The majority of rolling roads used to determine power figures in the UK are inertia dynamometers. Inertia dynos do not directly measure the force on the dyno rollers to determine power figures. Instead, these systems calculate the force on the rollers using the formula:

F= ma     where F is force; m is mass; a is acceleration

The mass and system inertia of the dyno rollers is known. In order to calculate the force applied, inertia dynos measure acceleration of the rollers by measuring the increase in current and voltage production when the dyno's eddy-current retarders are used as a power generator instead of a power absorber (as used to hold the dyno load when mapping engines).
Force on the rollers is therefore the roller mass multiplied by the acceleration determined by the voltage output. this force is multiplied by the radius of the roller itself to give torque at the wheels using the following equasion:

                   T= Fr     where T is torque; F is force; R is radius of application

Power is determined by using the formula:

                                          BHP= Torque (ft/lbs) x RPM/ 5252

This calculation is then used for the power at the wheels measurement. If an ignition pickup on the engine is used, these power figures can be used to plot a power curve. In order to determine power at the flywheel figures, a coastdown procedure is used which measures the deceleration of the rollers, and used this figure as negative acceleration and the F=ma calculation is used again to obtain the power losses through tranmission.
The major problems with these systems occur when changes are made to any of the rotating masses int eh system. This includes items such as the clutch, flywheel, or aftermarket wheels. These items do not change the power of the engine (obviously). However they will change the rate of acceleration of the vehicle. Therefore these changes will change the power output measured on an inertia dyno. This is another reason why a number of dyno manufacturers will not guarentee an accuracy greater than 5% for their dyno systems.