OPINIONS FROM OTHERS.
Page 29
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The Editor invites correspondence on all subjects connected with the use of commercial victors. Letters should be on one side of the paper only and typewritten by preference. The right of abbreviation is reserved, and no reseonsibility for views expressed is accepted.
Tractor Trailer Design.
The Editor, THE COMMERCIAL MOTOR.
[2030] Sir,—In your issue of October 24th you describe a. new tractor trailer in which the turntable of the tractor is mounted on trunnions, and I notice that you lay considerable emphasis on this feature as being novel. I would like to point out that this method was first adapted in the case of a number of trailers designed hit' the Ministry of Munitions during the war and which are still in use by the WA). in considerable numbers.
As it happens, this feature, amongst others, forms the subject of a claim which is now beforethe War Office and does not, therefore-, possess the novelty which your article would suggest—Yours faithfully,
Letchworth. DESIGNER.
[We think that if oar correspondent will again read our article upon the Watson tractor trailer (page 287 of the issue of October •24th), to which we assume he is referring in his letter, he will find no phrase which justifies his remark about " considerable emphasis on this -feature as being novel."— ED. C,21-.] 'Unsprung Weight in a Chassis."
The Editor, Thu 'COMMERCIAL MOTOR.
[2031] Sir,--I read with interest " The Inspector's ". remarks on pages 270 and271 of The Commercial Motor for October 17th concerning upsprung weight in a chassis, in which he says :—" There are examples, for instance, of very heavy back axles which have given remarkable results so far as maintenance is concerned, whereas it is easy to quote other instances in which a designer has evidently tried to cut his unsprung weight down to the minimum,•possibly with the result that axle shafts, bearings, axle casings and springs have over and over again given trouble."
He also states:—" Far less damage is done to vehicles of this kind when they are running. with loads that may even be somewhat in excess-of those for which they were scheduled when they left the makers' works. In any case, properly loaded machines will certainly -be less liable to trouble in many chassis components than will others which are run for long periods in a condition which produces the very worst effect from excessive -vibration."
If our chassis builders would only realize that the unsprung parts of every chassis, when running lightly loaded, namely, the tyres, wheels, axles, axle springs, and differential, can all be turned into wellsprung parts simply by equipping the chassis efficiently with good shockabsorbing tyres which " give" in road contact at least half an inch under minimum load, and three-quarters of an inch under maximum load, then the question of whether an axle was light or heavy would be a bagatelle compared with the fact of said axle being a well-sprung part, even when running lightly loaded (or when passing over a bad pot-holey road). Ample section in existing solid rubber band tyres will never give the shock absorbing qualities required under minimum load, but a good cushion or inflated tyre, which flexes under minimum load up to half an inch and under maximum load up to an inch, will eradicate all the unsprung parts of a chassis (whether pleasure or commercial). .
In a commercial _ chassis, where the difference between the vehicle loaded and unloaded is sometimes from 75 to 100 per cent., is it any wonder that far more damage is done to the said vehicle when it is running lightly loaded than when it is running with a full load (or even with an over-load), considering that the chassis, when running lightly loaded, is .practically untyred and unsprung, because the tyres cannot flex in road contact under minimum load and the metal springs (between the axle and the chassis frame) cannot ilex under minimum load'? How, therefore, can the chassis owner spring his chassis efficiently. when lightly loaded, considering that his chassis is running, say, half its time empty or lightly leadedi I reply—by means of a good shock-absorbing tyre which flexes under minimum load half an inch, and under maximum load threequarters of an inch, and by no other means.
It must be remembered that no type of suspension which is placed between the axle. and the chassis frame—no matter how perfect—can possibly turn the unsprung parts of the chassis, which are placed between the axle and the road, into well-sprung parts. These unsprung parts can only become well sprung, when the chassis is running lightly loaded, by means of a good shock-absorbing tyre which gives, say, half an inch under minimum load (which no existing type of solid rubber band tyre can do). If the tyre manufacliirer attempts to obtain greater "give" in road contact in solid rubber band tyres by manufacturing the tyre with softer rubber, he is up against the difficulty of creating an objectionable wave or hill of rubber in front of The • tyre when running on the road, which objectionable wave or hill of rubber is increased in height when softer rubber is used in existing types of solid rubber band tyres. .Another flaw, which existing solid rubber band tyves disclose, is that, the wave of rubber which is created in front of the tyre, when running on the road, is limited in its speed to the speed of solid rubber, and, therefore, when a vehicle is travelling at over 14 miles an hour, the speed of the vehicle is greater than the speed or frequency of repose of the rubber, with the result that the wheel is constantly leaving i the road so as to ease the rubber tyre, because of the fact that the chassis is travelling at a higher speed than that at which it is possible for the rubber wave to travel. In other words, unnecesary shocks are being transmitted to the chassis frame for the reasons mentioned above.—Yours faithfully, London. ENGINEER.
Assembled or Made Under One Roof.
The Hditor, THE COMMERCIAL MOTOR.
[2032] Sir,—Mr. Maughfling, in his letter under the above heading, in your issue of October 31st, seems to lose sight of the fact that standard components or units used by lorry manufacturers in the States are not built up specially for one design of chassis, but are stock productions, in just the same way that sparking plugs, magnetos, etc., are. If, therefore, an individual manufacturer of lorries goes out of business, it does not follow that the production of the type of unite which he was using willbe discontinued, as in all probability they are being used by several other lorry manufacturers, and the supply of spare parts will not be affected. With regard to Mr, Maughfling's remarks that service stations in the United States will not help users in this country, I quite agree, but, fortunately, nearly all leading components now manufactured in the States have their own service station or agents in this country, and I think there are very few component manufacturers worth considering who are not represented over here.—Yours faithfully, C. F. CLEAVER, A.M.I.C.E., M.I.A.E., FOUR WHEEL DRIVE LORRY CO., LTD. Slough, Bucks.