History Suspension 4
Apart from the front- and rear track-width, one dimension is particularly important, that is the wheelbase. The area between these measurements is by far the more expensive, because here, the stability and the
torsion are in question. If needs be, this can be extended to the front or to the rear by using a plastic component. The wheelbase determines the actual amount of space later available in the car, that means, the
distance from the front seat back-rest to the clutch pedal or from the the rear- to the front seat back-rest.
The overhang to the front and rear are actually only of importance in few vehicles, e.g., in off-road vehicles. However, even here they can be slightly slanted upwards, to ensure that the slope- or ramp angle is large
enough. Indeed, the appearance is then bound to suffer. Rear overhangs can sometimes be alarmingly large, particularly, e.g., when a light transporter is converted into a bus for 20 or more passengers. In this case,
the overall maneuverability is also restricted.
By the way, although the wheelbase is sometimes measured exactly to the millimeter, in most constructions these measurements are by no means fixed. When the rigid rear axle is guided by trailing arms it changes
if the springs are compressed. Only if the wheels were absolutely vertically guided, would there be no change in the wheelbase. This has been done on the front axle, but on the rear axle it can be excluded. Even leaf-
springing shifts the rigid axle a little, mostly to the rear.
The lateral movement of the wheels can be even more pronounced, because also the track-width is not constant when a compression takes place. This, by the way, mostly has a more marked effect on the
suspension, e.g., it can cause the straight-line driving performance to worsen and also further effect the steering, we won't even mention the increasing of the rolling resistance. A very important factor of the
suspension layout, is the changing of the camber and other parameters.
Let's remain, for the moment, with the center of gravity, the importance of which was also explained in the article about the position of the engine. It should of course, be as low as possible. Just how far the quest for a
low center of gravity goes, can be well seen in cleverly worked out racing car technology. The reason why a dry-sump lubrication was introduced, is that a smaller, or in fact, non-existent oil bath, would allow the engine
to be mounted lower. Using the same argument, a multi-disc clutch with a smaller diameter is sometimes installed instead of a larger single-disc clutch.
Let's take a closer look at the behaviour of the front- and rear axle. Looking at the two separately, the suspension components carry out pretty complicated movements, there is however, looking from the rear, one
point, around which everything turns when the springs are compressed, this is the 'momentary center-point'. Should one, e.g., have two wishbones, then this point would be the intersection of both extensions towards
the inside. If this point is joined to the center of the tyre-contact area, this line in the vehicle center, would go through the 'sway poles' or rolling center. Both the rear- and the front axles have this point. A longitudinal axis
running through these two points must be exactly in the center of the rolling movement. It joins all the points which do not change when the car body inclines.
Why is a specific point called the 'momentary center-point'? One 'pole each' is also part of the earth's axis and the expression 'momentary' refers to a quite unique situation which is taking place right now. If any
parameter is changed, and there are many, the point is somwhere else in relationship to the vehicle and must, by projection, be re-determined. The advantage is however, that with the help of this point the speed and
the acceleration can be be calculated at almost any point of the suspension. Indeed, one moment later the curve will have changed a little and possibly be more severely crossed, it must then be determined anew.
Using the above picture of the rear axle of the 1932 DKW-Front, I would like to explain just how important the two sway-poles are. You may consider this transverse leaf-spring to be unusually highly mounted. Due to
the missing boot-lid, it certainly does unnecessarily limit the access to the inside of the luggage compartment. Why then, did the designer still place it so high?
Lateral incline is felt to be unpleasant for the passengers in a car, particularly as soft springing at that time was normal, because of the bad road conditions. Should the rear-axle kinetics, through the placing of the
transverse leaf-spring be thus altered, so that the center of gravity coincided with the sway-pole, the additional incline momentum around the sway-pole could, at best, be zero. For this reason, this was called, indeed
somewhat exaggeratedly, a 'floating-axle'. 03/17