The Making of Roads.
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Excerpts from a Paper by the Consulting Engineer to the Road Board.
Colonel E. E.Crompton, C.13., a past president, yead a paper, distinguished by much originality of thought, before the Institution of Automobile Engineers on -Wednesday last. He chose for its title " The Wheel and the Eoad," but we may perhaps be excused if we point out, that little of his writing deals specificaily with the wheel, although he once again recommends, in the earlier part of his paper, the employment of large-diameter wheels, and gives expression to regret that the designers of commercial vehicles " have studied this question so little." The author considers that the diameter of rigid steel motorwagon wheels should be at least three times the diameter of rubber-tired ones carrying the same weight in order to do no more damage to the roads.
Now Colonel Crompton writes that the chief cause for the non-adoption of large-diameter wheels has in the past been their great weight, and incidentally, the difficulties of loading vehicles from the side when they are fitted with big wheels. These are not the only difficulties, in our experience. We yield to none in regard to the certainty of the lower destructiveness of large-diameter wheels, but we have a lively appreciation of the difficulties which confront designers who would, were it practicable, gladly embody such components in their standard chassis, if for no other reason than that the small wheel is destructive of chassia detail, especially of machines running at anything like high speeds. The designer and constructor find it difficult on slowspeed, high-load capacity machines satisfactorily to provide for the necessary large final-gear reduction for which big road wheels call.
The bigger the wheel, obviously the greater its weight. Large-diameter wheels are expensive to tire and to keep tired, and they are extremely awkward to handle for replacement purposes. Their weight, if of the modern cast-steel type, becomes almost inadmissible. whereas, if of the artillery type, their greater length of spoke renders it more difficult than ever to avoid creeping and distortion. So that, however desirable the big wheel may be from the point of view that it imposes a smaller tangential destructive effect on any given road surface, it must be admitted that vehicle constructors at any rate have much to justify their retention of the smaller-diameter wheel. To set against the facility with which the large wheel rides over small depressions, there is the fact that its greater unsprung weight must obviously produce a more destructive hammering effect upon the road crust.
This question of the most practicable qua the most desirable diameter of a road wheel is obviously one inviting controversy ; much has been written concerning it in the past. Colonel Crompton's welcome return to the subject we trust will result in even further careful consideration of its problems by chassis constructors as well as by those who naturally would have the vehicles built entirely to suit the roads.
The bulk of Colonel Crompton's paper is an interesting epitome of his acquired experience in regatd to the very latest forms of scientific road construction, and it is. therefore, from that point of view a valuable contribution to the literature available to those who use the road and the wheel.
He ridicules the india-rubber road---that favourite topic of the daily Press—and thereanent he writes in his latest paper:—
" Oar ' well-informed ' daily Press has frequently pointed ut that the ideal road of the future will resemble the carriage way under the entrance to Euston Station, where a short length of rubber-surfaced road has been laid down for
many years. We are told that the ideal road must eventually . .
be faced with rubber. What such a road would riet. even if
rubber was reduced to 6d. a pound, the author leaves to the imagination of the members."
The author considers that the aim of the modern read builder should be to secure the right amount of elasticity in the road material itself. In his opinion, so we read:—
The chief reason for the success of the methods which have been intredueed during the past eight or nine years, and which have resulted in reducing the mud, the dust, and the noise of travelling, has been really due to the small amount of elastic enshiiming which has been already introduced into the an chic,, of these roads."
Colonel Crompton writes interestingly of what in his opinion is the most important aspect of road wear. HP continues:—
" Alany people' including a considerable proportion of the road surveyors, have, up to quite recently, looked on the question of the wear of road surfaces rather superficially, that is to say, they have always considered that surface wear was the most important matter, and have selected for their macadamized roads the stones from those rocks which offer the greatest resistance to attrition. In this, however, they are wrong. The main source of the mud and dust found on our roads arises from the inter-attrition of the stones of which our macadam roads are composed. This inter-attrition arises from the rocking action above alluded to, which is caused by a rolling wheel, and which, again, causes the sides of the stones to rub against one another, and thus gradually removes the angles and changes the stones front the familiar angular shape in which new road metal is seen previous to its being spread on the road to the rounded form in which it is to be seen when a road is broken up for re-coating and repair; in other words, the wear of the road material, far from being confined to the surface as would be the case if the stones were firmly held in position and subjected only to attrition on their upper surface, goes on by grinding away the whole surface of the stones, and this affects the stones next the surface to the greatest extent; in winter weather, however, the inter-attrition extends to a considerable depth, the quantity of mud produced by this attrition increasing in-direct proportion to the depth to which the rolling or rocking action of the stones extends. All improved pavements have for their object the reduction of this interstitial wear, and the confining of the wear as far as possible to the upper surface of the blocks or of the sheet asphalt."
The very special characteristics of the sheet-asphalt road surface are succinctly.discussed in the following paragraph. Mitch which follows is of interest concerning the use of surface tarring and the various methods of using pitch in connection with road surfacing. The author also has much that is instructive to say with regard to the choice of suitable bituminous binders.
" A sheet-asphalt pavement may be considered as a macadam mad in which the particles are of extremely small size, and are prevented from rocking and consequent, interstitial wear by the interposition between each particle of a very thin film oh bituminous cementing matter, and as the surface of an asphalt pavement is thoroughly well sealed and the entrance of water prevented, it does not suffer as a waterbound macadam road does by the lubricating action of the water causing
• wet grinding ' of the particles which always goes on in a wa(erhmind macadam road during the wet season."
Augury of careful and skilful investigation of all the circumstances producing and accruing from road wear by modern motor and other traffic is the fact that the Road Board has sanctioned the design and construction, so Colonel Crompton tells us, of the first " largescale road machine." This will consist of "a building containing a circular track, on which experimental lengths of roads can be readily laid down and tested to destruction by the passage over them of wheels driven by motors which are guided and steered in a eircular track by a revolving framework."
!We shall deal further with this paper next week.)