Total Braking Force and Braking Potential
When a vehicle is subjected to retardation, the instantaneous acceleration is negative. In order to represent the braking intensity quantitatively, the deceleration z which refers to the ratio of the deceleration aB to the acceleration due to gravity g is introduced:

Eq. 3-16

The total braking force, which decelerates the vehicle, considering the road resistances and the sum of all the forces acting on the vehicle in case of a coupled engine, is given by:
Eq. 3-17
The required wheel brake force in order to achieve a deceleration aB is then given by:

Eq. 3-18


Stability in Downward Gradients
From the qualitative description in the figure, it can be noticed that in the downward gradient, a smaller traction force demand when compared to driving in the plane results due to the force Fgr acting in the direction of the negative slope, i.e., in the driving direction. According to the downward gradient, a brake force has to be created in the lower and average speed range, in order to keep a constant driving speed. In case of heavy commercial vehicles, continuous service brakes are used for braking in downward gradients. Their characteristics, in this case an engine brake and electric retarder, are also included in the diagram. If the magnitude of the brake force of the continuous service brake is larger than the resulting force demand FR + Fdrag + Fgr, a stable driving condition is said to be achieved.
The stability ranges in the figure refer to a driving condition in which the electric retarder is exclusively used. In the range of the maximum speed vperm prescribed by law, a higher constant speed can be achieved with the electric retarder than with the engine brake. In the lower speed range however, a stable condition can be achieved only using the engine brake.