A Criticism of Wheels and Wheel Construction.
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By R. G. L. Markham, M.I.Mech.E., M.I.A.E.
In the early days of motoring, and particularly of commercial vehicles—they were nearly all .steam wagons then— wheels and their fittings were the most negleeted of any part of the mechanical equipage. In the last few years they have received more attention, but a careful look round at Olympia indicated 'that there is room for improvement yet ; that is, of course, taking the show as a whole. There were individual examples of sound design and good construction, so far as the latter can be distinguished beneath a veneer of show finish, but there were also some specimens which, were they submitted to me in my professional capacity, I should feel bound severely 05 criticise, to put it mildly.
The Functions of a Driving Wheel.
As a general deduction from what was to be seen at the show, and from what is to be heard outside it—the creak, creak, creak, of nearly every motorbus wheel that passes, I can only conclude that the functions of the wheel as an integral part of the propelling mechanism of a heavy motor vehicle are not sufficiently understood or considered by those responsible for the general design. In the lighter types of vehicle, the weights and powers are generally less, whilst the service conditions are not so severe, and the factor of safety in the construction of wheels is greater ; consequently, the same troubles do not arise. For the present, therefore, these light wheels may be passed over.
With regard to heavy vehicles, one must have a more or Jess clear comprehension of the strains to which a driving wheel is submitted, in order to appreciate the points of dif
ferent methods of construction. In the first place, the wheels have to carry the whole weight of the vehicle and load. In the case of the back wheels,, this may amount to ten tons or so on the axle, or five tons per wheel, as some margin is necessary over and above the legal eight tons on any axle. Now, if the wheels thus loaded were to run continually upon ideally smooth paving, there would be little difficulty in this direction, but, unfortunately, the authorities have very little consideration for the wear and tear of motor vehicles and their wheels. The Borough of Hammersmith is one of the worst offenders in this respect, as the state of the roads westward from Olympia testify. Over roads such as these, full of lumps and hollows, a wheel is constantly bumping, for bad wood paving is worse than bad macadam. It is like a series of hammer blows on the wheel, with a mass of from three to five tons, and with a force in proportion to the speed of the vehicle. Think, for a moment, what this means when going on all day long. C'onsequently, and apart altogether from the driving and braking effects, it is not merely dead weight that has to be considered in proportioning the strength of a wheel to carry its load.
Strength in Relation to Load.
It may not have occurred to everyone why rubber tires are used upon motorbuses and other vehicles. The reply, of course, will be that it is in order to obtain greater speed, an easier running. Yes, but why? Upon absolutely smooth paving, it would be possible to run as fast and as smoothly with steel tires as with rubber. The real object of the rubber is to minimise the shocks due to the bumping over uneven roads ; it is, in effect, a buffer placed between the wheel and the road, which may be regarded as continually hitting it. And this brings us to other considerations respecting the life of tires, but I fear there is no room to deal with them in the present article.
Dealing with the wheel itself, the load has to be borne by the spokes, and the road shocks have to be transmitted by them to the axle. The feline and tire, or rim, are merely accessories for other purposes, though, if strong enough, they may serve to some extent in the distribution of the strains and stresses. In a wooden wheel, the spokes above the line of the axle have no part in this duty ; those in, or near, the horizontal line may be practically neglected ; and, of those below the axle, the ones nearest the vertical bear a greater part than the others, Therefore, it may be accepted that the whole of the load and bumping strains are borne by three, or at most four, spokes at any one moment. Practically, and particularly as regards shocks, this may be reduced to one or two spokes. So far as the strength of the spokes is concerned, it is obvious that there is a great divergence of opinion. Taking two examples at random, the Ryknield " five-tonner " has twelve spokes, whose crosssection (neglecting any widening for the support of the felloe) is 5 inches by Ili inches, whilst the Maudslay "threetanner" has 14 spokes each 6 inches by 31 inches in section. It thus appears that the vehicle for the lighter loads has spokes more than 2!; times as much in cross-section as the other, whilst regarding the wheel as a whole, the former should be more than three times as strong as the latter.
Both vehicles may be reckoned in the first class of motor construction, and they would probably agree more or less in the proportionate strength of their other principal parts, but, obviously the wheels cannot both be correct in regard to strength for loads. There is no doubt the Maudslay has erred on the right side, but the resulting wheel is heavy and ungainly.
This general divergence is further emphasised by the fact that some makers have weakened the spokes by drilling through—in a few cases right at the root, just outside the hub flange—for a brake-drum attachment, which is undoubtedly bad practice. In the case of the cast-steel wheels, which seem to be coming more into vogue, and with which I will deal presently, there seems to be nearly as much diversity in the strength of spoke as in wood wheels. Some of them, as on the Milnes-Daimler vehicles, are thin and neat, whilst others are, apparently, unnecessarily heavy and
unsightly. „.
The Effect of Road Shocks.
The cross-sectional strength of spoke is not the only point to be considered in respect of load and shock. Wood doc.!s not readily shrink or compress in the direction of the grain, as in the length of a spoke, but across the grain it is another matter. Each spoke end butts against the felloe across the grain, except for the tang, which enters into a corresponding hole in the felloe. Now all the weight and the shocks are transmitted through the feriae, and there is naturally a tendency to crush it where the spoke bears on it. In course of time, a very slight crack or space may appear between the two, and the trouble then becomes greatly augmented, for, at the top of the wheel, the spoke tends to draw from the felloe, while, as it comes to the bottom, it is crushed and
hammered into the felloe, the crushing of the latter being aggravated by the space through which the spoke is able to work. Once slackness commences here, " working " will occur at the hub, and then arises the creaking which is so noticeable in motorbuses, though other reasons contribute to produce it.
The greater the bearing surface of the spoke on the felloe, the less the likelihood of trouble, and, in this respect, some of the spokes at the show were unduly light at the felloe ends. In the Stagg and Robson wheel, which was shown on the Wallis and Steevens steam lorry, there is no tang, the spoke end being carried in a shoe attached to the felloe, and, therefore, a greater bearing surface is obtained, which is one of the advantages of this wheel. A similar advantage attaches to the " Tangent " wheel, examples of which were shown on one of the Dennis stands, and by T. C, Aveling, Limited. In this case, the spoke is wedge-shaped at the end, the whole of the width of the spoke forming the tang.
Weakness of Spoke..r.
Passing, now, to the question of driving strains, many of the wheels in common use must be considered mechanically bad. This is the fault of the designer of the vehicle, rather than of the wheel builder. Several designers seem to have thought that, when they have applied power to the wheels, they have done enough—the wheels will go round! They seem to have overlooked the fact that the power has to be conveyed through the wheel to its point of contact with the ground ; that, in fact, the wheel is an essential part of the transmission mechanism, requiring as much thought in its design as any other part. With live-axle driving, and also with chain driving when the chain ring is part of, or is attached to, the hub, the power is conveyed direct from the hub to the spokes, and so to the rim of the wheel ; yet, there are examples where the diameter of the hub plates is so small in proportion to the size of the hub barrel as to give a quite insufficient length of spoke anchorage in the hub : this is a matter that could easily be altered, and, fortunately, the examples are few.
Rather more numerous are the instances where the spokes have been weakened by drilling for the attachment of brake ringS or chain wheels, in some cases right at the root of the 'spoke—the very worst place. It must be remembered that a spoke transmits power in a similar way to that in which a fishing rod is used to land a fish. It is held at the root in the hub, as the fishing rod is held in the hand, and power is applied in such a way as to tend to bend it. This power is transmitted to the felloe in such manner as to tend to shear
off the tang, or to push the attachment, whatever it be, along the felloe and around the wheel. The spoke, in fact, is a cantilever, and it should be strongest at the hub, decreasing as it nears the felloe. It will be seen, therefore, that drilling the spokes, especially near the hub, weakens it for the transmission of strains in bending, which it is least able to support at best, and the swelling of a spoke where the hole is drilled does not adequately make up for the breaking of its grain at the middle.
Somewhat similar criticisms apply in relation to side strains, though, as the spokes are generally much thicker through (in line of the axle) than they are wide, they are better able to resist them. Staggered spokes, of which an example was to be seen on the Dennis stand, are used in some cases to give greater strength in this direction, whilst the Shrewsbury and ChaHiner Tyre Company showed a new type of wheel, with spokes interlocking at the hub to give greater strength and rigidity at this point. An illustration, showing the construction of this wheel is given at the foot of this column.
Transmission of Power.
The artillery wheel, like all other wood wheels, is intended to carry the load, and it is not primarily designed to transmit power, for which function it is mechanically incorrect. This has been recognised by some of the best makers, who relieve the spokes of all such strains by attaching the chain rings either direct to the felloe, or to the spokes at their x tre mities —where they enter the felloe. In the StrakerSquire vehicles, the chain ring is attached to six brackets, which are secured direct to the felloe, but there are also bolts, through the spoke ends, whose introduction might easily be avoided. The Thornycroft chain ring is secured by strap bolts, of a very neat pattern, which clasp every spoke
right at the felioe ends, and the spokes are [lot weakened in any way. In the case of the live-axle, Stewart-Thornycroft, steam wagon, the power is conveyed through a pair of plate springs, direct to brackets on the felloe, and the spokes have no part in the transmission of the power of the engine, but merely take the axle load.
Mechanically considered, as a wood wheel designed to transmit pewter, the " Tangent " wheel already named must be considered the only correct form. In this, the power from the hub is transmitted along and not across the length of the spoke, which, also, in any given hub, has a longer spoke root than would a wheel of ordinary type. For these reasons, among others, this type of wheel can be built rather lighter than usual for equal strength, but, of course, it is intended to drive only in one direction ; it cannot be equally strong both ways, though it is strong enough to stand the amount of backward driving, at slow speed, which occurs with a vehicle in ordinary service.
Braking Effects.
As the artillery wood wheel, mechanically considered, is weak for the transmission of power, so also is it weak in the matter of braking effects. Putting on the brake, whether upon a countershaft or upon the wheel itself, is in effect only a reversal of the direction of the application of power. Whether the wheel be driven from the hub, or retarded by braking at the hub, the power in either case has to be transmitted through the spokes to the rim of the wheel,
where it finally lakes effect. There were several examples to be seen, even when the spokes had been left intact-so far as the drivine, was concerned, where they had been drilled through for the attachment of a brake ring, and this must be regarded as a weakness, even though these wheel brakes be not much used.
The power required to start the vehicle is always very much greater than that required to keep it going, and the power required to pull it up by the brakes is nearly as much as that necessary to start it up from rest to the same speed in the same time. Now, think how often in the course of a day a motorbus is quickly started from rest, and as suddenly brought to a standstill ; remember, also, that all the power for both these operations has to be transmitted through the wheels, not continuously in one direction only, but repeatedly in opposite directions ; and bear in mind what has already been said as to the effect of weight and road shocks. One will then understand what a motorbus wheel has to put up with and will not wonder that it should complain loudly in a short time. Yet, as one manufacturer admitted to me (and I think nearly every other might have done the same), their wheels were not designed : they were made as strong as was thought necessary; if they -do not prove strong enough, then make them stronger ! if necessary, increase the dimensions still further.
The artillery wheel is right enough, and as good as any other type for weight carrying ; but, for the transmission of power, it is wrong. Many patents have been brought out, with a view to strengthening the wheel, making it more rigid, etc., but they have nearly all been in the nature of tinkering at it : with one exception, no one seems to have looked to root causes. Nor have vehicle designers generally bridged the difficulty altogether, as some have done, by applying the driving and braking power right at the Tim and leaving the spokes out of it entirely.