COMFORT IN THE MOD PASSENGER VEHICLE.
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The Luxury of the Present Day C Suspension has Played Its Part1,. annot be Questioned. Improved bra tion has also been Reduced. CCOMFORT, that factor moth sought by humans, is nowadays expected by all and sundry in every degree of the social scale. The millionaire, in his 40 h.p. limousine, is not more insistent in his demands for vibrationless, noiseless and shockless travel than the ordinary workaday member of the public who rides in coaches and buses. The exhibits at the recent Commercial Vehicle Show at Olympia made this apparent, even to anyone taking quite a superficial glance around the various passenger chassis and coaches.
The chief object of this article is to show how comfort—luxury would perhaps be a better term—is not entirely a matter of elaborate or deeply sprtuig seating accommodation, but is dependent to a very large extent upon the design and weight distribution of the chassis—to a much greater degree, in fact, than is usually supposed. For instance, it does not seem obvious to those who have not gone deeply into the matter why engine design should affect a passenger's comfort when travelling in a coach, but that it does there is no doubt. as will ,be explained later on.
Let us first of all examine the ethics of what really constitutes the term "travel comfort." Quite apart from the accommodation provided for the passenger to enable him to adopt a natural reclining attitude, there are many forces at work which tend to upset the passenger's ease of body and mind. Vibration, sway, pitching, rolling, noise, etc., all play their part in causing a temperamental disturbance. Considerable discomfort can result from any one of the foregoing annoying features, especially if it be pronouriced.
Now, the causes of discomfort obviously are forces, and if a person be subjected to any large extraneous force it is likely to cause either discomfort or, worse still, illness. A really heavy impact, for instance, might result in death, whilst an often repeated, slow, swaying motion, although in no degree violent, may, with some people, be the cause of sickness. Again, people possessing certain physical characteristics are capable of experiencing quite severe vibration without in any way being adversely affected. Carrying this statement a little farther, we may allude to the fact that the effect of electric shocks on people of vastly different physical endurance and strength is very noticeable.
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The Effects of Noise and Shock on Different People.
Most people are aware that with electric-shocking machines, used both medically and for competitive purposes, often a slim girl can, with moderate comfort, stand many times the amount of. current that would make a big, physically fit man squirm. Now, if the girl who can stand successfully the electric shock takes a cruise on a boat in which the movement, due to the rolling of the sea, is very small indeed, she may be liable to violent sickness. Expanding this theme a little we can go one step farther. Noise can, with certain types of people, be the direct cause— or at any rate a strong contributory cause—of discomfort, especially if the person happens to be enclosed either within a building or the body of a motor vehicle at the time the noise is going on. This can be proved by taking anyone susceptible to noise for a ride in a ear or motor coach first with the silencer in position and then with it removed, care being taken in the latter case to avoid any awkward contretemps with the police. As regards the public coach or bus, of these the coach is more important, of course, as people ride in it for longer periods at a time than in the bus, so it is important that all disturbing elements such as have been outlined should be eliminated. It can be seen, therefore, that, quite apart from the actual suspension of the body and chassis, the coach must be quiet in operation and free from vibration, the latter comprising vibrations of both high and low frequency and great or small amplitude. Now, the source of• high-frequency vibration in any motor vehicle is nearly always to be found in the engine, as this is the only unit in the chassis in which any part is moving
with a rapid reciprocating motion. There are, of course, other components from which noise and vibration are likely to emanate, but the possibilities of serious vibration with these units are not very great.
The modern vehicle is, of course, a vastly different affair from the type of vehicle in use just after the war, for instance. Comfort, as has already been outlined, is vastly -superior ; all the desirable attributes such as noiselessness and lack of pitching, rolling and vibration are now expected. What have been the causes of such a distinct alteration?
In the first place, engines have vastly, different performances nowadays from what they had in the past. The power output has, in a good many instances,
been doubled—capacity for capacity—engine revolution speeds are up by almost a like amount, and the unit runs with far greater silence and with practically no vibration. One would be tempted to say at first Sight that with the increase in engine speed and efficiency, noise and vibration must have increased also. but anyone who questions the matter of silence anti Smooth running has only to take a ride in one of the up-to-date coaches for his suspicions to be allayed.
The Problem of Attaining Smoothness in Engine Operation.
Smoothness in the internal-combustion engine is a matter of both design and manufacture. The design must be right and the manufacture must be accurate in order to prevent noisy operation. Both these factors have improved, possibly the progress of design being the more marked. When designers found that by improving the shape of the combustion chamber, providing larger valves and raising the compression ratio efficiency could be increased and higher revolt'stine speeds obtained with a corresponding increase in
power, they were forced into investigations on the score of silence and smooth running, and it was soon discovered that with pistons weighing anything up to 5 lb. or 6 lb. and connecting-rods, etc., proportionately heavy, smoothness (in the modern sense) was quite impossible of • attainment.
. This is where the metallnrgilts stepped in. They provided aluminium alloys which, weighing only a fraction—volume for volume—of cast-iron, thus provided a means whereby the weights could be reduced enormously. With the overall increase in efficiency, too, it was soon discovered that smaller engines could be used to provide quite a satisfactory road performance. This automatically assisted in keeping down vibration and maintaining a smoothness in running which, at the high speeds then developed, had been previously unthought of.
Then came the six-cylindered engine with a further substantial improvement in the degree of silence and smoothness with which it functioned, all due very largely to. the decrease in the weight of the reciprocating parts. To-day, many manufacturers are equipping
their engines with Durahnnin connecting-rods, aluminium-alloy pistons, and, in sonie instances, where the valves are of the overhead type and are operated through the medium of push-rods, the push-rods themselves are tubular and made from one of the light alloys. Parallel with the advances made in the reduction of the reciprocating weights, crankshafts have also been improved. Most crankshafts are now fully balanced and, as they are mounted in substantial and numerous bearings, are much more immune from vibration than in the past. One might be tempted to say that engine vibration is concerned chiefly with the power unit itself, and is more or less separate from the body of the vehicle. Such a theory, however, is, of course, quite wrong, because vibration when once started in such a comparatively " springy affair" as a passenger chassis, is nearly always amplified, due to the flexibility of the structure. That is partly the reason why, say, a window will often vibrate in sympathy when vibration is taking place, at certain speeds, within the engine, as, of course, each part has its own period whfch may correspond with those of the engine. As there are
many windows in a coach, and the engine runs at greatly varying revolution speeds, it is quite easy to see that it might be difficult to find an engine speed at which sympathetic vibrations could not be found in some other component in the vehicle.
Now a great deal of the vibration that may take place in a motor coach, for Instance, is prevented from causing discomfort to the passengers by the springs of the seat cushions, which, of course, are capable of dealing effectively with a small high frequency motion such as that from the engine. On the other hand, however, vibration of this type usually causes noise, or at any rate a disturbance of the air confined within the body space of the vehicle, which, in turn, has its effect upon the ear drums of the passengers and so causes them to suffer from symptoms usually associated with the effects of noise.
The importance of smooth running for the power unit cannot, therefore, be over-emphasized, whilst it would seem apparent that some form of insulation should be used between the engine and chassis. De velopment in this direction has been taking place for some considerable time, with the result that, to-day, engines are usually mounted in the frame with rubber or fibre-insulating blocks interposed between the engine bearer arms and brackets attached to the chassis frame members. In this way, any vibration that does happen to be involved is damped out in the initial stages, due to the "dead beat" quality possessed by rubber or any compounded rubber substance such as.is used for shock-insulating purposes.
Suspension of the vehicle itself has already been dealt with in a • comprehensive manner in a
series of articles which has already appeared in The Commercial Motor, and so it would appear to be redundant to dwell at any great length on this subject, despite the fact that it has such a great bearing upon comfort for the passengers. There is, however, one aspect of the problem that might receive the attention of designers—the provision of an adequate arrangement for graduating the softness, or otherwise, of the road springs.
It will be realized, of course, that certain combinations of loads, speeds and road conditions require certain qualities in suspension adequately to provide a shockless and non-pitching motion for the passengers. For instance, a rough pot-holey road if negotiated fairly slowly would require the suspension to be definitely on the soft side, whilst a comparatively hard and dead-beat action for the springs is desirable when the vehicle is travelling fast over a long-pitched wavy road surface —two absolutely opposite sets of conditions. • Now this "suspension millennium" can only be obtained—with springing systems as they are at present— by varying the damping action of the springs. A light spring with a variable damping device seems to be indicated. Perhaps the hydraulic, so called shock absorber, offers the best solution, with a control which can be varied easily and quickly by the driver. We include illustrations giving an outline of the suggestion.
We cannot do better than conclude an article such as this by paying tribute to the achievements already attained in the matter of passenger comfort in the modern coach or bus. That there is still room for improvement there can be no doubt, but much can be learnt from a brief survey of recent developments, coupled with a study of the psychology of the people who pay to ride. Designers, manufacturers and users alike are, or should be, interested in what the public appreciate, and an analysis of conditions often throws light upon a subject which may, on the surface, appear to be somewhat obscure.