The Road Conference at Olympia.
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Abstracts from the Papers (continued).
THE DESIGN OF MODERN MOTOR VEHICLES IN RELATION TO EXISTING ROADS.
lily Cot. R. E. CROMPTON, GB., RE., M.Inst.C.E., etc.
It is evident that much may be done by the designer of modern motor vehicles to reduce the wear and tear of the road surfaces on which they are intended to run, At the present time this matter is of great importance, and I think it would be useful to set before you a few of the points which designers are now studying, but which in order to render them as successful as possible need the closest co-operation of those responsible for the design and upkeep of the roads themselves. I recently prepared a paper which was read before the Institution of Civil Engineers on modern motor vehicles, and I then pointed out that, whereas the designer of a railway locomotive has to deal with a roadway over which he has some control, the designer of the road locomotive is compelled to design the wheels of his vehicle, and to design the vehicle itself, so that it can adapt itself to roads over which he has no control, and which, under existing conditions of road management, differ ‘videly in their general contour and in the character of their surfaces. These surfaces necessarily vary in different parts of the country with the nature of the materials used as road metal, and in the amount of money that the local authorities responsible for the roads can afford to spend on their upkeep. The chief point which must be observed by a designer who has this fact before him is the adapting of the wheels of the vehicle so as to roll over the surfaces of the roads with as little crushing and abrading action as possible, to arrange his wheels so as to spread his weight over as large an area as possible; and in doing this he must consider the effect of the relative diameter of the wheel he uses, the number of wheels over which the weight must be distributed, and the methods whereby he obtains the requisite road adhesion or tractive effort.
Load per Unit of Tread.
The regulations issued by the Local Government Board for motor wagons and similar vehicles lay down the maximum crushing load on the road per inch of width of tire at 74 cwt., or 810 lb. [This is for wheels 3 feet in diameter.—En.] It is a very much lower figure than has obtained in the past, and there is reason to believe that as this regulation is now being strictly enforced a noticeable reduction will be found in the wear and tear of the roads traversed by these vehicles, but this is only one limitation. The strains on a road surface produced by a horsedrawn vehicle are of two classes—the crushing strains which for equal weights and for equal tire widths are the same as those for motor vehicles, and the other or traction strains produced by the angular impact of the iron-shod horses' feet, which is a well-known cause contributing to the destruction of the surface, particularly when the road is soft after rain or after the breaking up of frosts.
When the vehicles are self-propelled and the tractive effort has to be transmitted through the wheels, it becomes important to know what will be the horizontal stress or tendency of the driving wheels to move the road metal backwards, also measured in pounds per inch of surface. Now this tractive effort varies greatly. It reaches its maximum when heavily loaded vehicles are climbing gradients. Experiments have shown that this horizontal or nearly horizontal strain transmitted to the road metal cannot much exceed one-third of the crushing strain, for the simple reason that the adhesion with smooth wheels is, under ordinary circumstances, limited to one-third of the insistent weight. It is probably slightly increased on hard roads by the cross strips which are used on traction engines, and which are allowed in a modified form by the regulations of the Local Government Board, but I do not think that the increase of tractive effort, and hence horizontal sftains on the road metal, are greatly increased by the cross strips when the road is dry and -the co-efficient of adhesion is at its maximum. The cross strips are much more useful for soft ground, or when the hard surface is covered with a thick greasy mud, as at such times they are the only portions of the wheel which come into close contact, and therefore really have the effect of somewhat reducing the surface contact. From this it will be seen that the designer cannot benefit the road by increasing his bearing surface to an unlimited extent, as he is always compelled to put such weight on his driving wheels as will prevent the possibility of their floating on a layer of liquid mud, for under these circumstances the wheels will from time to time partly revolve without dragging the train forward, and when they regain their tractive effort the strains on the road metal are doubtless momentarily much greater than they would be if the wheels had never commenced to slip. Larger Wheel Diameters Important.
Hitherto both designers and road engineers have apparently thought the only way out of the difficulty is by increase of width, and to some extent by increase of diameter, of the wheels. The Local Government Board regulations allow an additional 1 cwt. per inch of width for each foot of increase in diameter of the wheel, and it is probable that they are correct in doing this, but, unfortunately, increasing the widta of the wheel does not produce the increased surface contact that is desired. This is due to the irregularity in the surface of the road, and more especially to the extreme camber which is too frequently given and does not allow of a very wide wheel ever coming into line contact with the road throughout its entire width. Far more commonly these very wide wheels are very heavily loaded at their inner end, and the crushing strain gradually diminishes towards their outer edge, in many cases there being practically no weight carried by the outer one or two inches of the wheel. It is evident, therefore, that if we take a very common case of a 10-inch wheel, 3 feet in diameter, which under the Local Government Board regulations would be allowed to carry a weight of 8,400 lb., better results will be obtained if we can distribute our insistent weight and our hauling stresses over two wheels 3 feet in diameter and only 5 inches wide, for we must remember that, as the surface of the road is to some extent elastic, the road material would have to move, or be crushed, to only half the extent to enable the wheel to come into full line contact with it, throughout the line of 5 inches, as would be the case if the road had to accommodate itself to one 10-inch wheel carrying the same weight. From this it appears that it is most desirable that constructors of road vehicles should endeavour to distribute both weight and driving strains over as large a number of wheels as possible; and this is now being carried into practice in the Renard train which has recently been shown in this country, and in two ways. The Renard train not only distributes the weight over four vehicles instead of one, but it makes these vehicles six-wheeled, and uses the centre pair of wheels of each as driving wheels with compensating evices so arranged that about half the weight on each vehicle is brought on to the central pair of wheels, the front pair and the back pair only carrying the remaining half between them, This arrangement is evidently an excellent one from the point of view of the road engineer. The strains are distributed over a large number of wheels, and these can be reduced in width to the 5-inch minimum allowed by the Local Government Board. The introduction of six-wheeled vehicles has, too, another important effect, as it will be seen that when one of the wheels of a four-wheeled vehicle passes over an obstacle that corner of the vehicle is lifted to a much greater extent than would be the case if that wheel, instead of being one of two wheels on one side of the vehicle, were one of three. As a consequence, the blow given to the piece of stone, or the brick, or obstacle on the road, on climbing over it, is reduced, and the blow given to the road in descending from the obstacle is also similarly reduced, so that both the shocks to the vehicle and to the roadway are greatly reduced. It is said this reduction of shock to both would appear to be only 50 per cent., that is, in the proportion of the three wheels to two wheels, but the reduction is really found to be greater than this.
Design of Leading Wheels.
Another point of importance to' designers of road vehicles lathe consideration of the steering gear. So long as the steering gear was of the old traction-engine type, where the leading axle is pivoted at its centre, the damage done to the road by the act of steering was practically nil, but since the general introduction of the Ackermann system of steering, the two front wheels being each of them attached by vertical pivots to the two ends of the front axle, it is evident that unless the vertical pivot is directly over the centre of the tread of its wheel considerable strains are brought on the road surface by the act of moving the wheels through an angle, and this is particularly the case with heavy motor wagons. With lighter vehicles also, if the steering wheels are out of adjustment so that the axis of both wheels when the vehicle is being driven straight forward are not truly in line, one or other of the steering wheels must be always slightly slipping sideways with results both disastrous to the rubber tire and hurtful to the road, for this side movement at high speed has the effect of stirring up an extra quantity of dust or the loose particles lying on the surface of the road, with the result that they are blown away by the great draught or air current which passes under the body of the vehicle when it is driven at any considerable speed, or even at a very low speed when meeting a head wind. Excessive Camber.
As has been said in the early portion of this paper, a great number of roads are made with far too great camber, and as it may be many years before this camber can be cut down to a correct figure, so that the wheels can rest throughout their entire width of line in contact with the road, designers of road vehicles have in the past considered, and they are still considering, methods by which the tires of the two wheels, instead of forming one continuous cylindrical surface, may be allowed to accommodate themselves to the camber of the road. This, of course, can only be done by dividing the axle and putting springs inside and outside each wheel, but the arrangement introduces considerable complexities in design, and for this reason is not likely to have much success. The true remedy for this trouble is for road engineers to realise how important it is for their own sakes that the excessive camber should be cut down.
It is probable that road surfaces also suffer in the same way as the rubber tires do from the irregularity of the driving power introduced by the use of the four-cycle "Otto" internal-combustion engine as a driving power. This is very notable with a one-cylinder vehicle. The irregularity of the driving impulses undoubtedly produces increased wear of the tire, and in so doing must increase the disturbance of the surface of the road. This irregularity in driving entirely disappears in the case of electrically-driven vehicles, and almost entirely in the case of steamdriven.
THE PLANNING OF THE ROADS FOR THE NEW TRAFFIC.
By REES JEFFREYS.
Foresight is a characteristic of individuals. It is a quality in which corporate bodies are notably deficient. History affords many illustrations of this fact, but no department of collective activity furnishes more examples than road-making and maintenance. After the Great Fire of London Sir Christopher Wren planned out a new city. Wide arterial thoroughfares were to be connected at proper intervals by cross-streets ; provision was made for the existing traffic, and the growth of traffic for many years to come. Unfortunately neither the City Fathers nor Parliament could foresee the advantage of rebuilding London according to some definite and well-thought-out scheme. The City was permitted to become a collection of mean streets and alleys.
Costly Street Widenings.
Succeeding generations have had to pay millions in street improvements, and tens of millions have been lost to the public by reason of street obstructions and traffic delays because Sir Christopher Wren's plan was not adapted. Experience did not teach. Again and again the authorities of London have had opportunities for laying down arterial thoroughfares. They were all permitted to slip. Only when the congestion of traffic had got intolerable were clearance schemes carried out, and Marylebone roads, Shaftesbury avenues and Kingsways built at enormous public expense. Still the lesson has not been learned. In every suburb of London—Brentford, Kingston, Croydon, Wimbledon, Bromley, as well as in the north and east —streets have multiplied without reference to any well-considered scheme. No new arterial thoroughfares have been built. The new streets only serve to feed the existing highways which are inadequate for their existing traffic, and have been for years past. The Metropolis is faced with the necessity of not only widening its Piccadillys and buildings, its Kingsways in central districts, but also with the necessity of widening its Bath roads, North roads and Brighton roads, and constructing alternative thoroughfares through many miles of suburban areas.
When will the intolerable burden of street widenings and street improvements reach that point when even public bodies will show a little foresight and learn the necessity for widening their arterial thoroughfares and planning new ones before builaing operations commence? This is the practical question which I should like to bring before a body of experts this afternoon: Some millions of capital invested in underground railways and tubes; these are most expensive substitutes for good broad arterial thoroughfares upon which traffic could travel without being delayed. When look at in a purely intellectual light, it is an absurd state of affairs that road vehicles have now been constructed that can travel conveniently at a mean speed of 30 miles per hour, yet in no wise are they able to maintain this speed in urban or suburban centres. The problem is to adapt the existing roads, and to build new roads, so that the public can obtain the maximum advantage of the improved means of locomotion at a minimum of expense. How is it to be done?
A Central Authority Wanted.
A close investigation of past history shows that the primary cause of London's failure to deal adequately with the traffic difficulty has been its large number of authorities, and the conflicting nature of their functions. No single authority has ever been constituted to aclise or deal with the roads in the metropolitan area. The roads in the metropolitan area are divided for administrative purposes among 100 local authorities. The number of authorities and corporations having statutory rights and powers in connectinn with roads amounts to a very large number indeed. There is no attempt at unity. The first thing essential in any general scheme of highway improvement is to create a central highway authority for the metropolitan area. Among its first duties should be to consult with all local authorities, and to draw up a plan of greater London to which all new building developments and further improvements should he required to conform. Greater London should be required to develop according to some carefullyconsidered plan. A great artery should be widened before building operations commence. Alternative arterial thoroughfares should be planned so that, as estates develop, one new street would continue another. The result would be that, in 20 or 30 years time, instead of having a chaos of mean streets all feeding existing arteries, alternative arteries would be automatically constructed. As time and finances permit, alternative Bath roads, Portsmouth roads and Great North roads should be constructed. As a general rule it will probably be found that the construction of an alternative road costs, in the long run,. far less than widening an existing one. Compensation for property taken for road purposes should be on a reasonable scale. Procedure for taking land compulsorily for road improvements should be much simplified. This, then, should be the first duty of a central highway authority.