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## Center of gravity

#### Assignment

To put it simply, one could say that the center of gravity is the point where one can imagine the total mass of a vehicle to be. If one starts with the car (preferably with a flat) roof, its center of gravity can be determined by balancing it on a sharp-pointed object. If a single wheel is now added, one would have to place its mass, in relation to that of the roof, which would move the center of gravity slightly in the direction of the wheel. It would be shifted, not only to the side, but also downwards. Adding all the wheels, it would lie even lower, but once again, in the center of the vehicle. The front mounted engine, together with the gearbox, would be heavier than all the aforementioned parts together and would cause a shifting of the center of gravity, of more than half the distance from its previous position, in the direction of the engine. If all the components are considered in this calculation, the center of gravity is determined at which point one could, theoretically, raise the vehicle without it tipping to the side or to the rear.

In reality, this is of course, not possible, unless one installs a cage into the vehicle, with a fixed point in the center, since, as a rule, the center of gravity is found somewhere in the interior. In the above shown motorcycle, it should be found somewhat below the rear cylinder head. In a front-wheel drive motor car, it could lie just above the hand-brake. By the way, its position is not always determined solely by the position of the engine. There are vehicles which, despite having the engine up front, have a weight-distribution of 50:50. In this case, it would lie smack in the middle between the axles.

#### Function

Dynamically seen, things look very different. The moment a somewhat corpulent person gets into the car, the center of gravity moves to the side. It can sometimes be seen, in vehicles which have been used for years to carry only one person, that even without passengers, they have a tendency to tilt to one side. The changing of the center of gravity when accelerating, braking and particularly when cornering, is far more important. There are old-timers with rear mounted engines and a weight distribution of approx. 30:70 which is corrected, when braking hard, to almost 50:50. To prevent diving, e.g., in two-wheeled vehicles, help can be had by, among other things, the construction seen in the above picture, here the front wheel is guided by a small, sprung trailing arm up front. The braking forces turn this lever in an anti-clockwise direction, thus counteracting diving. Vehicles with a particularly high engine performance and relatively soft suspension, (e.g., the earlier versions of the S-class Mercedes) have a similar construction on the rear axle, to offset the sinking movement at the rear when accelerating.

When cornering, the center of gravity moves outwards. How far out it is shifted, depends on the rolling angle of the vehicle and on the suspension. Should the center of gravity, in two track vehicles, shift further than the outer curve contact point, the vehicle will start to tip (as in the Elktest). This problem is made even worse if a, possibly unsecured, load is on board. In this case, a car doesn't differ very much from a ship, which can capsize through sliding cargo. Because there is no fixed center of gravity during normal driving operation, the expression 'momentary center' is preferred when the construction calculations of a vehicle are made. Changes to the vehicle dynamics however, generally take place through the axle construction. Thus, one differentiates between the momentary center in the front, and that at the rear. They must not necessarily be at the same height.

Drivers of two-wheelers (motorcycles) experience the center of gravity or the momentary center even more directly. The steering procedure alone, differs from that of the motor car, the rider must first of all, bear slightly opposite to the curve direction and then start banking into the curve. Basically, a two-wheeler is not actually steered in a curve at all. Here a resultant force is developed, between the weight- and the centrifugal force, which is almost equivalent to the incline. There is actually, no special resaon for the installation of V-engines in two-wheelers, except that with in-line engines, a lower center of gravity, and thus a more acute angle of incline (and a higher speed) is possible when cornering.

There you have it again, to leave the center of gravity out of a dicussion about handling characteristics, would be unthinkable. It should be low, also in trucks. This is why one has more reasons to make (high) superstructures especially light. Also, a (genuine) off-road vehicle, because of the necessary ground clearance, can never have good handling characteristics, not to mention the tyre problem. In racing cars, one haggles over almost every millimeter that the center of gravity can be lowered. Instead of using single-disc-, multi-disc clutches are installed, to allow the engine to be placed even lower. Where do you think the additional weight is placed if the racing car is below the regulation weight? Of course, its placed on the lowest possible spot under the monocoque. Is that enough to emphasize the importance of the center of gravity in vehicle manufacturing?

There is one other almost playful, although not at all safe manipulation of the center of gravity. In biker circles its known as a 'wheelie', riding on only the rear wheel, with the front wheel in the air. It all started with the powerful engines, in the lower gearspeeds it was a simple matter to lift the front wheel from the ground. The older motorcycles never had the finely controllable engine performace of the modern machines. With the developement of the braking systems, the 'stoppie' was born, balancing on the front wheel after braking the front wheel especially hard. One can safely assume, that also here, a good chassis technology plays a decisive part. Whatever the case may be, one can, with only a few square centimeters of tyre/road contact, determine the center of gravity exactly. Only the height is then still unknown.

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