either from our two letters columns, Forum and Technimail, or from the questions we get asked over the […]

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Forces and Vectors 1

Forces cause either linear (straight line) or rotational motion, depending on where they act on a body, and whether the body is free to move in any direction. For example, if you push a small box lying on a table the box will in most cases not only move linearly but also rotate slightly. Only […]

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Newton’s Laws of Motion

An understanding of vehicle testing requires a few basic facts from the scientific study of motion, which is called dynamics. This study was initiated by Galileo Galilei (1564 – 1642) and Isaac Newton (1643 – 1727). Newton’s three laws of motion make it possible to understand the way forces influence objects. The content of these […]

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Jake Venter is a Freelance automotive journalist, a world engine of the year judge, an Automotive Technology lecturer and an automotive historian. He was also the Technical Editor of Car Magazine (SA) from 1996 to 2011 and is currently the Technical Editor of new SA Automotive Magazine "Accelerate".