The torque produced by the car's engine is not applied to the road directly; first, it is passed through a set of gears in order to optimise the engine speed for the road speed. The effect of the gearing is to change the number of times the engine rotates for each rotation of the road wheel. In addition to the gearbox itself, the differential (or `final drive') may alter the gear ratio as well as dividing the torque between the two front wheels. The `drive train' is illustrated in Fig. 1.
If the engine rotates many times for each rotation of the road wheel, the torque on the road wheel will be larger than that produced by the engine.
with the gear ratios of the differential and gear-box ( and ) defined as shown in Fig. 1.
The aim of the gearbox is that, by changing the gear ratio , we keep the engine turning close to its maximum efficiency (or power) even though the speed of the vehicle may change.
Figure 1: The torque produced by the engine is fed through the gearbox and differential before being converted into a force at the road surface.
The following table gives the gear ratios for an Alfa Romeo 145 QV.
To calculate the force that is applied to the road in order to accelerate the car, we need to multiply the torque at the wheel by the radius of the road wheel (including the tyre). This gives: