Structural Considerations in Motor Cars.
By Thomas J. Fay, E. E.
The first cost of a motor car should be high enough to insure a low cost of maintenance. The best “coefficient of economy” is that which represents primarily the lowest possible cost of maintenance, not per gross ton-mile, as it is oftentimes considered, but per unit of useful results; and in the second place the lowest possible first cost. The least expensive to maintain of two “white elephants” would be a bad bargain at any price. Likewise we may say that a motor car, to be economical, must not only operate at a low cost of maintenance but operate well with a maximum burden. Another way of putting it is to say the first cost is a matter of no serious moment, provided the cost of the useful result is quite within bounds.
There are many details in a motor car, each one of which must be thought out with a degree of care wholly at variance with the demands of any other class of machinery. A car may be well built and prove to be very serviceable indeed, and still be a very bad choice, for the reason that the design may not be in accord with (a) the materials afforded by the market; (b) the methods in vogue in the up to date shop. Hence the first cost may be inflated. To illustrate: A coin, for instance, might be gold, of full weight and flawless; yet the methods of manufacture might be such as to make the cost of production more than the value of the coin.
Motor cars up to the present time have successfully eluded the grasp of the poor man, and it is believed that the poor man will never be rich, so that “Mahomet must go to the mountain." Sometimes it does seem as if motor cars would ever be costly, both first and last, but history repeats itself, and it has ever been a trait of history to see the unexpected.
On the Continent the "poor" man receives no consideration, hence no attempt is made to make the cost of the car conform to the man's pocket. In America the poor man has greater wants, and demands every innovation—at a price. It is for this reason that "cheap," half baked cars are not the product of successful makers; and, also, on this very account American manufacturers are bound to lead in the long run, because the majority of buyers (a) cannot pay a high price; (b) will not buy an inferior product; (c) do offer a wholesome opportunity to makers of car who continually strive to lower the cost, but who positively will not lower the standard of quality.
It costs approximately $25,000 to design and construct one single runabout, provided the design is new and worked out in promoting and jobbing shops. On the other hand, cars of this class are furnished to buyers by responsible makers for not far from one-fiftieth of that cost, while in so far as quality is concerned the experiment car, not withstanding its enormous cost, is likely to prove a flat failure.
To a limited extent the commercial field has been invaded, but the road ahead is long and rough. Critics are wont to complain about tires. As a matter of fact, tire troubles are brought about to a vast extent through the wholly bad practice of making cars much heavier than need be. A setting hen will bring out a brood of chickens and not fracture a single egg, while a small boy and a brick would shorten the chicken crop. Tires, too, will wear depending upon treatment; that is to say, tires are thoroughly commercial just as they are, but heavy, cumbersome cars are not. Improvements in cars have been marked, and many more im-
Fig. 1.
provements may be expected. The first big electric truck the writer had to work upon weighed something like 7,000 pounds, and felt very much abused under a burden of 1 ton. The first big steam truck the writer was connected with weighed nearly 6,000 pounds and refused to run at all.
It cost a lot of money to learn that conventional machinery designs would not suit in motor car work; and it was hard to forget that a bridge building factor of safety was not a factor of safety at all in automobile construction. As illustrative of the changes wrought, consider the following: According to Seaton the thickness of cylinder walls should be:
in which P is the maximum pressure on the piston in pounds per square inch, and D the diameter of the cylinder in inches. For a 6 inch cylinder this would give (for cast iron}
Now let us compare with this the actual present practice in automobile gasoline motor construction, which is to give the cylinder walls a thickness
so that for the case in point
Considering cast iron with an ultimate tensile strength of 18,000 pounds per square inch and 3,600 pounds per square inch non-shock working load, it would be impossible to build a commercial motor car and at the same time take Seaton's advice. On the other hand, there are a vast number of cars in which the motor cylinder walls are quite as thin as that given by the above formula. It is believed that, notwithstanding the apparent safety of the above formula, some allowance should be made for deformities in the casting. and with this in view the formula stands revised, as follows:
Illustrative of the use of this formula reference may be had to Fig. 1 of a cylinder in which
thickness of walls about the piston in its bottom position. The thickness of the wall below the piston head with the piston at the bottom of the stroke was left
This cylinder was tried out at some length in a runabout type of car during the year 1903, and, in so far as this phase of the problem was concerned, the results were very satisfactory. It is possible, of course, to consider a slight thinning of the cylinder walls, without assuming great risks, but for thoroughly good work it is believed the walls are thin enough.
The maximum pressure given, i.e., 379 pounds per square inch, is none too high when the shock effect is taken into account. Referring again to Fig. 1, it will be found that the port walls are three-six-teenths inch thick. This thickness is very much more than what a calculation would dictate, but here the "foundry question" is paramount and the port walls are made just as thin as possible consistent with the foundry chances. In the case of this particular cylinder the head is covered by “combustion chamber covers," and is so proportioned that. considering the strength of machine steel covers, a rupture of the head would not he imminent. If, however, the head of a cylinder is made integral the shape of the head must receive consideration, else deformation, due to pressure, may become a source of trouble, at least in cylinders of large proportions.
A very good shape, economical both in point of weight of metal required as well as space occupied, is that represented by a flattened ellipse. The thickness of the head
