Oscillating Front Axles.
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A Discussion on What Happens When a Back Wheel Mounts an Obstruction.
It of ten happens in mechanics that great .ptirts are made on good ideas only half thought out. The good point is seen. It interests its advocates, and its advertising features have the necessary business glamour. If buying members of the public are not likely to look further, there is no necessity for the promoters to do so; and so the thing goes. Now it seems to me that "the oscillating front axle" is one of these cases. It looks very nice to be able to say " no matter whab,unevennesses of the road this axle goes over the goods deck keeps steady," and, at exhibitions, practical demonstration models are shown in proof of the statement. But has anybody seen or heard of a maker who accounted for the effect such an arrangement had on the clerk when the hind axle reached the same obstruction? It is the investigation the writer now proposes to make.
Two cases must be taken : these are of vehicles built exactly alike, with the single exception that the deck of the one is supported on four springs connected to the axles as directly as possible, whilst the other is fitted with some contrivance for carrying the front end of the deck freely in the middle, so that its level is entirely controlled by the hind axle. Referring to the drawings, Fig. i (page 5) shows the four-point supported deck, and Fig. 2 the three-point supported deck. The dotted lines show the position of the car with all the wheels on the ground.
Let us consider case one with its " off " front wheel going
over an obstruction, the point of vision being in front of it. We know at once that, on the wheel's rising, either the deck will twist or the springs will deflect unequally. For simplicity of investigation we will suppose the deck to lie absolutely rigid, and we will further suppose that the car is fully loaded with two-thirds on the hind axle and one-third on the front axle, the springs, of course, being loaded in proportion. Under these circumstances, if the rise of the wheel causes the axle to assume an angle of 90 with the road (Fig. t), the deck will assume an angle of only 3°, since it has two-thirds resistance against can tings from the stronger springs of the hind axle. Following the process, and watching the hind axle (Fig. 3) go over the same obstruction, we get the case as illustrated. There is the same cant of 9° in the axle, but the deck has assumed an angle of 6°, or twice as much as before, because we now have the stronger springs inducing the cant against the weaker ones in counter-action. From a static point of view, then, the maximum cant throughout the process is 6°. Following case two—the pivoted front axle—as illustrated by Fig. 2, in a similar way, on the "off " front wheels arriving at the top of the obstruction, we have the same angle of 9' in the axle ; hut, since the deck is pivoted on the centre of this axle, it remains level, and, so far, seems superior to case one. But let us see it through, and watch the " off " wheel of the hind axle mount the same obstruction. The axle assumes, as before, an angle of 91', and, since the front end of the deck has now no check, it follows the axle, and takes up the full angle of 9°. In case two, then, the maximum angle of the deck throughout the process is something over 9, considered statically, or more than 3' worse than the first case of the four-front supported deck, The springs, in this case, do not influence the question. The front pivot puts them out of action so far as twisting the deck is concerned. The whole deck must follow the hind axle as to angle. [No doubt this is so; but is it not some advantage to avoid this twisting action? Also, the back springs cannot allow the deck to tilt the full 9°.-1:13.1 Where, then, is the benefit of the oscillating front axle? Instead of saving the deck from canting, as has been supposed, it actually allows of a bigger cant, and a greater strain on several of the parts, when the dynamic conditions Come to be allowed for. Imagine the real conditions of going over these obstructions without stopping. In case one each axle is a check on the other against the canting
of the deck ; but in case two, although the front axle goes over the obstruction one-third better, so to speak, the hind axle gives the deck a good throw over, and the impetus of this throw may make the strain on the springs and deck fully twice as bad as the more simple arrangement of the four-point supported deck, And there can be no doubt whatever that this is the steadier, the lighter, the cheaper, and, with less parts to go wrong on the road, the better machine. In short, is not the oscillating front axle worse than useless? HERBERT GUTHRIE.