U.S.A. War Department Specifications for Motor Transport Chassis.
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Forecast of the British After-the-Peace W.D1 ,Model. Freedom of Choice for Final Drive. Emancipation from Fussy Insistence on Adherence to Detailed Dimensions. Twelve Good Point's and True.
The specifications just issued by the American Quarter-Master. General, to which heavy motor vehicles for use with the U.S.A. Army must in future be constructed, cannot fail to be of interest to us at any tune. They are particularly important at the present juncture. The influence of the British specificatien on them can readily be detected; it has evidently served as a basis upon which to build an improved and up-to.zdate design. Those who read them, and who have any considerable knowledge of the behaviour of heavy transport in the present European war, cannot fail to realize that these specifications differ only to a slight extent from any which could. at this time be issued by our own War Office— So far as can at present be seen, they afford excellent Means of enabling us to forecast the British specification as it -will be amended at the earliest opportunity.
To be Revised Annually.
Space, unfortunately, forbids -our 'giving these specifications in detail. We will, however, endea,vour briefly to summarize them, emphasizing the important points and the novel features, commenting where needful, and dealing with them generally, by way of a, comparison, with the existing British subsidy specification which, it should be noted, was prepared in 1912 and has not officially been subject to any comprehensive revision since. Incidentally, it may hero be mentioned that it is decided that the American specifications are to be subject to revision annually.
No Interchangeabie Parts but Tires.
With the exception of the wheel rims, the sizes of the tires and the wheel tracks, no definite sizes te which adherence must be had are given in these specifications. There are several instances, such as in regard to the wheelbase, where limits, maxi/1)nm and minimum, are set to the measurements allowed. Apart from these, freedom is given to the manufacturer to .proceed upon his own individual lines. As we proceed with our summary, it will be realized that the maximum of opportunity for constructing vehicles useful alike commercially and in a military sense is afforded. It is evident that the policy of forming convoys of one make of chassis, so that any trouble
030 with regard to spares could be reduced to a minimum, has proved so satisfactory in practice as to eliminate, in the opinion of the American authorities, the necessity for having certain parts of the vehicles interchangeable throughout. It has therefore not been thought advisable to standardize the fittings of such parts as radiators, magnetos, brake rods. petrol tanks, nor to insist upon all wheels and wheel bushes being interchangeable.
To Carry One-and.-a-hall and Three Tons, but to be Constructed as for Two and Four Tons.
The two sizes to be ace-opted are, as in the case of the British, for loads of one-and-a-half tons and three tons. (In considering these specifications, it must be remembered that the U.S.A. ton is equivalent to 2000 lb. avoirdupois only, and not 2240.) In order, however, that the rigorous use to which these vehicles are likely to be put on war servico may not affect them adversely, it is recommended that they be dcsigned and constructed as though for loads of two tons and three-and-a-half to four tons, respectively. This is undoubtedly a -wise precaution. It will possibly result in the one-and-a-half ton chassis being sold commercially as a two-tonner, and in the threeton chassis being sold commercially as a four-tonner. The question of subsidy is not discussed at all in these specifications; we cannot, therefore: venture an opinion as to whether the sale of these chassis as two and four-tonners respectively would adversely affect any subsidy which may he offered. In the specifications as issued, full and complete particulars are laid down as towhat is expected from the one-and-a-half ton machine. The corrections for the three-ton specification are then added as an appendix. We propose to deal with these specifications in the same manner. It must, therefore, be understood that for the time being anything we say must be read as applying to the one-and-a-half ton chassis.
The Twelve Main Points,
There are twelve items which are considered to be of' vital importance. We will deal first with these twelve, in the order in which they appear in the specification, afterwards discussing other interesting details in their turn. 1.—Low Gear Reduction.
Besides laying down a minimum ratio for the low gear reduction as between the engine and rear axle, which ise stated must net be less than 40 to 1 (the corresponding figure in the British specification is 37), the specification also calls for what is described as a " Tractive Effert Factor." This is the figure obtained by dividing the net tractive effert at the tire surface of the driving wheels by the total weight of the fully.. loaded truck. It is expected that this shall not be less than .0775 on the high gear, nor less than .338 on the low gear. The tractive effort is based on an efficiency of the transmission of 85 per cent, from eegine to road wheels on the high gear; or 70 per cent, efficiency on the, low gear.
2.—Large Engine.
The specifications eall for a minimum piston displacement in the engine of 312 cubic inches. The corresponding figure in the British specification is not so definitely stated. It calls for a minimum cylinder diameter of 4 ins. If we presume a 5 in. stroke engine, this is equivalent to 250 cubic inches. The American engine is, therefore, approximately 20 per cent, larger than the British. This engine must develop a torque of not less than 2000 inch peunds at not more than 1000 piston speed per minute • furthermore—and this is an important clause—the torque at 500 ft. piston speed per minute shall not be less than 90 per cent, of the rnaxinaum torque. The British specification merely asks that the h.p. shall not be less than that given by the R.A. C. rating when run for three hours at normal full speed. In the case of the 4 by 5 engine mentioned above this would be 24.8 h.p., which is equivalent to a torque, at a 1000 'piston speed, of only 1300 inch pounds-. The U.S. specifications call for greater engine power, in relation to size, than does the British.
3.—Four-speed Gearbox.
No particular ratios are specified in the box ; it ismerely insisted that there shall be four, and that the least shall be such, as stated above, that the total gear reduction is not less than 40 to 1. It is insisted that ball or roller bearings must be provided and not plain bushes; gearboxes constructed as inlegral parts of the rear axle will not be accepted.
4.—Ground Clearance.
The minimum ground clearance at all parts of the truck except under the sprockets of chain-driven vehicles, or under brake drums, shall not be less than 91 ins. The ground clearance in the middle of the chassis between the front and rear wheels shall not be less than 18 ins; the clearance under the engine pan must net be less than 11 ins. The British specification calls for a minimum clearance of 12 ins, underneath the axle.
5.—Demountable Tires to Specified Sizes.
All tires are 36 ins. diameter. The wheels are to be cast steel, and the tires are to be demountable. This is 'evidently regarded—and rightly so—as an important feature, and only steel fastenings for demountable tires constructed in accordance with drawings supplied by the Quartermaster General will be accepted. It is interesting to note that on this size chassis 26 in, by 4 in. single tires are called for on the front and 36 in. by 7 in. single tires on the rear
wheels. On four-wheel drive chassis, which are covered by the same specification, 36 in, by 6 in. single tires are required all round.
6.—A Large Fuel Tank.
The petrol tank must have a capacity of 30 gallons and be provided with partitions so arranged in conjunction with a, three-way cock that a reserve supply of six gallons is always anaiThble. when the main compartment is fulyntv 7.—Electric Lighting System.
A complete electric lighting system is to be fitted, including 'generator, -batteries, searchlight, side lamps, dash light, and tail light. Provision is to be made for a starting motor, and the flywheel is to have teeth cut thereon to the S.A.E. standard, so that the motor can readily be applied. Dual ignition to the engine is also specified, the high-tension magneto being supplemented by battery and coil. The battery may be the same as that-used for lighting, and it is not essential that a separate distributor, apart from the magneto, be fitted. 'The British specification does net call for electric lighting, but acetylene; the ignition is by magneto only, with the pro-, viso that a second magneto, hOwever, has to be carried as a spare.
8.—Three-point Engine Suspension.
"The engine must be mounted with aethree-point suspension or on a sub-frame having a suitable threepoint suspension which will effectively prevent the distortion of the frame from straining any part of the engine, clutch or transmission."
9.—Locking Diffekential Gear.
It is worthy of note that a self-locking differential gear is desired. We quote verbatim: "The differen-. tial shall be of such a type as automatically to permit the wheels to revolve at different speeds without complete loss of torque on either wheel, and also to apply at least normal torque to whichever wheel shall have traction. These requirements shall be met when the vehicle is running either forward or backward."
10.—Springs to be of Alloy Steel.
The physical properties.of the steel employed for the manufacture of the springs are stated. The steel must have a maximum elastic limit of 170,000 lb. per square inch, a tensile strength of not less than 210,00Q lb. per square inch, an elongation of not less than 5 per cent. in 8 ins. and the Brinell hardness factor must be between 390 and 470. The deflection • of the springs corresponding to any given load is also clearly defined. There are no fewer than 20 paragraphs devoted to the specification of springs and suspension details. There is not one in the British specification. It is no secret that perhaps the most troublesome feature of vehicles of al_ makes overseas has been in connection with the springs. It is, therefore, interesting to note the importance, in view of war-time experienCes, that the U.S. authorities attach to this point.
11.—Wheel Track.
The wheel track of all vehicles is to be the same, 58/ ins, being specified in connection with 4-in. tires (front wheels), 601 ins. for 6-in. single tires (as used on four-wheel drive vehicles), and 61i ins. for 7-in. tires (rear wheels).
12.—Large Radiator.
The twelfth item in connection with the radiator is perhaps more than any other derived from deductions formed from the performance of American vehicles in the European war. Not only is a particularly large unit insisted upon, but it is specified that the cooler shall be of the fin and tube type, with cast or pressed upper and lower headers, and bolted-on cooling section or sections. The radiator is to have a front area on the cooling section alone of not less than 2.25' square inches per square inch of piston swept surface. The total radiating surface in the radiator is not to be less thimi 90 ins. per square inch of swept surface. The British specification gives the actual overall dimensions of the radiator to be fitted, and specifies the diameter of the tubes. The trunnions in which the radiator is carried are also in our own W.D. specification standardized.