Aims

Fortunately, the rate of creep within the contact patch is small, and the layer of air carried along with the body surface – known as the ‘boundary layer’ – is thin. But both play a significant role in vehicle dynamics, and in what follows we shall spend some time looking at the different kinds of interface and the forces they convey, which (a) support the vehicle’s weight, (b) control its direction of motion, and (c) determine the resistance that the engine must overcome. We shall also be looking at the converse problem, which is the impact of the vehicle’s motion on its surroundings together with the traces it leaves behind. In short, we are dealing with transport technology from the ground up.

The web site is a work in progress. At the time of launch in 2013, it consisted of nineteen Sections, some dealing with road vehicles and others with more general matters. Since then, the content has grown to include railway vehicles and ships together with a review of fluid mechanics as applied to vehicle motion. The process has taken longer than anticipated, and we expect the final tranche on aircraft flight to be uploaded in 2022. Some of the content is new, though most is adapted from books and papers published by other writers, and is therefore second-hand. To the extent that many of the sources are themselves are re-workings of other people’s material, it is third-hand. We are building on the work of others. This is inevitable because transport engineering spans across a number of disciplines and no-one can be expert in them all. Our excuse, if we need one, is (a) that not all the topics are easily available in published form, and (b) some kind of overview might be useful. And from time to time we do attempt to tie up loose ends (such as the ‘correct’ alignment of the Ackermann steering linkage that connects a car’s front wheels), or to re-interpret an obscure topic in a way that the newcomer can more easily understand.

Content

We’ll be delving into the mechanical sciences with a dash of physics and chemistry, and where necessary we’ll use mathematical equations to help explain what is going on. Engineers use mathematics because they have to quantify things in order to make things work. But the maths needs a foundation to build on, namely an intuitive understanding of the phenomena one is trying to describe. This entails thinking in pictures – mental images that are unique to every individual. After all, physical phenomena can be understood at different levels, no one of which tells the whole story.

A good example is what makes an aircraft fly (figure 2). There are at least six different explanations, and scientists are not entirely happy with any of them [1]. One of the simplest is that the wings deflect the air downward, and the force they apply to the air creates an equal and opposite reaction that we call ‘lift’. But how to explain the deflection – why does the air go down rather than up? Here, we’ll focus on the role played by friction, which leads to a curious asymmetry in the flow above and below the wing surface.

Figure 2

Another would be the rolling resistance that slows down a railway train, some of which arises because the track is not completely rigid. You can tackle this either (a) by writing down equations for the energy that dissipated in the ground beneath as it deforms under the weight of the vehicle, or (b) by painting a simple mind-picture: the track sags beneath each wheel and the depression is dragged along, a little behind the wheel because the ground is sticky and takes a moment to recover. The wheel is therefore travelling uphill. Even if you can’t predict the forces involved, at least you can make sense of what happening. If the maths doesn’t work for you, the pictures might.

Structure

The web site covers four modes of transport: road, rail, water, and air. Within each mode, the material is divided into Sections, each numbered with a prefix to indicate the mode to which it belongs. The prefix C denotes roads together with the vehicles that travel on them such as cars, while R denotes railways and railway trains, M stands for marine craft, and A for aircraft. There are also two areas that span across all the transport modes. In the first area, all Sections carry the prefix G; they deal with vehicle dynamics and a number of general topics relevant to most vehicle types. Those in the second carry the prefix F, because they relate to fluid mechanics, again an area that spans across all modes, and a rich area of study in its own right (figure 3).

Figure 3

Each Section title (for example R0418 Hunting) contains a number that reflects its position within a notional structure. Like the roots of a tree, the topics divide and subdivide as they penetrate into the soil beneath. Topics located high on the stem are concerned with the vehicle as a whole, while those lower down have to do with matters of detail. In fact there are really two kinds of root. The first kind relates to hardware: the mechanisms and materials that make up a working vehicle (figure 4). The other relates to principles, specifically, those that govern how objects behave.

Figure 4

How to use the site

The structure is not particularly important because the topics are self-contained. You can dip into them at random, or if you prefer, tackle them in the order in which they are set out. We start at the bottom, not the top, that is to say, we start with the contact patch and work upwards in stages to the behaviour of the vehicle as a whole.

Within limits, you are welcome to use the material for your own purposes. This includes the text and the diagrams, which are copyright under the Creative Commons framework. This means you can download them, copy them into your own documents and change them if you want. But please acknowledge where they came from and don’t use them commercially for gain of any kind. The photographic images are copyright under a different arrangement, and you must contact the owners separately for details.

As explained earlier, most of the material in this web site is a re-interpretation of material produced by other people. There is always a risk of mistakes, so please don’t take anything you read here for granted. There are plenty of mistakes (or misrepresentations) in what follows. It’s just that we don’t know where they are yet. If you find some, please let us know. We would greatly welcome your comments, criticisms and suggestion for improving the site. Please email christopherwright1944@gmail.com.

Production