THE STABILITY OF PASSENGER VEHICLES.
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The Allied Problems of Skidding and Overturning on Corners. Influence of Centre of Gravity, Tractive Effort and Braking.
N0 ONE will deny that, from every point of 'tiew, it is highly desirable that the passenger vehicle should be made as safe as possible, and in the question of safety, stability, under all reasonable conditions, is a prime essential Using this word in its broadest sense, it includes not only freedom from danger of actual overturning, but also the ability to maintain a steady course, without skidding, on slippery roads.
These two considerations—overturning and skidding—can conveniently be dealt with together, because each depends upon the outward centrifugal force which is produced when a vehicle is moving along a curvedgpath. The pure scientist dislikes the . term " centrifhgal force," but, nevertheless, itis very convenient when illustrating the state of affairs which obtains when a vehicle is cornering. This force acts outwards along a radius drawn from the centre round which the vehicle is turning, and it may be supposed to act through a point known as the centre of gravity. As most people are aware, the centre of gravity. (e.g.) is de point at which the whole weight of a body can be regarded as concentrated. In the average passenger vehicle it will be situated on the centre line, and, when viewed in plan, its distance from the front aide will be, perhaps, 60_per cent, of the length of the wheelbase. The height of the e.g. above the road is of great importance, and this dimension should be reduced to
the sm` allest possible magnitude. This has, of course, been very successfully accomplished in the latest London buses, built by the A.E.O., by dropping the frame and employing a. double reduction gear in th6 transmission. Even when a double-deck body is mounted upon a. more or less normal type of lorry chassis, however, the weight of the chassis is so great compared with that of the body, that the centre of gravity will cmly be about 3 ft. above the road. Generally speaking, the height of the e.g. can be taken
as about half the wheel track for many buses, passenger coaches, and chars-banes.
The effect of the height of the e.g. can readily be illustrated by tilting a wooden block, as shown in one of the illustrations. It will be found that overturning is just about to occur when the e.g. is vertically over the corner upon which the block is resting. This result is also found to be true if a composite block with aheavy base is used for the experiment, but here the angle of tilt will be much greater, because the e.g. is very much lower. In the ease of an actual vehicle taking a corner, a double advantage is obtained with a low centre of gravity, because not only is the safe angle of tilt greatly incredsed, but also, by reason of the fact that the overturning centrifugal force acts through the e.g., its effectis minimized when this point is brought as low as possible.
Reverting to the experimental block of wood, it will be found that if this be placed upon a slippery surface, then the force applied with the object of overturning it will simply cause it to slide along without tilting. Precisely the same effect occurs in the case of a motor vehicle, and under normal conditions Thus, if we assume that the height of the centre of gravity is equal to half the track, then overturning cannot occur until the outward centrifugal force is equal to the weight of the vehicle. Now, even on dry roads, the adhesion between the tyres and the surface does not amount to more than about three-quarters of the weight of the vehicle, so that it is clear that an outward skid will occur so soon as the centrifugal force amounts to this proportion of the total weight ; consequently, overturning is hardly possible. However, if the vehicle skids violently into a kerb, the resistance to further sliding is very greatly increased, and, in consequence, overturning is likely to occur.
Another factor to consider is the excessive camber found on certain roads. The effect of this is shown in one of the illustrations, from which it will be seen that the vertical line along which the weight of the vehicle acts is brought nearer to the near-side wheels. If, under such conditions, a right-hand corner is being negotiated, overturning is quite liable to octur if the point at which the load line meets the road is very much nearer to one wheel than to the other. Suppose, for example, that we consider a motor coach with a track of 6 ft, tilted by the camber until the load line is only 2 ft. from the near-side wheels, instead of the normal 3 ft. The righting effect available to counteract overturning will be reduced to two-thirds of its normal value. Consequently, the speed at which a given corner can be negotiated with safety is reduced in proportion to this ratio squared —i.e., four-ninths. In other words, the safe cornering speed becomes, roughly, one-half of its normal value.
It may be of interest to mention that the magnitude of.the centrifugal force depends upon the radius of the curve and the square of the speed. Some figures may illustrate this point,. and, as an example, we may consider a bus following a curve of 40-ft. radius at a speed of 25 m.p.h. In these circumstances the centrifugal force produced will be just equal to the weight of the vehicle. However, as this force varies as the square of the speed, quite a small reduction in velocity will be sufficient to reduce the force to quite a safe amount. Thus, if the speed is reduced from 25 m.p.h. to 14 m.p.h., the force is reduced to nearly one-quarter of its former value.
As regards skidding, the position of the centre of gravity is not of very great importance, a far more potent factor being the suspension system. This should be so arranged that the wheels are kept iii contact with the road for most of the time, even When the vehicle is traversing an extremely rough' surface. Many systems are faulty in this respect, a large bump resulting in the wheel concerned being left in the air for quite an appreciable distance. If this occurs when the vehicle is negotiating a sharp corner at speed, it is obvious that the serious loss of adhesion will make a skid extremely likely. For this reason independent springing of each wheel possesses great advantages—a fact which has been amply proved by tests carried, out on various experi mental chassis embodying this type of suspension in various forms.
Another point which affects adhesion is the torque, or twisting effect, applied to the wheels either by the engine or by the brakes. The effect can be witnessed in -London on any wet day by watching the buses, and it will be seen that when they are accelerating away from the kerb, or braking in order to draw up at the kerb, the torque on the rear wheels reduces the adhesion and the tail of the bus slides bodily sideways. Whilst a four-wheel drive would. halve the acceleration effect, it may be considered as quite impracticable at the present time, but there seems no reason why four-wheel brakes should not be adopted more generally. They have been used in Paris with marked sucens.
The writer has no wish to enter into discussions upon stopping distances in this article, but would point out that if the four-wheel brakes are designed to give exactly the same stopping ability as a pair of rear-wheel brakes, then the torque on each wheel is exactly halved, and the reduction of adhesion, caused by braking is greatly minimized.
As a practical example of what can be done tO minimize skidding, one cannot do better than refer to the unique Holle chassis, which was described in this journal some months ago. Here the combination of a low centre of gravity, independent wheel springing, four-wheel drive, four-wheel braking, and fourwheel steering produced a stability when cornering on greasy surfaces which was really astounding.
Without adopting all these 'novel features, a great improvement in stability can be obtained by employing one or other of them. Four-wheel braking has already been referred to. Independent springing for each wheel is likewise very beneficial—on rough roads in particular. The writer has recently tested two other types of independent axle-less suspension, and in eaeh ease the road-holding qualities of the. vehicle were far above the average.