A convenient way to show the forces acting on a vehicle (or individual tyre) is the `traction circle' (Figure 8). The arrow shows the vector sum of the acceleration and turning forces being exerted on the car. The circle shows the maximum force that can be exerted by the tyre. This depends on the force pressing the tyre onto the ground as discussed in Part III.
Unless the engine is exceptionally powerful, there will not be enough torque to reach all of the forward traction circle. In this case the acceleration is limited by the engine torque.
In this lecture I'll consider the combined traction circle of all four wheels. We discuss the balance of the weight of the car between the wheels in Part V.