Opinions from Others.
Page 15
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What Causes Road Waves?
Sprung and Unsprung Weight.
The .Editor THE COMMERCIAL 'MOTOR
1112971 Sir, ••I have heard a lot of opinions expressed on the above subject. .Following last week's criticism in tile." C.M." on Colonel Crompton's popes, I should like to ask for more consideration et toe reIntir•e effects on the road of sprung and unsprung -weight. Their relative importance in respect of the classis has been fairly well discussed ; surely, as a:lion and reaction are equal and opposite,
tz,,•Is should be looked for upon the roadway • ' sur What is the periodicity of the unsprung weight ot a motorbus.? " is the question I should like to put to Colonel Crompton. That is the part. of the vehicle reass which does all the damage.—Yours faithfully, tmedoo, W.C. • " Loa:Do NI SI: PER, What Causes Road Waves?
Waves Where There are Few Motors.
The Editor THE COMMERCIAL Moron.
2[181 have perused with interest Lhe article
on the above subject in your last issue. It has oeyarred to me that there must be many roads in Loudon which are practically unused by motor traffic, old which yet show signs of this waving effect. One outstanding example of this comes to my miud as write this letter. There is a. street leading into the -Borough Road, which I think is called Lancaster Street, one end of which is paved with asphalt for alonit 50 yards. Nov. despite the fact that very little motor traffic passes through this street. the paving is a series of very marked and regular waves from end to end, How do you account for this
--Yours faithfully, " Sit:DENT." Finsbury, E. C.
What Causes Rod Waves?
The Lack of Similarity in Springs The Editor, THE COMMERCIAL MOTOR.
[1299] Sir,It was with great interest that I perused the article entitled " What Causes Road Waves 1" which appeared in your last issue. As the subject is one about which a great deal or controversy has taken place recently, possibly some notes from a practical engineer may be of interest, to your readers. • Colonel Crompton gave it as his opinion that road a. ayes are formed by the equal periodicity of the springs on various fleets of m•otor vehicles, aed he brought forward, as proof, the fact that waves quickly form on roads where motorbus services are continually running. Now I contend that the periodicity of bus springs is not constant ; it is not even eonstant fer an individual bus let alone for a he go fleet. The periodicity of a. spring depends upon its load ; when the load is light the neTiodieity will be higher than when it is heavy. Thus it will at once be seen that a bus which is continn•ally altering its sprung weight, by reason of passengers getting on and off, cannot possibly have a constant. spring periodicity. If we take the average speed of a bus as 12 m.p.h. aid the average periodicity of the springs as 100, which is very near to the actual, we find, that it travels 10.5 ft. during •oue complete vibration of the springs. Therefore if road waves wet e actually coused by the harmonic spring vibrations of a fleet of bases we should expect the crests of the waves to be 10 ft. apart, bet as they are more nearly 2 ft., this seems to me to be conclusive proof that spring period has very little to do with the phenomenon. Another proof of the above is that if bus springs eause road waves, the latter would surely synchronize with the period of the springs when the vehicle is running at its normal speed, so that, since the vertical vibrations would then receive an impetus as the wheels pass over each wave, driving at. that speed would become almost impossible. If Colonel Crompton's harmonic percussion period is correct why is it that the waves develop or remain ? It appears to me that the crests ale the parts which would have the most wear. In my opinion road waves arc usually formed during the construction of the road ; this particularly applies to macadam roads. After the road is laid and while it is soaking with water, a. steam roller is used to bed it down. This roller pushes n. wave of loose stones in front of its wheels until the wave is sufficiently large to resist the roller, when it is surmounted and ilattenc•d down into a hard ridge. This action continues all the time the roller is iti motion, hard ridges with sofi hollows between being formed at poetically equal distances along the road. It has b•een noticed that the wave is usually composed of the larger stones, while the smaller drop back into the hollows.
During the time the rolled road is drying a gradual substdence takes place, this being less marked in the high. ridges than in the hollows. In this manner a series of waves is formed even before the advent of traffic, other than the original roller. Once road waves are formed every vehicle passing over the roads tends to accentuate them. This is particularly the case in wet weather, as when the hollows are full of water and wheels rush through them, water and mud are splashed out in waves to either side, and the hollows rapidly enlarge. Another cause of extra wear in the hollows is that. they amo kept soft by the water, while the -crests remain hard and almost. dry. In some respects a new road resembles an undeveloped photograph, the action of moisture and wear acting as the developer which brings the waves into prominence.--Yours faithfully, Streatham Common.
What Causes Road Waves?
The Method of Laying the Ballast.
The Editor THE COMMERCIAL Mcsroa.
[1300] Sir,---Your critique under the above heading, published in your last issue, opens up a problem which it is the undoubted duty of our road makers to use their best efforts to.solve.
I must differ from that point brought forward by Colonel Crompton where he states " the deformation of road surfaces, caused by modern traffic, is due to the rolling action of the wheels at the speeds now employed for self-propelled traffic being no longer true rolling, but more in the nature of a rhythmical permissive action, the wheels striking the ground at regular intervals." Surely, with the maze of traffic of vehicles ranging from the parcelear to the machine of heavy axle-weight, there can be little uniformity or rhythm in the percussive action?
I consider that a. feature which helps to a very considerable extent to deform our roads is the method utilized in constructing the under layer of ballast. The tar-covered pieces of granite which are used in. modern road construction cannot, until subjected to consistent high pressure, take up their natural position. Consequently, it takes a period of time for the high points to be worn off, and thus, in my opinion, are the waves generated, the initial corrugations haying been started by the method of rolling.
This is some of the problem which devolves itself upon the mechanical engineer. A part remedy seems to me to be to utilise finely-screened granite chippings, so that there is less tendency for space to be left between them when first laid, and what little there should be is more readily negatived by the action of the roller.—Yours faithfully, Aston, Birmingham. J'. W. WALKER.