OPINIONS FROM OTHERS.
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Advantages of Big Tractor Wheels.
The Editor, THE UOMMERCIAL • MOTOR.
• [1570] Sir,—In his reply to my letter on the advantages of big tractor wheels " Agrimet " shows himself rather impatient of any opinion differing from his own. I suggest that my contention cannot be fairly described as a quibble because one of .the first things to do im studying any phenomenon is te try to get an exact understanding of its causes.
"Agrimot" considers that the horse-power of a tractor working on soft land is absorbed in compressing the soil. I suggest that this is not exactly the case, but that it is absorbed in the attempt to lift the tractor out of the groove which it 'forms for itself. The compression of the soil is one of the consequences of this attempt. I stated that the depression of the soil' and the hill-climbing effort of the tractor are simultaneous, which " Agrimot '.' does not believe.
Reference to the two little diagrams reproduced below-will make my meaning more clear. When.a.wheel is travelling on level graund, the lowest point 'of the wheel is the instantaneous centre of rotation. When., as in Fig. 1, it is travelling up a. slope (BA),
the instantaneous centre of rptation is the point (D). In turning about this point the axle at C is raikd, and power is consumed in driving the machine uphill. Now refer to Fig. 2. The horizontal line shows the level of the ground, and the dotted line indicates the depth to which the tractor wheel sinks in. As the tractor moves forward, it is faced with the slope (ABE), and the centre of rotation is therefore about where the line (CD) cuts the circumference. The movement round this centre involves an effort to
raise the axle at C. We know that no actual lifting of the axle takes place, therefore it follows that tie effort to.lift the axle and the compression of the soil below D occurs simultaneously. Supposing the .soil refused to be compressed, the raising of the axle would actually take place, and the engine power would then be consumed in an obvious hill-climbing effort. This is then, I think, what is taking place, the depression being. merely the result of the attempted climbing of the tractor, and not the direct cause of the loss of Power' since. we see that the power would still be lost if the compression did not occnr, until the wheel had lifted its lowest point on to the level of the ground. In stating that some power was doubtless used in .driving in the spuds on a tractor wheel; I was only quoting " Agrimot " himself, though he now states that little or no power is expended m this way. When I wrote that I could not see that much work is wasted in rolling over the spuds and lifting them out, I did. so not out of any desire to contradict " Agrhnot," but with a view to drawing from him some arguments or exPlanation hearing out his Contention. What I should like is some proof of his present argument that the main loss is occasioned by rolling over the spnds; and not a mere repetition of his Assertion to that effect. I can assure him that I am quite amenable to argument, and wrote mainly for the purpose of try
.1354 ing to clear my own ideas, and perhaps those of other. people, as to what is the relative importance of the various causes of the loss of power when a tractor . works on soft land.
I should also like to know, why, while agreeing that ease of attachment and detachment of spuds is essential for many reasons, he considers that the leatt of these reasons is connected with the need for running upon the roads.. It seems to me that this need must often arise when the tractor has to move from one
field to another.—Yours faithfully, VEcTis.
The Live Axle—Is it Dying?
The Editor, THE COMMERCIAL MOTOR.
[1571] Sir,—An article which I contributed to THE COMMERCIAL MOTOR some weeks ago, drawing attention to the undoubted difficulties which had been encountered on active service in connection with certain heavy Army lorries embodying live-axle designs of special type, has brought, down on my head the criticism of at least two designers. I must admit that I am not surprised that Messrs. David Brown and sons, of Huddersfield, refuse to enthuse as to my prophecy with regard to the possible demise of the live axle, or, at any rate, of its considerable Supersession by the chain drive for Same of the heavier models. . Dealing first with Messrs. David Brown's letter, which appeared in, your issue of 22nd November, I have again read my article, but I regret that I cannot • detect the portion of it in which I am supposed to have .contradicted myself. I would remind the writers that I did not specifically criticise the worm drive. There are other types of live back axles. The writer of this letter is, I fear, not sufficiently informed of' the actual U.S.A. Government purchases to be ac_ curate. it is not correct to say that the United States Government " is buying a few chain-drive trucks, and them in a very limited way." I am informed that the numbers are large enough to warrant the assumption, that the State Department concerned did not consider, the risk of such a choice a dangerous one. Nothing . is said in this letter to explain the decision of the Freneb Government, for which, to those who knoW, there a very excellent reason.
With regard to "Driver Designer's" letter in your 6thDecember issue, I must concede that the chaindriven Wolseley owed much of its efficiency to the crosswise positioia of the engine and to the absence of bevel or worm gears, such as one finds in modern ears —including the live-axle ones I still consider, however, that a factor , in the early Wolseley's unique efficiency was due to the use not only of the final chain drive, but also to the silent chain drive between the . engine and gearbox. So far as brakes are concerned, I think " Driver.Designer " w.ill, on reflection, recall that the latest practice is to put both the brakes on the back wheels : they are thus not rendered inoperative if a chain breaks.
Can "Driver Designer" melitien a standard lorry,
live-axle design, of which any considerable number are on service at the Front, on which, to his definite knowledge, no trouble whatever has been experienced, beyond that due to normal wear and tear or to sheer accident ? If so, will he name it? His analogy of the certain' make of car on which the bottom of the engine case is frequently knocked out through hitting the pave is not a good one. This mishap developed solely on account of the wrong position of the engine, The live-axle failures which have occurred on active service have, in the majority of cases, not been due to any such fault or to the striking of road obstructions, but to the unequal and severe stresses set up in the complicated structure of the live axle itself. It is a • difficult. matter to adapt the design to absorb these stresses tinder the circumstances.—Yours 'faithfully, THE INSPECTOR.