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UNSPRUNG WEIGHT.

12th December 1922
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Page 12, 12th December 1922 — UNSPRUNG WEIGHT.
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Which of the following most accurately describes the problem?

In His Reply to a Recent Editorial Article on this Subject, "Engineer" Discloses His Ideas Concerning the Best Means of Arresting Shock and Vibration at the Point of Contact With the Ground.

. THE ARTICLE on unsprung weight which

appeared in the issue of The Commercial Motor for November 28th deals with a subject of great importance, particularly at the present time, when the foundations of our roads and bridges are being prematurely destroyed because of the unsprung weight in our heavy types of chassis.

In order to comply with your invitation to hear any practical suggestions from "Engineer " or anyone else on this important matter, it Is necessary to ask two questions, namely :—

(1) What is "unsprung weight " 7 and • (2) What is " resilience " in tyres l From the article referred to it would appear that

entirely different meanings can be attached to the phrase "unsprung weight" and the word "resilience " applied to tyres, and, therefore, in order that the article may be properly replied to, the true meaning of unsprung weight and resilience must ha defined and agreed upon before this inoortant matter of unsprung weight can clearly be understood by every chassis owner.

I think you will admit that unsprung weight means something which is not sprung. In'other words, when a vehicle or chassis's is running on the road shod with steel tyres, every part of the chassis between the axle and the road. is an unsprung part, because there is no spring or cushion placed between the axle and the road. When, however, you shoe the wheels of the chassis with solid rubber band tyres, you convert the unsprung parts between the axle and the road into badly sprung parts, because the solid rubber band tyre can only flex, say, a maximum of + in. when the chassis is fully loaded, and considerably less when the chassis is running empty or lightly loaded. if, however, you reshoe the same vehicle with good shockabsorbing tyres which flex or give in road contact, under minimum load, in., and under maximum load t in. up to 1 in., as and when required, then you spring every unsprung iart between the axle and the road by simply shoeing the wheels with good shock-absorbing tyres. My contention is that it is absolutely impossible to spring the tyres, wheels, axles, axle springs, and 3328

differential of any type of chassis, whether pleasure or commercial, except by means of good shock-absorbing tyres.

You may attempt to reduce unsprung weight in, say, an axle by reducing the total weight of the axle, but that is not the proper way to reduce the unsprung weight of an axle because, by doing so, you run a very serious risk of interfering with, not only • its efficiency, but its length of life. The unsprung weight of an axle, however, can be reduced, effectively and enormously without reducing its actual weight, by simply shoeing the chassis with abnormal shockabsorbing tyres which sprit's. the axle even when the chassis is running lightly Laded, and by no other means. •

There appears to be as great a difference of opinion as to the meaning of the word " resilience " when referring to tyres as with regard to the meaning of " unsprung weight," and, therefore, I would like definitely to state that resilience,means bounce, which is the opposite of shock-absorbing qualities. What every chassis owner wants is good shock-absorbing tyres instead of resilient (bouncing) shock-transmitting tyres.

Solid rubber band tyres are all good resilient (bouncing) tyres, but they are very poor shock-absorbing tyres, and because they are good re. silient (bouncing) tyres, the chassis wheels are constantly leaving the road instead of clinging to it.

In the past, it was considered of great advantage to have good resilent (bouncing) tyres, and many of us must have attended motor shows in which wonderful resilient (bouncing) pneumatic tyres were displayed by means of an apparatus which shewed how much resilience (or bounce) the said tyres had, proving beyond any doubt that, in the past, a resilient (bouncing) tyre was consideresI by our tyre manufacturers to he an efficient tyre.

At last, our tyre manufacturers have scrapped their resilient bouncing tyres and produced an inflated tyre having a cord foundation which is capable of running with low air pressure which means that, instead of the tyre bouncing off' road, it clings to the road because of the extra amount of "give ' or flex which it has in road contact. In other words, the new cord foundation inflated tyres of to-day are being con structed to be good shock absorbers, instead of being resilient (shock-transmitting) tyres, and this revolu tion has bean brought about because the foreigner has found out the increased advantages of shoeing a chassis with good shock-absorbing inflated tyres, and that good shock-absorbing inflated tyres could not be constructed unless the foundations of the tyres were of cord construction instead of canvas.

Unfortunately; however, because our British tyre manufacturers did not at first agree that there was a practical advantage in running tyres with low air pressure and thereby springing efficiently the unsprung parts of a chassis under both minimum and maxinium lead, an unduly large proportion of the in flated tyres used in this country are manufactured abroad, and I contend that the new inflated tyres of to-day with their low air pressures, although vastly superior in every respect to the canvas-foundation tyres of yesterday, must be increased in width on every type of light chassis if we are to have the luxury and comfort in a light chassis when running on the road which is obtained to-day in, say, a Rolls-Royce. Figs. 1 and 2 contain a practical suggestion as to how the efficient springing of a light chassis can be brought about.

In your article the following sentence appears :— " Engineer' must have a very poor opinion of the bus companies and firms owning large fleets if he thinks they still cling to solid tyres from want of knowledge, or sheer cussedness. It is not wise to think that such men have not investigated the merits

of all kinds of tyres. Engineer' is probably 310tf aware of the huge sums 'spent annually by -certain companies in experimental and research work in connection with tyres and every part of the commercial motor."

In replying, I beg to ask why it is that the companies to which you refer, if they have been experimenting in research work in connection with tyres and every part of a commercial vehicle, have, during the last 10 years, utterly failed toltreduce4he unsprung parts of existing types of heavy chassie ? If they have been attempting to do the impossible, namely, to reduce the unsprung parts of a chassis by means of axle suspension instead of by means of shockabsorbing tyre suspension, then they have been undoubtedly working on wrong lines, and the sooner they change their tactics the better in the interests of chassis upkeep, road upkeep, comfort, and safety when travelling over our badly cut-up, potholey roads.

The question naturally arises, who is to blame for the present state of affairs? Is it the chassis manufacturer or is it the tyre manufacturer, or both? I contend that it is both, but the remedy lies with' the chassis manufacturer.

In your article you say "the matter of reducing unsprung weight has received the attention of designers for years and, if it were possible to reduce the weight of such parts by any appreciable amount, it would have been done long ago."

In reading your article carefully, I take it for granted that this opinion is really that of the manufacturers, otherwise the last paragraph would not appear in this valuable article on unsprung weight, and, therefore, I will give you some information which you may not know, and which is the real cause of our heavy chassis running on destructive solid rubber band tyres instead of on efficient shock-absorbing cushion tyres.

Fact No. 1 is much cheaper per mile to run heavy chassis when using solid rubber band tyres than on any type of good shock-absorbing cushion tyre. In other words, the tyre cost per mile per bus on heavy chassis is often as low as under id. per mile for solid rubber band tyres and, therefore, solid rubber band tyres are used, in spite of the fact that said tyres are known prematurely to destroy the foundations of the roads and bridges and also the foundations of the houses at each side of the roads over which heavy traffic chassis pass.

Fact No. 2 :—Because of fact No. 1, the engineers and chassis manufacturers manufacture their wheels so narrow on their outer circumference that an efficient shock-absorbing tyre cannot be fitted to them, because any type of shock-absorbing tyre (no matter how constructed) must be much wider in its crosssectional base than an existing solid rubber (shock. transmitting)tyre ; therefore, the question of reducing unspring weight in any type of heavy chassis must first lie with the chassis manufacturer, who must produce wheels which are sufficiently wide to take goo(' shock-absorbing tyres.

I venture to prophesy that, if the chassis manufacturers will only produce the correct type of wide wheel required to enable every type of heavy chassis to be efficiently shod with good shock-absorbing tyres, the day is not far distant when every chassis (no matter what its weight) will travel over our roads having every part of the chassis well sprung, whether running lightly loaded or fully loaded, but that day can never come until our chassis engineers or designers increase the present widths of the rims of their wheels:, and thereby allow wider good shockabsorbing tyres to be attached thereto.

The illustration shows how every light type of chassis can be as luxuriously and as well sprung as the heavier type of chassis, but, to accomplish this, the tyres must he increased in width in order to obtain the same amount of give in road contact in the tyre, under both minimum and maximum loads, as is obtained in the tyres: atted to, say, a Rolls-Royce ; and, working on the same principle, if the chassis engineers or designers will only increase the rim widths of their wheels to enable wider, better shock-absorbing tyres as clearly shown in Figs. 2 and :3, Lc be attached to their chassis, the present troubles in connection with .unsprung weight in any type of chassis will disappear, and then it may be possible to reduce the actual weight of,say, the axle and other heavy parts used to-day, because the axle, by means of its tyres, will become a well-sprung part under both minimum and maximum loads,instead of, as at present, being an unsprung, or badly sprung part, particularly when running under minimum load.

One of our big tyre manufacturers has realized the enormous importance of good shack-absorbing tyres, and has already shod over 5,000 of the one-ton Ford chassis with these shock-absorbing tyres, using abnormally low air pressure in the tyre, which low air pressure not only adds to the shock-absorbing qualities of the tyre, but increases its speed. If such a tyre can be manufactured for one-ton Fords, there is no reason why, working on the same principle, efficient tyres should not be manufactured by the same British manufacturer to carry any chassis load up to 8, 10, or 12 tons, and if you will take the trouble to interview the tyre manufacturer to whom I refer you will find that the great difficulty any British manufacturer has to-day is the width of the commercial vehicle wheel, which has been manufactured so narrow that it takes only solid rubber band (shock-transmitting) tyres, and cannot, unless widened, take good shock-a,bsorbing tyres.

In this letter I consider springing, cushioning, and tyring as synonymous terms, altholigh I quite understand that it is commonly accepted in the motor trade that springing means axle springing, but, as the best place to spring every part of a chassis is at the wheel's circumference, I use the word springing in its widest sense, which covers cushioning by means of a tyre. In my opinion if the word •springing were always used in its widest sense when referring to any part of a chassis which has a spring or cushion placed between the said part and the road, many differences of opinion re unsprung parts would disappear. We cannot get over the fact that, when you use a good shock-absorbing tyre, the axle spring of to-day becomes much more efficient because the many smaller road shocks are absorbed by the tyre before they reach the axle spring. ,

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