Useful Charts and Tables. - No. 5.
Page 19
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By George Watson, A.M.I,Mech.E.
Engine and Gear Ratios and Speeds in Miles per Hour.
The chart given herewith is extremely useful both to the motor engineer, and to the automobilist who desires to know all the details of his car, and to be able to speak with certainty regarding such points as the engine speed, etc. Without any calculation whatever, the speed which the car is geared for may be read at a glance from the chart if the following figures are known :—Revolutions per minute of the engine; diameter of the driving wheels; and the gear ratio of engine to road wheels. In order to make it quite clear what is meant by this ratio, reference should be made to the diagram given below, in which it will be seen that all gears, whether change speed, bevel, or chain gears, must
be taken in.to account. In the diagram, let the wheels (A) and (C) each have 20 teeth, the wheels (B) and (D) 30 teeth, the bevel pinion (F.) r5 teeth, and the bevel ring (F) 40 teeth, 20 20 15 I
then ratio =
30 30 40 6
therefore, the ratio of engine to road wheels = 6 to r.
Example r.—If the engine speed equals r,2oor.p.m., and the gear ratio equals 6 to r, what will be the speed of the ,r1r with 32-inch driving wheels? Commence at the top of the chart at 1,200, and drop down
Lo the point of intersection with the diagonal line for ratio of 6 to I. Next, read across the horizontal line until the diagonal for 32-inch wheels is cut ; then drop to the base, and read the answer, rg miles per hour.
Example 2.—Taking the same figures for wheel's diameter, and gear ratio, but with a known speed of zg miles per hour, find what will be the engine speed in revolutions per minute.
In this case, commence from" 19" at the base, and read up to the point of intersection with the diagonal for 32-inch wheels, then, trace along the horizontal until the diagonal for ratio of 6 to / is bisected, and the figure vertically over this will give the engine speed in revolutions per minute, viz., 1,200.
Example 3.—Still keeping the same figures, but, this time, knowing the engine speed, the car speed, and wheel diameter, what will be the required gear ratio?
From " 19 " on the base line, read up to the diagonal for 32 inches, then, trace across until directly under the required engine speed. The diagonal passing through this point will give the required gear ratio, viz., 6 to T. This example is useful, as one can see at a glance, when reading descriptions of chassis and their performances, whether they were fitted with very low, or average gears.
The chart is also useful for finding the speed of a vehicle in miles per hour, when the diameter of the driving wheels, and the revolutions per minute of the same are known. The following example shows this application of the chart :
Example 4.--If the road wheel diameter is 42 inches, and the revolutions per minute of the same are 275, what will be the speed of the vehicle in miles per hour? From " 275," at the left-hand side of the chart, read across to the point of intersection with the diagonal for 42-inch wheels, now, drop to the base, and read the answer-34* miles per hour.
The above examples show some of the principal uses of the chart, but the engineer wilt find many other uses to which it may be put.