RAPID ACCELERATION

NZ Truth, Issue 1175, 7 June 1928, Page 15

RAPID ACCELERATION

Weight-power Ratio Is Determining Factor In Quick Up-take AUITH the daily increase of traffic on our highways the need for rapid acceleration becomes more and more pressing.

THE driven whose car accelerates 1 rapidly can often pass others and return to the left side of the road m safety. The same manoeuvre attempted on a slower vehicle would be fraught with danger, or even disaster. Motorists generally express varying views concerning such topics as suspension, steering, and types of body construction, but it is pretty safe to say they ail relish quick "getaway" as a feature of a car's performance. In speaking of rapid acceleration, most people naturally think of engine power as being the most important requirement, whereas only a little reflection should be needed to show that the weight of the vehicle has an equally potent influence upon its liveliness. Acceleration, m fact, is exactly.proportionate to what is known as the power-weight ratio, and this ratio can obviously be bettered either by increasing the power or by reducing the weight. In many small cars of medium size, the engine will develop about 40 b.h.p., and the total weight will be about one ton, giving m round .figures a ratio of 2 b.h.p. per hundredweight. To get a better acceleration than is possible with such a car the designer has two methods available: to increase the output 'of the engine or to reduce the weight of the vehicle. Thus, if the weight can be cut down to 15 cwt., the acceleration would be exactly the same as though the weight had remained unchanged and the power had been increased to 53 b.h.p. This point is emphasized, because ill many cases the performance of a car has been quite spoilt by overloading it with cumbersome bodywork, and then, to bring the acceleration .back to normal, the designer has been forced to wring a few more "horses" from an unwilling engine, a practice Avhich may easily make a smooth-running power unit into a rough one. Nowadays, however, with light sixcylinder chassis and fabric bodywork, power-weight ratios show signs of an all-round improvement. . :It is also worth mentioning that when an engine is causing a car to accelerate it must also provide power to increase the speed of the main rotating parts, such as the flywheel, transmission shafting and road wheels. The rotational inertia of these parts is not very great compared with the inertia of the car as a whole, but -nevertheless make a difference to the performance obtainable.

It is very interesting to study how the acceleration of a car m top gear varies according to the speed. However, before explaining how and why the acceleration changes from moment to moment, it ' is necessary to devote a little space to description ■ of the forces which are available to cause this acceleration. The explosions m the cylinders, < transmitted through the connecting : rods, produce a turning effect on the crankshaft known as torque, and this is reduced slightly by the friction prei sent m the transmission, but greatly ; amplified by the back-axle gearing, finally emerging m the form of a torque tending to revolve each o^ the rear wheels. ' . This torque produces a backward kick on the road surface, tending to i cause wheel-spiri, and a forward propulsive force acting upon the car through the back axle. Now an important characteristic of the petrol engine as used m private cars is that when the throttle is opened wide the torque remains practically constant from about 500 r.p.m. up to 2500 r.p.rrt., and then falls off' slowly as the peak of the power curve is approached. f Consequently, the gear ratio and other factors not being subject to variation, the propelling force acting upon the car when driven m top gear will be practically constant from 10 m.p.h. up to about 45 m.p.h., and will then fall off only slowly. The resistance which this propelling force has to overcome, however, increases fairly rapidly with increases of bpeed, and is made up of rolling friction and windage. . Consequently, the surplus or excess of propelling force over resistance, which is the effort available for the acceleration of the car, grows less and less as the speed increases.