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Investigation of a Remarkable Case of Attrition.
The problem which we set to our readers in a recent issue concerning the remarkable and unusual wear which had occurred to the rubber tire on one of the front wheels of a steam wagon has raised a great deal of interest, and brought to us a large volume of correspondence on the subject.
Naturally, a great number of these correspondents have contented themselves with a suggestion that the wear in question was brought about more or less solely by the vibratory action net up on account of the slackness inherent to chain steering, but there are a number of other opinions which it will no doubt interest the readers of this journal to peruse, and we have set aside a considerable amount of space in order to reproduce a selection of these communications. The problem is one which has attracted the attention of a large number of drivers and others in the past, although in no case have we a record of the actual wear being quite so defined, so symmetrical or characterized by the particular features which were included in the photographs we reproduced when we brought the matter to the notice of our readers, and which, in order to refresh their memories, we again include in our present pages.
A Tire of Well-known Make.
This particular tire was submitted to us by one of the principal solid-tire manufacturers in this country, with an expression of their desire for our own unbiased opinion as to the cause, and perhaps in conjunction with the letters which we reproduce below it may be appropriate now to include the principal paragraphsof the report, which we submitted in accordance with that request :—
Our Own Report.
"We acknowledge your further letter dated the 29th inst., which confirms our suggestion that the machine to which this tire was fitted was a three-ton Foden. We note that the tire on the other .front wheel was well worn down before the new tire was fitted, so that thete was a very considerable difference in the overall diameters of the tired wheels when this new tire was first put into service.
"You do not appear to be able to give us the information for which we asked as to the possibility of the bushes in each of these front wheels being more or less worn. This is to be regretted as, in our opinion, such wear would have had a very considerable bearing on the problem, which is a very interesting one.
"The conclusion at which we have arrived is that the wear is either wholly due to the running of these two front tires at unequal diameters, or generally due to that cause and the worn condition of one or both of the front wheel bushes.
Wear Not Due to Tire. • "There is certainly no reason for us to suppose that the wear is in any way due to the tire itself. The eubber appears to be in excellent condition. its resilience is good and equal all over. "There is a big line of demarcation running ciremnferentially round the whole of the worn tire. This, in our opinion, clearly indicates lateral sliding, which has been arrested by the crowding up of the rubber as clearly shown on the surface about the same distance across the tread of the tire throtwhout its whole circumference. 'iThis is evidence of side sliding, and confirms our general opinion as to the cause of this wmarkable wear. suggest that, on a wagon provided with chain steering, in which there is necessarily considerable slack, the running of a pair of front rubber-tired wheels of unequal diameter will inevitably cause a tendency for the larger wheel to run round the smaller wheel with the point of contact of the latter with the ground as a centre. This will go on so long as the slack in the chain steering will permit, and will then be brought up short, and this sudden jerk will, in all probability, produce violent vibration that Will jerk the whole axle back to its normal square position. This action will be repeated continually at very short intervals, the less the slack in the chains the smaller the intervals. We suggest that the flats on the tire are the result of constant phases of this action, the wear taking place both owing to the running out of line and to the jerking back of the axle into its normal position. The length of the flat is, we suggest, direct measure of th.e time the whole phase takes ; in other words, the slack on the chain the longer the flat and the fewer of them on the circumference.
"With regard to the actual wearing down of this tire, it will be seen that it is decidedly conical, showing the evident effect of running on a pair of tires of unequal diameter, this again being confirmation of the theory we put forward.
If the Bush be Worn.
"Should it be granted that the bush in one of these two wheels—or in both—was badly worn, a similar action might take place as between the wheel and the axle itself, not taking into account the effect of the slack of the steering chains. In other words, the whole wheel is tilted over to commence with on account of the uneven diameters of the two tires, and this condition in conjunction with the wornbush will tend to .allow the wheel to keep its normal vertical position while, owing to the slackness of the bush, it will tend to run inwards. Of course, this will not take place in the direct line of advance of the whole vehicle, but more or less diagonally. When the wheel cannot run in ' any further, owing to the slack being taken up in the bush by contact with the collet or shoulder in the axle itself, the shock of such arrested movement will tend to force the wheel back into its normal position once more. This also might partially or wholly account for the flats.
"It. will be seen, therefore, that either the slackness of the chains alone or that in conjunction with the worn state of one or both of the bushes can produce the running-out-of-line effect, and the quite periodic return to normal straight running would, in our opinion, be sufficient to account for the initial wearing of the flats. Once they were produced, the tendency to go on wearing on the flats would, of course, be aggravated."
A Zigzag Track.
We hare awarded the writer of the following letter the ten shilling pri2e offered in our issue of the 26th ult.
The question raised in your issue of 16th September with regard to a solid tire which has distinct flats upon the periphery is one which should be of great interest to all users of commercial vehicles.
In practically every instance where flats have appeared upon a tire, excepting rear tires which have developed fiats through undue brake action, it will be found that one flat portion extends diagonally across the tread, whilst the next will also be similar, but worn in the opposite direction. Obviously, this peculiar manner of wear upon the tire cannot be a defect in its manufacture, and we get our first clue as to why the tire has worn in this fashion from the fact that the flats appear in the manner pointed out in the preceding paragraph. Further to this, the flats are symmetrical and equidistant. All this points to something wrong in the actual running of the wheel. Another point which supports this theory is the fact that the tire invariably appears to have worn exceptionally well, although the actual mileage run may have been small.
From these facts the following deduction is quite clear, i.e., the wheel in running has covered practically a zigzag track, which has in consequence produced a " buffing " action upon the tire.
In the case under discussion where both wheels are fitted to an axle which works upon a pivot, the only possible seat of trouble must he in the wheel itself. This, therefore, confines us to the hub, which, if carefully examined, will be found to contain a loose or worn bush, which has caused the wheel to turn in and out of track, thus producing the 'flats. F.S. (Erdington). ' Tire Too Tight on the Wheel.
Having observed in your last issue a request for an explanation of unusual wear of a solid tire, I submit the following, which I consider are sufficient reasons. If the wheel on which the tire was running be examined, it will be found to be altogether out of truth. The diameter of the wheel, if measured from points between the spokes, will be found to be a millimetre or so less than when measured from head of the spokes. The cause of this, I imagine, would be that the hand of the tire was too tight a fit for the wheel, and when pressed on drew the rim in between the spokes, or else the wheel was too weak and gradually flattened out. The flats being worn at an angle, provided the axle and wheel were running true, is harder to explain, and the only reason I fancy would. be this. Most wheels have a flange on the inside of the rim, against which the band of the solid tire is pressed, and this flange considerably strengthens the rim on that side, and would possibly keep it true. This would doubtless cause the flats to wear at an angle, which would be accentuated as the flats in creased. R. E.R. (Londonderry).
Ten Distinct Snatches.
I am greatly interested in the " tire-wear " article ; the symmetrical wear is certainly unique. (a) I consider that the chain-actuated steering on the pivoted front axle causes one wheel to "mark time, and crunch round on a road-contact line parallel with the axle, the slope of I in 16 being caused by road camber near the kerb.
A44 (b) The equi-distant pitch of the flats seems to-imply that there are 10 distinct snatches at the steering chain during one lock or the rounding of a corner ; they may also be aggravated, possibly, through the front axle's trying to straighten itself out (to the wheelbase), owing to chain slackness on the idle side. H. A. S (Coventry).
Has It a Bent Axle ?
In my opinion, the reason for this tire wearing, as per illustration, is due to a bent axle. If the axle were bent slightly backwards, it would cause a creeping or scrubbing motion between road surface every few inches, the distance which it would run straight being determined by the amount of movement it had on axle and the slackness in steering gear.
Cause Was Evidently Overloading.
As a draughtsman, your paragraph re "Remarkable Tire Wear" interested me. I conclude that the wheel has 10 spokes (probably a Foden), and that the wearing has taken place between the spokes.
The main cause is evidently overloading. In the wheel the compression due to the load is borne by the spokes to the axle. The portion of the rim between the spokes has "given " under the load, and consequently 10 flats have resulted.
As the tire wears the wheel develops an increasing tendency to push along these, surfaces when starting and stopping instead of maintaining the true rolling motion.
In addition, unequal loading or overloading would deflect the centre of the axle downwards and wear the inside edge of the tire, as shown in the illustra tion. E.L. (Warrington).
In my opinion there are only two solutions which will fit this peculiarity of wear, and from the information given it is not easy to say which is correct. Flats on a tread will result from weak felloes, which allow a certain amount of spring on the portions between the spoke heads. In this way the tire will be held up to its work opposite the spoke heads only, and will wear flat on these portions alone. This is probably not the solution here, as one of the illustrations indicate a side-crushing effect being displayed by the rubber. This effect, and also that of the symmetrical flats, can be produced by faulty alignment of the wheels, and, seeing that the wheel is a, front one, this will probably be the solution. When running, the wheel endeavours to respond to progression along the same line as that of its fellow wheel—you must have one wheel direct to steer by— and there is just enough give in the rubber to enable this to be done over a short portion of the tread. This lateral strain gets too pronounced, and the tread slips sideways, thus giving rise to friction where the flats are shown. V. D. (Glamorgan).
The Front Springs Blamed.
I attribute the " flats " on the solid tire shown in your illustration to three factors :—
(1) The radial eacillation allowed by the chain and pivot type of steering. (2) The depth and freedom vertically of the springs of the front axle, which allow the front wheels to travel under alternating light and heavy loads. (3) The use of a ten-spoke wooden wheel.
You do not indicate the type of wheel, but as a tenspoke wooden wheel is common on this type of vehicle, I assume it has been used in this case. I suggest that the extra stability of the wheel at the junction of the spoke with the felloe causes the wheels to run with a bounding motion instead of a true rolling, and that excessive wear is sustained at the point in contact with the road on the rebound. This is accentuated by the type of springing adopted and intensified by the radial motion which is always observable when the pivot steering is employed. F.C.