A Criticism of Wheels and Wheel Construction.
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By R. G. L. Markham, M.I.Mech.E.,
Seeing the trouble which makers and users have had with wood wheels in the past, partly due to indifferent workmanship or material, but, to an even greater extent, attributable to lack of appreciation, in design, of all the functions and limitations of a driving wheel, a number of manufacturers have discarded wood wheels entirely in favour of steel. This applieS both to omnibuses and light vans, where rubber tires are used, and also to wagons running on steel tires.
Steel Wheels with Rubber Tires.
Of the lighter vehicles, the Milnes-Daimler have been the chief exponents for a long time past. These wheels are all of steel, cast in a piece, with fairly light spokes, and a deep rim to carry the gear ring : there is a side flange to hold the rubber tire. There were several other examples of caststeel wheels in the recent show, mostly on foreign vehicles. Some of these were heavy and unsightly, as witness a wheel with roughly-cast, H-section spokes, while others were neat and light, as those on the Berna and Berliet vehicles.
Here, again, as in the case of the wood wheels, there seems to be a divergence of opinion as to the strength re quired, indicating a lack of calculated design. A certain saving in weight is effected in the Berliet and Berna wheels, by the fitting of the rubber tires direct upon the rim of the wheel, without any retaining channels or side flanges. Thetires are pressed on, and are held by a shallow rib running around the rim. (It is claimed for the Dunlop, endless, steel-band, rubber tire, that nothing of this kind is necessary to hold it in position.) The Fiat vehicles were fitted with some wheels constructed of a pair of 3-16-inch, dished plates, riveted up to a flat plate rim. They do not look pretty, and are, I should imagine, .unnecessarily heavy. As usual, the attendants seemed to consider the wheels a quiteunimportant part of the vehicle, and I could glean no information as to the merits claimed for this particular type, save that it appears to have been evolved for use in the desert._
From the nature of the material used, steel wheels, especially when cast in a piece, are better able than wood wheels to transmit power through their spokes, but that does not make them, mechanically, any more correct, and, unless, as in the Milnes-Daimler vehicles, the power is applied at, or close up to, the rim, the frequent and sudden reversal of
force applied to the spokes at the hub will engender the risk of " fatigue " and ultimate fracture, except the spokes be unnecessarily strong to carry the weight. The only correct form of metal wheel for transmitting power from the hub undoubtedly is the wire wheel with tangent interlaced spokes in tension, though I see no reason why built-up steel wheels, with tangent spokes placed one way in compression, similar to the wood wheels of that type, should not be employed where it is practicable to drive them in one direction only.
While steel wheels seem to be coming more into vogue for buses and similar vehicles where rubber tires are employed, they appear, so far as the show indicated, to be going Out of fashion for the heavy lorries. Still, there were some examples, and these chiefly of the " cast-in-one-piece-withthe-tire " type, as on the St. Pancras and Straker-Squire steam wagons.
Now, here, we have food for cogitation. Why is it that steel wheels are not so much employed where steel tires arc used, while they are being more largely adopted in conjunction with rubber tires? Also, why is it that cast wheels are favoured for, say, certain makes of steam wagon, whilst built-up plate-spoke wheels are retained for tractors? The first question leads to the query as to why steel wheels are not universally preferred to those of wood, whether with rubber or steel tires, and it forces us back to a comparison of the two types. For weight carrying at speed, and particularly for the transmission of power, steel wheels are undoubtedly the stronger; in the case of heavy lorries, they are also generally the cheaper ; and, if only the wheels themselves need be considered, they would probably be more generally employed than they are. It is the vibration to which a vehicle is subjected, by the impacts between the wheels and the inequalities of the road, and which are transmitted more or less through the wheels themselves to the springs, that we have also to consider. The springs do not take up all the vibration, though they do " damp" the heavier bumps. It might not be so thought at first sight, but there is a very considerable amount of " give " in even a heavy-wagon, artillery, wood wheel, as compared with a steel one : this is usually termed resiliency, and, consequently, the minor, but continuously repeated, road shocks which cause the vibration are, to a certain extent, absorbed in the wood wheel instead of being passed on to the vehicle as they are by the absolutely rigid steel wheel. It will easily be appreciated that, in a fast-running vehicle, this excessive vibration would be very destructive ; but, where rubber tires are employed, they neutralise the lack of resiliency of the steel wheel, permitting its use without undue vibration.
Cast Steel and Plate Wheels.
With steam wagons and similar heavy vehicles, rubber tires, except in special circumstances, are impracticable, on account of the expense, whilst, owing to the lower speeds, the vibration is not so severe, though still quite considerable. Wood wheels are, therefore, to be preferred, but, owing to the heavy loads carried, and the great width of tire required, they are expensive to build.
When steel wheels were first employed for this class of wagon, they were of the built-up, riveted, mild-steel, plate type, similar to those used on traction engines, but, strange as it may seem, it has been amply proved that the cast-steel wheels (Krupp, supplied by Aug. Reichwald) stand the vibration very much better than did the built-up, mild-steel type. The preference for the latter upon tractors is due to the fact that in such large diameters cast-steel wheels would be both heavy and expensive as compared with the others.
These larger diameters, of course, and the much less weight which such tractor wheels carry, are of considerable effect in reducing the vibration to which the wheels themselves are. subjected.
Composite Wheels.
With a view to retaining the advantages of the steel wheel and reducing its extreme rigidity, wheels are made wherein the usual woad felloe is fitted on a sole plate which is cast in one with the spokes and centre. This also enables an ordinary steel tire to be fitted and renewed when necessary.. Examples of these are the Leyland and Hercules wheels, both of which were to be seen at the show. In the " Samsonian " wheel (illustrated last week in "THE COMMERCIAL MOTOR_ "), which is a modification of the " Herculean," the centre is enlarged so that the usual type of hub may be fitted, this being held tight with wood wedges driven between the hub and the centre ring of the wheel proper. I believe this wheel has stood very well in practice, though I have not had personal experience with it over any length of time, yet, to me, it seems that, if the wood is to serve its purpose of giving resiliency to the wheel, it must suffer in the process. There is comparatively little wood in the length of a radius, and it is held between the solid tire and the unyielding centre it is apt to be crushed—for that is where its resilience lies—beyond its ability for complete recovery.
The " Sentinel " wheel, shown on the wagons of that name, has a cast-steel centre and spokes somewhat similar to the " Leyland " wheel, but the sole plate is coned. The under side of the wood felloe is also coned to agree, and the felloe, separately built up into its tire, is drawn laterally on to the wheel, and held there by bolts passing through suitable lugs cast on to the wheel. Should any slackness arise in use, it can be taken up by tightening the pulling-on bolts. This arrangement also admits of the steel tires' being replaced by rubber on the same wheels, when required for special purposes, and, though this system has its advantages, I am doubtful whether its practical utility would warrant the expense, and also whether it would not be likely, except in careful hands, to cause more trouble than it would obviate,
Messrs. Scammell and Nephew were showing in the gallery another type of wheel with a similar objective, though in this case it is not possible, so far as I could see—as usual, there was no one capable of explaining—to change the tire. This wheel, known as the Hindley, was illustrated in our issue of the 2nd inst. It is built up of two, steelplate, outwardly-flanged, and double-coned discs, so that what would correspond to the sole plate forms a shallow concave V with the point cut off. The two discs are attached to the respective sides of the hub, and they do not meet at their peripheries, but are bolted through. The wood felloe, V-shaped to correspond, with its tire, being placed in position, the bolts in the discs are tightened up, thus causing the disc flanges to grip the V of the felloe, and so to tend to tighten it outwards in its tyre. This seems to me a neater and possibly lighter job than the former of the type just referred to, but it has also the disadvantage of comparison which I have mentioned. The Smith Flexible I-Iub wheel (see " THE COMMERCIAL Moroa " of the 2nd April) was the only type of the truly resilient or " spring " wheel en evidence at Olympia, if the "Tangent" wheel be excepted, but the subject of " spring'' wheels and elastic hubs, etc., is one by itself, and it is impossible to deal with it in the present article, but I may possibly return to it on another occasion.