I can only hope you gave some recovery to arms meantime. Because now we have another task for you. You can learn a lot with this two steering knuckles. Before we go on with our thought experiment here a very simple term.

It is called 'track width', defined as the distance between the center lines of the wheels of an axle. So simple it is, without reason it is not included in the datas of modern cars. If you compare it on the front and the rear you can see, where the car needs more support, where the most mass is gathered, at the front or the rear side.

From this preliminary remarks you may realize that the two track widths mostly differ a little. In earlier times there may have been an identical formula for normal cars often combined with rigid axles pretending a certain track width, some bodys protruding the wheels very much.

Then the first stage of selectable wider tyres came giving the whole construction a more harmonious concept. But with the winter tyres being smaller and should be, the old gruesome impression was back again. All this changed a lot with the concept of conciseness between wheels and body.

That is due to the fact that there are different offsets in the rims possible which move narrow wheels more to the outside. Offset means the distance between the middle level of the wheel to the lay-on surface of the rim nave. It's value is positive if the lay-on surface is outside the middle of the rim, becomes smaller the more it makes the wheel move to the outward, can of course be negative, too.

Here you see an extremly big offset at a truck wheel. It is needed here, because two of them can be combined to a twin wheel usually at the rear axle of trucks and lorrys. And the very unusal example of a wheel mounted the other way round which is disallowed in road traffic and causes further problems at the front axle.

The track width is inter alia dependent on the offset. It can be altered by mounting other rims. That the biggest track width is always at the powered axle or at the one carrying most of the masses is not always correct. There are many technical possibilities of impact. Nevertheless we have prefixed the picture of the Citroën ID which offers an extreme difference between front and rear because of its front drive and much weigth on the front axle.

Now you can ready yourself for a second experimental procedure. This time again the arms very wide apart from each other. You already know that you present a front axle being left today only in real off-road vehicles and trucks. Please hold the two steering kunckles not exactly vertical but both by the same amount obove tipped backwards.

Yes, what the heck is that anyway I hear you say. The front wheels turn during being steered a little around an axis at its top inclined towards the rear. Expressed in technical terms the angle of the wheel to the vertical alignment changes during steering. It is reffered as 'camber', becoming positive if the top of the wheel is angled outwards and negative if angled inside.

It's a pity that there is nearly no positive camber left. But sometimes on television you see at the front wheels of a F1 racing car significantly negative camber, strange if you take in account the wide tyres. Or the sexed up suspensions of old VW Beetles. If they are always driven straight ahead and not roared enough through curves, the running surfaces of the tyres are shut down inside and nearly like new outside.

We don't want to overestimate the negative camber as the fans do but we only want to say that it helps to compensate the inclination of the car body and it keeps the tyre contact area as large as possible. It would be disastrous, if the angle of the wheels would be equal to the body. That could increase the tendency for tilting. In other words the car supports on itself better with negative camber at the outer wheels.

Here you see the F 400 Carving from Mercedes with an extremely negative camber of up to 20°. It certainly doesn't just compensate for the lateral inclination. About 3° is considered to be the absolute limit for any meaningful negative camber, and this prototype has more on the outside. However, this is due to a special chassis technology.

With normal road vehicles, adequate side camber must also be achieved, otherwise lateral support in curves actually deteriorates. It's crazy but true: if you only drive long distances on the motorway with a lot of camber, you not only wear the tyres on one side, but also have a much worse straight-line stability.

But what does that have to do with those damned steering knuckles that you may still be holding in your fists, tilted backwards? Quite simply, as a cornering inner wheel it gets more positive camber and as a cornering outer wheel it gets more negative camber. In order not to have to work with two terms, the inclination of the steering knuckle axle at the top towards the rear is called the (always positive) 'caster angle'.

In addition to this angle, there is also a trailing distance whose effect should not be underestimated. The picture above shows it very clearly as the distance that the wheel runs behind the piercing point of the pivot axis through the roadway. In the past it was called the principle of the tea trolley, today it would be that of the shopping one. If you reverse its direction of travel, the wheel realigns itself accordingly.

Incidentally, the point where the swivel axis pierces the roadway is called the 'track point'. Its distance from the centre of the tyre's contact patch is the 'caster distance'. The greater this distance, the greater the wheel's willingness to run behind its track point.

Unfortunately, even in parts of the so-called technical literature, the two terms 'caster angle' and 'caster distance' are mixed up to 'caster'. This is then claimed to help stabilise straight-ahead running. Let's look at this blatant example above and below: On a two-wheeler with undoubtedly a lot of caster angle, the handlebars are turned 90°.

What do the two drawings tell us? Will the front frame lower or raise as a result of the steering? I hope you agree, it will lower, which means the weight acting on this steering will cause it to deflect rather than reset. So all these comments about the effect of caster would have to be wrong.

You might object that you could not ride a bicycle hands-free then. But you can. Here is an experiment that you can do yourself on nearly any bicycle. You simply measure the distance from the front frame to the ground, starting from straight ahead to extreme steering angle. What will you observe? That at the beginning very little changes in this measurement, the height of the front frame remains more or less constant. Only when the steering angle has reached a level far beyond hands-free driving does it drop to the 90° position. How can this strange effect be explained?

The two pictures above and below show it: Although the caster angle is the same in both cases, the caster distance can be quite different. And it is precisely this that ensures stability, e.g. when riding hands-free. By the way, that would be impossible with the wheel in the picture below. It would probably not even be possible to go straight ahead without swerving. By the way, the trail distance is also called 'custer offset'.

The wheel therefore runs behind its track point. Now even the technical literature gives the impression that the vehicle is lifted by the caster when steering. However, if you look at the picture of the bicycle and think about turning the front wheel by 90°, you will easily recognise that the front structure of the bicycle is not lifted but lowered.

In this picture we have to work out once more that an additional caster distance belongs to a caster angle. If not, the steering axis and the perpendicular from the centre of the wheel to the ground would have to come together. And now we can explain why you can ride a normal bicycle hands-free and why the handlebars do not suddenly flip to the side when you let go of it.