What Causes Road Waves ?
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Is Colonel Crompton's Harmonic-percussive Theory the Correct One? Are not Traffic Units Too Varied to Yield Cumulative Results on the Roads?
Much of an interesting paper which Colonel R. E. Crompton, C.B., the well-known consulting engineer, read on the' 19th December, before the Institution of Mechanical Engineers, corresponded with the contents of his contribution to the proceedings of the Manchester Association of Engineers on the previous Saturday. We, therefore, find it inexpedient to devote much space to this further technical contribution to the discussion of road-construction methods and their relationship to the vehicles using the highways. We have, however, some criticisms to offer.
The recent London paper opened with a further reminder of the nature of the pioneer work which was carried on by Col. Crompton in India in the "sixties" of last century, but the greater portion of the contents consisted of a record of current practice in respect of plant and methods which are employed for the most modern classes of road making. These particulars, although of general interest, are not specifically important to the vehicle user arid manufacturer, and we, therefore, do not propose to discuss them at any length, contenting ourselves rather with referring those of our readers who wish to go into these aspects of the subject more deeply to the paper itself, copies of which will no doubt be forwarded to genuinelyinterested inquirers by the Secretary of the Institution of Mechanical Engineers, or by its author.
The Analogy of the Rail.
'What we again find of much interest in these further utterances of Colonel Crompton is his praiseworthy attempt definitely to theorize as to the cause of road-wave Formation. He spoke at some length on the same subject at Manchester. It may be recalled that, in his previous paper, Colonel Crompton drew a useful analogy in respect of wave formation as between the effect of rail vehicles on the permanent way and that of road vehicles on the highway surfaces. To our way of thinking this is a very useful line of thought, and one that should considerably help towards the elucidation of what is undoubtedly a very puzzling problem. As the lecturer said at Manchester, corrugation of rails, the cause of which, of course, has been a problem for permanent-way engineers for many years past, is now with considerable unanimity accepted by authorities as being due to the percussive effect of steel-tired wheels, initiated by preliminary bumping over points and rail joints.
The Diversity of Loads, Speeds. Tracks, etc.
One is naturally disinclined without hesitation to accept the 'theory that road waves are due solely and entirely to similar action on the part of read vehicles of multitudinous types and of very varied weights and loadings, travelling at wide ranges of speeds and at times when the roads so affected are subjected to extreme differences of atmospheric condition. Colonel Crompton, of course, does not claim that wave formation is due in the first place to this percussive effect ; he relies on the proof, which he has secured with great and commendable care, that all freshly-rolled roads exhibit this characteristic to a. greater or lesser extent. He —and with considerable reason we must admit—asserts his belief that it is this initial waving, in itself a constructional defect, which gives rise to the first bouncing of wheel on road which, he then believes, becomes and continues to be harmonic, The considered conclusion of tramway and railway engineers which we have recorded above goes very far towards confirming Colonel Crompton's theory, but we cannot admit to entire persuasion thatwe have ise arrived at the root of the matter. Subsequent to the last paper, on the 19th ult., Mr. Dryland (Surrey County Surveyor) drew the lecturer's attention to the fact that this road waving was not only not a new thing, but that it had been troublesome to surveyors for many years past, and there are other observers who are willing to testify that the same condition of affairs 'actually existed, although probably, on account of the slower speeds and the lower axleweights, it .then was less marked, long before the advent of the. first motor vehicle.
Only One Homogeneous Fleet of Any Size on Regular Routes.
Complete agreement with this harmonic-percussive cause of road-wave formation makes it necessary to assume that a very large preponderance of the vehicles using any particular surface under observation must correspond, in their combined effect in regard to their periodicity, and this regardless of varying loads and varying speeds even in the one outstanding example of a homogeneous fleet using definite routes, such as that of the " B-types " and Daimlers belonging to the L.G.O.C. group. There is practically no other example of the same kind which can be quoted. There are other considerable fleets of machines of similar types, but they are not used in regular service to anything like such an extent over the same routes, nor is the incidence of their passage of such moment in respect of the bulk of the other traffic.
One could accept the percussive theory with more readiness were the roads so affected exclusively used by machines of identical type, of similar age, shod with the same tires, and loaded identically. With the data so far available, it is difficult, however, to reach the conviction that the percussive effect arising from so many other types of traffic, varying from the lumbering, slow-speed, heavily-loaded steam wagon, to the high-geared, heavy touring car fitted with big steel-studded pneumatics and long springs, does nothing to destroy the wave effect which it is claimed is presumed to be produced by any fleets of identical units.
Do the Wheels Hammer Depressions Deeper ?
Properly to adopt the theory that wave formation of made roads is due to the harmonized bumping of the vehicles using them, it is necessary also to suppose that each machine will exactly bounce into each trough and over each peak, that the full effect of each blow and the limit of its intensity is reached just as the machine arrives over the geometrically-formed depressions, which would appear to be a most unlikely regular happening. It is conceivable that if there be bumping on a wavy road, it will be the crests which will receive the harmonized blows. Observation of a bus wheel on a badly-waved road confirms this optically. In respect of the rail analogy, the axle units there, hundreds of thousands of them, are identical, as are all other conditions. The corrugationm are of short period only, if we recollect rightly. The whole subject is one which is worthy of very considerable discussion, and is of exceptional interest, not only to road builders, but to vehicle users. Colonel Crompton's painstaking efforts to investigate this subject are, 'therefore, particularly welcome. The section of Colonel Crompton's I.M.E. paper which records the interesting investigations which he has made into the movements of the stones or other components of road surfaces is quite absorbing, and it is exceptionally interesting to learn authoritatively that the bottoms of the depressions in these wavy roads invariably reveal the presence of the smaller components in the stone formation, whilst the larger ones go to build up the peaks. It is not quite obvious, however, that such re-arrangement of pieces is not traceable first of all to the road rolling during construction, and subsequently to similar action arising to a less extent from the. rolling action of all vehicles.
Colonel Crompton, in discussing Mr. Lanchester's theory, advanced in 1907, that vehicles of weight moving at sufficiently slow speeds should automatically reduce the wave effect, considers that speeds low enough to produce this are not likely ever to be observed, and that, therefore, such effect is negligible. The reader of the paper proceeds as follows: The wave-forming action of the modern self-propelled vehicle is far more intense than could have possibly been the ease x-itb horse-drawn traffic, one mason being that the speed being greater, the change from true rolling to pulsating action is therefore greater. The wheels as a rule are of smaller diameter, are more equal in size, and the general harmonic characteristics of the mechanically-propelled vehicles re,ctidile one another far more closely than was ti0 case with horse-drawn vehicles; for of the latter many were slow-moving two-wheeled spcingless carts with wheels of large diameter, others were light carts or four-wheeled wagons with unequalsized wheels. There was also the blow of the horses' feet whieh must have considerable modifying effect on the road. surface; therefore, in all essentials, the conditions of horsedrawn traffic were far more heterogeneous and non-harmonic then the new traffic.
The Horse Has Not Yet Disappeared.
It will be seen from the above that the assumption is made that the traffic now almost entirely consists of moor-drawn vehicles. The two-wheeled springless carts, with wheels of large diameters, etc., are spoken of as haring existed. It must not be forgotten that hca'se-drawn traffic still constitutes a very large proportion of the volume of present-slay traffic.. In a ft w instances it is still the major portion.
Further on we read :
Again, coinnercial vehicles now consist of large fleets of wagons or vans which are practically identical in all their icirmonie features, and as these run over reads as regularly as a train service on a railway they have a severe harmonic effect on their surface. If we examine the effect of a line of motor omnibuses all having the same characteristics of total weight, of unsprung axle-weight, period of springs, length of wilpelbase,, vibration of engine, and Nilich traverse e road at approximately the same speed at regular intervals of
time, it is not surprising to find that, with such traffic, wave formation amounting to cross corrugation is rapidly set up, and that roads of the old type become broken up from this cause in a few months, and that the wave formation produced is very regular and definite in wave length.
As we have mentioned above, we know of few "large fleets of wagons or vans which are practically identical in all their harmonic features and which run over roads as regularly as a train service on a railway." The large delivery fleets all run in radial directions, and but few similar vans follow each other. Again, in 'only a few eases does one user's fleet correspond in type to that of another.
VIle repeat 'that the only maniple from which it is possible as yet to draw more or less conclusive data, on this percussive theory is that of the London Genera] Omnibus Ga.'s concentration of similar vehicles on regular routes.
The Three-axle Roller.
The rest of the paper, as we have said, dealt in interesting fashion with Colonel Crompton's three-axle roller and with its behaviour during constructional work, and with yet another hint to vehicle manufacturers that road wheels of large diameter are advantageous both from the point of view of the roads and the vehicles. This, of course, has long been accepted by manufacturers and users alike. Constructional limitations are serious handicaps.
Bad Roads Increase Running Costs.
Colonel Crornpton concluded with summarized tabulated particulars which are intended to demonstrate that the improved road should save the vehicle user es much as id. a mile on vehicles of up to 2,1i tons gross weight, and 4d. per mile on heavier machines. As, however, the figures are based on the assumptions that the average .cost for the lighter class at present (running on pneumatic tires) may be taken as 8.2541, and that of the heavy five-ton vehicle as 16d., and that the corresponding costs " as they will be" have been ascertained as "those now obtainable from vehicles running most of their mileage on well-surfaced urban roads, and due not only to savings in fuel and repairs, but to increased speed and increased mileage," we fear that these figures can only be taken as an 'attempt to put into writing what has all along been accepted, viz., that bad roads put up the costs very severely for users.