OPINIONS FROM OTHERS.
Page 29
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The Editor invites correspondence on all subjects connected with the Me of commercial motors. Letters should be on one side of the paper only and typewritten by preference. The right of abbreviation is reserved, and no responsibility for views expressed is accepted.
Thermo-sipbonic Water Circulation.
The Editor; THE COMMERCIAL MOTOR.
[1932] Sir,--We have been following with some interest the correspondence regarding pinup v.. thermosiphonic water circulation in the cooling of commercial motor vehicle engines.
As makers of radiators of both • types, we can claim to speak with impartiality, and we must say that our experience goes to prove that, of the two, the thermo-siphon system is to be recommended, provided that certain salient facts -are taken into consideration when designing both the radiator and the water jack-eta of the engine. Space would not permit of going into great detail in this matter, but we have found the thernab-siphon system quite satisfactory for all motors of monobloc construction, arid, in fact, superior to pump circulation, providing that the radiator is designed to give sufficient head above the cylinder tops and that the internalcirculating resistance of the radiator is not too high.
With reference to your correepondent's statement regarding the furring of the water pipes, this is practically non-existent with the vertical tube radiators such as are generally fitted to commercial motors and, in addition, should the tubes fur as stated, the trouble does not consist .so much in the water circulation being impeded as from the fact that. the furring of the tubes restricts the transfer of the heat from the water to the cooling air. A water pump makes practically no difference to the cooling, providing that the connections to and from the engines are of good capacity and without restrictive bends.
Assuming that the water in the radiator boils, we have proved conclusively from our own test that, when the water in the top of the radiator is boiling, the water in the bottom tank is never more than two, degrees below boiling point. It, therefore, follow that, in these circumstances, a pump can only circulate the water through the system rapidly and it will be seen that no difference can occur in the .temperature from such circulation We have designed and assisted our customersin the design of thermo-siphon radiators for motor 'vehicles of all sizes up to 4-tan capacity and engines up to 40 hp., all of which are running both at home and in the colonies with every success. One great advantage of the thermo-siphon system which is generally overlooked is that, in the case of extremes of temperature, the engine gets to its working temperature much quicker than when the pump is fitted, and that, consequently, theperiod during which the engine is not working efficiently is much smaller than it is with the pump.
The principal trouble in thee design of the cooling system of the average motor vehicle of to-day is the insufficient provision made for a free passage for the air after it has passed through the radiator. It
often be found on inspection of the chassis that the area at the dash where the heated air is to pass through is less in cross-sectional area than the radiator itself. In these circumstances the fan is having, in addition to sucking the air through the radiator, to pump the air from • underneath the bonnet, We .think it will be found that in hot weather 50 per cent, of the vehicles are running with either the side of the bonnet removed or open toe allow for a free escape of the air after passing through the radiator. It will be observed that the
continental vehicles, generally speaking, are much better equipped in the matter of louvres in the bonnet and general_ arrangement for getting hot air away than are the English vehicles, and we are of the opinion that, if this point is carefully watched in the design, the pump, and its consequent troubles in upkeep and its first cost can be dispensed with.
In our opinion, the pump is only necessary in the ease of engines having cylinders in separate units in which the pipe to the radiator is preferred to be of small size to give neatness in general a,ppearance.—Yours faithfully,
. TEE SPIRAL TITRE AND COPrIPONENTS COMPANY, A. WILKINSON, Manager.
The Ten-wheeled Chassis on Rough Roads.
The Editor, THE' COMMERCIAL MOTOR.
[193.3] Sir,—In your able articles on Multi-wheel chassis, when dealing with the design that I have put forward, you mention certain " obvious difficulties," one of which is that, on an uneven road, as, for example, on approaching and surmounting a hoghacked bridge, grip would be lost through several of the wheels becoming " floating " at once. But, as the springs of a multi-wheel chassis are, of course, only required to support a small proportion of the total load, they can be made much less stiff than when required to support the much greater proportion that an ordinary back axle is called upon to carry, so thlit, when the wheel under any spring one, multi-wheel chassis is raised by a hump on the road, the naturil flexure of the spring allows the wheel to rise right up to its buffer springs without unduly increasin,g the load on the wheel (which is, 'of course, practically impossible when only four wheels are employed). This permits of a much greater vertical movement 'being allowed each wheel as regards the frame, so that, even when one wheel is 6 ins. or 8 ins, above another, they can both be carrying load, and in this manner theestrams on the chassis are minimized.
True, the springs are stressed under the tie-back principle, but' this is not a great stress in proportion to the total load, so that, when more than two wheels tend to become " floating " at once the weight on the remaining wheels causes them to flex until the
load is more evenly 'spread. A " road " which would cause six wheels to float would have to be humped 9 ins, in every 3.ft., which is to say that itis not a road!
Before even four wheels could become floating the hump would have to he over 6 ins, in every. 3 ft. •
Nave as regards vehicles travelling light, when the springs are tied back, the normal stress imposed on the latter is not such as will Cause them to be stiff when they strike a projection, or, to express it diff&ently, when the wheel is lifted by the projection on the road it, of course, tends to lift the whole vehicle, but the spring is arranged only to carry a small proportion of the load and, consequently, will flex considerably before raising the vehicle, and, as the periodicity is damped by the oil dash pots in the tie-back the vehicle will ride comfortably even when empty.
Five minutes spent 'in trying various types of humps, hollows, and 'rough surfaceswith the model
which in London will do more to convince an engineer that the principle is rie,,ht than a whole book Of explanations.—Yours faithfully, • • Sydney,/CS.-W. L. E.' DE MOLE.