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10 - Brake Test SW Glossary of terms

Commonly used for applications such as measuring side wind susceptibility, and vehicle deviation during various manoeuvres; the centre line deviation function can be used to show the deviation of a vehicle from a specified centre line to an accuracy of between 1 and 2cm (over a 30 second time period).

The high accuracy, Doppler derived velocity channels are measured and used to determine the lateral deviation from a specified track heading at every sample using geometric theories. This calculated deviation (lateral displacement) is an accumulative distance measurement, so it is most accurate when measured over short periods of time.

The above diagram simply displays how the accumulative deviation from a given centre line (H) is calculated.

Once the centre line deviation value starts calculating (S1), it will initially take the heading value recorded by the Doppler shift, and create a heading difference value (h1), relative to the defined Centre line deviation heading. Knowing the velocity at which the vehicle was travelling between samples (v) allows a sample measured deviation distance to be accurately determined. Using trigonometric relationships, this deviation distance is calculated using the following equation.

(Sin (heading diff) * Speed(m/s))*sample time

For successive samples a heading difference is then determined by comparing the current heading sample to the one previous.

The sample measured deviations are then accumulated throughout the test to produce a final centre line deviation result.

The corrected distance measures the point at which the brake trigger is activated to the test end speed, corrected by multiplying the total distance by the ratio of the square of the trigger speed to the nominated ‘corrected distance start speed’

Corrected distance =

Where:

*Nom Speed* = a specific speed or the nearest 10km/h (or mph) of the trigger speed.

In the real world it is very difficult for test drivers to hit the brakes at a set speed within 0.1km/h repeatedly. The Corrected trigger distance calculation compensates for the over, or under speed application and normalises the result to a set start speed value maintaining the original initial braking reaction time, as if the brakes had been applied at that exact set speed.

This deceleration figure is used to show the maximum deceleration figure a vehicle can achieve. It is usually the deceleration between 80% and 10% of the trigger activation speed, the time at which the vehicle is loaded up and braking at its highest achievable level.

The MFDD is calculated by the following formula:

MFDD = ((v_08)² – (v_01)²) / (25.92 * (s_01 – s_08))

Where:

v_08 is the speed at 80% of the brake trigger activation speed.

v_01 is the speed at 10% of the brake trigger activation speed.

s_08 is the distance at which the speed is v_08.

s_01 is the distance at which the speed is v_01.

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