INTENSIVE study of vibrations has had to be made by transmission manufacturers who were seeking a means of obviating the car noise that has become known as “high-speed rattle” and which makes itself heard through the transmission case, although it is caused mainly by periodic vibrations in the engine and propeller-shaft. Out of this study has come a revival of the idea of using a “fast” rear axle, that is, one having a low ratio between the engine and driving axle, whereby the rotative speeds of the crankshaft and propeller-shaft can be reduced greatly and the vibrations at a given road-speed consequently diminished.
To make the fast axle, or low rear-axle ratio, usable satisfactorily with present cars the four-speed transmission has been developed to provide more reduction in the transmission without making the steps in ratio between the several speeds too large. Four-speed transmissions have not been satisfactory in the past because of the noise caused by spur-gears and the wear of the teeth. Recent developments in internal gearing have shown that it is entirely feasible to design a transmission having internal gearing for third-speed that is both reasonably quiet and highly efficient. Such a design is illustrated and described in detail by the author, who explains its operation.
It is important to arrive at a rear-axle ratio that is the best compromise for all operating conditions. This should be as “fast” as possible yet avoid the necessity of much more gear-shifting than the public is accustomed to at present. A fairly close approximation of the best ratio to use, as made by plotting engine revolutions per minute against car miles per hour for a given size of tire and with a variety of ratios, shows that most cars produced today do not operate at the most advantageous torque of the engine horsepower-curve and that engines are run at too high speed. This causes excessive periodic vibrations, noise, wear of the engine and unnecessary fuel and oil consumption. The relation of various axle ratios to engine horsepower and torque, in third and fourth-speeds, for two cars of different size, are shown in charts and discussed at some length.
In the discussion, members favoring retention of three-speed transmissions advocate securing the desired results by more powerful engines and less gear reduction than in present cars, calling attention to some new European cars with three-speed transmissions. Others who are favorable to the newer four-speed transmissions develop points about weight distribution, gear-shift arrangement and bearing mountings, and outline an ideal four-speed transmission.