SPRINGING FOR SPEED.
TN the previous article an attempt was made to show 4the direct relationship between springing and speed, and to indicate how it is that a rough road is more difficult to travel upon than a smooth one, considering the matter from the point of view of the power which is absorbed in propelling a vehicle along that road. In the course of the argument, attention was drawn to the importance of reducing the proportion of unsprung weight, and a new method of reckoning its effect was demonstrated.
Now, when a springless vehicle, with hard tyred wheels, hits an obstacle, it loses energy, the amount lost depending upon a number of factors, prominent amongst which are the speed and weight of the vehicle itself. These are the factors which, taken together, determine the actual energy possessed by a moving vehicle, hence their importance in any calculation to ascertain the loss of a portion of that energy. A vehicle weighing, for example, about two thousand pounds, and travelling at thirty miles an hour, hitting an obstacle two and a half inches high, first with the front wheels and then with the rear, would lose as much as a couple of miles an hour of its speed, which has to be made up by the expenditure of power which might very well be employed to increasing that speed beyond the thirty miles per hour. The object of the springing with which a racing car and, in a lesser degree, an ordinary sporting car is equipped, should therefore be to absorb these shocks in such a manner as to reduce to a minimum their effects on the car speed.
The immediate effect of a bump is two-fold. The vehicle is lifted into the air, and receives energy dissipating shocks. As, in the ordinary way at any rate, the vehicle comes to the ground again, without any excessive loss of time, the diminution in speed consequent upon the first effect is not very great. That which results from the second effect is much more important. The first effect is due to the vertical component of the shock 0/ the bump : the second results from the horizontal corn
By H. SCOTT-HALL, M.I.A.E.
7 he second article dealing with this important subject.
ponent. The latter is therefore much the more important from our present point of view. Before going any further into the matter it may be as well to explain precisely what is meant by the vertical and horizontal components of a shock. When a wheel meets an obstacle as shown in Figure I, the blow which it receives is not purely vertical, nor is it altogether horizontal. It is almost invariably a slanting blow, as indicated in the diagram by the sloping line AB. Now all these blows are not, unfortunately, directed at the same angle of slope ; • if they were the problem of springing would be considerably simplified, and all that would be necessary would be a spring, or series of springs, disposed in direct opposition to the line of action of the shocks. Instead of that happy state of affairs, however, it is actually the case that almost every different blow takes a different direction, as may
well be understood by reference to diagrams 1, ia, and Ib. In the first, an obstacle of medium size has been presumed to have been encountered, and the blow received by the tyre is seen to be at an angle of about thirty degrees with the ground. In the second one, a large object has been struck, and the shock is more horizontal in its direction, being inclined at no more than fifteen degrees to the ground. In the third diagram, Fig. ib, a small object has been encountered and the shock more nearly approaches the vertical, being directed along a line making an angle of about seventy degrees to the ground. Now, in view of the wide difference in the effects of the blows from these objects of different sizes, some simplified method of calculating their effects is necessary, so that they can be resisted by springs which will act in such a manner as to meet them. One single spring can hardly be arranged to perform that function, since it could not, within itself, contain the means for regulating its angularity according to the size of the obstacle. For
that reason it is customary to resolve the blow, mathematically, into two equivalents, of which one is purely horizontal, and the other purely vertical. On the diagrams, the lines AC represent the horizontal effects, or the horizontal components, of the blows which the wheels are supposed to have received, while the lines BC indicate, graphically, to the same scale, the vertical components. It is to be understood, of course, that the line AB, in the first instance, has been drawn to represent to some scale, the total effect, both as to direction and amount, of the blow itself. It may be appreciated, by reference to these diagrams, that the horizontal effect, which is the speed reducing one, is greater in proportion when the obstacle is large, than when it is small. Most car springing systems are designed to take care of vertical shocks only, or at least, they can absorb but a very small proportion of the horizontal components. They are not, therefore, so effective for helping to maintain speed as they might be were that not the
case. The chief value of this vertical reaction, which is afforded by the ordinary type of spring, is that the wheel is kept into closer contact with the ground. When an obstacle is encountered the spring delays the action, slowing it down and therefore reducing the intensity of the bump, thus decreasing the height to which the vehicle jumps, as well as delaying the start of the jump ; it also has the effect of restoring the wheel to the ground more speedily than would be the case without springs.
In a touring car, the horizontal component of a blow is taken by the tyres, if they are not inflated to too high a pressure. In a racing car, this cannot be, to any appreciable extent, as highly inflated tyres are essential to the best speed results.
Shock absorbers are introduced for the purpose of remedying the deficiencies of the normal springing system, and amongst them that of absorb ing horizontal components of shocks is prom CAMBERED minent. They are also useful to provide for the absorption of those small rapidly-recurring shocks which come too frequently for the laminated springs, and which, in an ordinary touring car which is not customarily driven at high speeds, are absorbed by the pneumatic tyres. In a racing car, for reasons which have already been discussed, the tyres cannot perform this function to the same extent, and that is one of the reasons why shock absorbers, which are only used on a minority of touring cars, are almost
universally favoured for racing purposes. Another purpose which all types of shock absorbers serve, is that of checking the rebound of the spring, and, incidentally, countering the effects of the periodicity which is the unfortunate property of all springs. Shock absorbers which really take proper care of the horozintal shocks are, however, sufficiently rare to be considered as nonexistent, and I am of opinion that the next step in the development of springing for racing cars will be in the direction of making adequate provision for these shocks.
One simple method of overcoming them would be to use deeply cambered springs, which, when secured to brackets on the frame at the front ends, and shackled at the rear, give in a horizontal direction when subjected to a blow. The objection to the employment of Such springs, as is fairly widely known, is, however, that they increase the tendency of cars to roll. In fact, that tendency has been one of the deciding causes in makers using flat springs for all types of car. The reason for this is partly indicated by the diagrammatic illustration which is designated Figure 2. Whenever the car has a tendency to sway, it must swing about the springs, at the points where they are secured to the axles. Th411 dimension ” x ” on the diagrams is a measure of the freedom to sway, for it expresses the leverage which the body has over the springs, and the greater that leverage the more readily can the body roll. Consequently, other things being equal, the vehicle to which those springs are fitted which have least camber is least likely to develop objectionable swaying motion. Deeply cambered springs too, are very frequently more flexible than the flatter ones, although this is not, of course, always the case, or necessarily so. When they are morç flexible, then rolling is accentuated by the spring on the side to which the body is, for the moment, tending to roll, deflecting to an excessive extent, while the spring on the other side is not deflecting at all.
Deprived of this means of reducing the speed-destroying horizontal elements of his road shocks, the racing expert has been compelled, as has been stated, to turn to supplementary springing devices to give him aid, and the consideration of this equipment, generally referred to as shock abgorbers, becomes correspondingly important. A more scientific and comprehensive examination of all the various devices which, as regards many of them, ” masquerade ” under that title is long overdue. Until such time as it comes to be undertaken, we may only take the opinions of those who have used such devices, deducing, to some extent, causes from results, and thus arrive at conclusions which may be of assistance in determining what direction improvements should take. Apart from the fact that the best of all places for the absorption of road shocks of all kinds is the rims of the wheels—coupling with that the circumstance that no one has yet devised a practicable means of achieving that end without involving excessive loss of speed ; the most effective springing is obtained by a combination of springing elements which afford a progressively descending flexibility, that is to say, the resistance of
the springs to deflection increases as the load or the shock increases, and that is the underlying principle of the best systems now in use.
For a satisfactory springing equipment three things, really, are essential. First, semi-elliptic springs as a foundation. They must be as long as practicable, flat and must flex as easily as the circumstances and conditions of the design will permit. Second, rebound checks or snubbers, capable of stopping the vibration of the above springs almost instantaneously when they first return to the position of equilibrium from which they are disturbed by the shock. Thirdly, some device which will absorb, as nearly as is practicable, the horizontal shocks.
A combination of this kind, suggested some time ago by an American springing expert, is illustrated in Figure 3. The spring is supported at the rear end by a shock absorber of the rebound damping type, while in the middle, above its point of connection with the axle, the *spring bears against the underside of a pneumatic device. The latter, in conjunction with the arrangement of the spring, which is not horizontal, but is sloped upwards, from rear to front, will have the effect of absorbing a good proportion of the horizontal shock. Figure 4 shows a similar arrangement applied to a cantilever spring.
It is pleasing to notice that this season is attracting several newcomers to the racing wheel, mostly private owners. When the fascination of motor racing is more generally known, and the fallacious idea that the sport is very expensive is removed, we shall see a large increase in the amateur ranks at Brooklands.