Article | REF: BM2582 V1

Automotive transmissions - Role and determination of gear ratios

Authors: Elian BARON, Philippe PESCAROU

Publication date: July 10, 2011, Review date: June 29, 2023

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2. Resistance to vehicle travel

As we have seen, the automotive transmission adapts the power delivered by the engine to the power required at the wheel, by reducing torque and engine speed. The power required at the wheel is a function of the force resisting the vehicle's forward motion, itself the sum of various components which are detailed in this paragraph.

2.1 Aerodynamic resistance

Bernouilli's law states that the resistance exerted by the air on a moving body is proportional to the air's density, the body's frontal area and the square of its forward speed. We also need to take into account the fact that the force required by the body to move through the air depends on its ability to "push aside" the threads of air in its path. This ability to penetrate the air is reflected in a coefficient...

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