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.