# 03 - Coastdown test theory

The test is performed in accordance with European Directive 70/220/EEC – Annex III and is based upon statistical evaluation of a minimum of 4 test runs.

The test results are considered to be valid when the statistical accuracy of the average test values is ≤ 2%.

The test can be performed at any speed according to the manufacturer’s requirements, but must follow the general procedure below:

1. Accelerate the vehicle to a speed 10 km/h greater than the test start speed (user definable – 125 km/h in the diagram)
2. Place gearbox in neutral
3. Measure the time taken for the vehicle to decelerate from the start speed to the stop speed (user definable – 85 km/h in the diagram). This is time t1.
4. Repeat steps 1-3, but in the opposite direction. This is time t2
5. Take the average time of these two test runs (t1 + t2)/2. This is the time T

Repeat steps 1 to 5 until such time that the statistical accuracy of the average is not more than 2%

The statistical accuracy (p) is defined by:

Where:

= Coefficient given by the table below,

n = Number of tests

s = standard deviation

n
4 5 6 7 8 9 10 11 12 13 14 15
t 3.2 2.8 2.6 2.5 2.4 2.3 2.2 2.2 2.2 2.2 2.2 2.2
1.6 1.25 1.06 0.94 0.85 0.77 0.73 0.66 0.64 0.61 0.59 0.57