Explanation of Results
There are 8 variables that can be measured using the BRAKEBOX
- Average acceleration
- Mean fully developed deceleration (MFDD)
- Path distance
- Forward distance
- Deviation distance
- Direct distance
Of these time and speed are self explanatory. Average acceleration is simply is calculated from:
Average acceleration = change in measured 3d speed / time interval
The sign convention used is that acceleration is positive, and deceleration is negative.
Mean fully developed deceleration (MFDD) is defined as the average deceleration between 2 set proportions of the vehicle speed at the start of the test; 80% to 10%, 80% to 20% and 70% to 30% are all typical values. The MFDD thresholds can be set by the user using the configuration menu. The rational behind the use of MFDD is that is measures the deceleration during the fully braked part of the test, without including the entry to the test and the final stopping of the vehicle, both of which are affected by pitching moments, and in the case of entry to the test by the time to develop full braking pressure. Only one value of MFDD is produced per test, and hence this variable is not tabulated. The sign of MFDD is opposite to that of average acceleration (above) in that the deceleration value is positive. Some tests may not produce a valid MFDD value, such as when the end speed of the test is set higher than the lower proportion of the start speed, and in this instance no value of MFDD will be shown.
The last 4 are four distinct distance measurements, as shown in figure 4, and described below:
- Path distance. This is the distance the the car actually travels between the the point at which the test starts and where the vehicle comes to a rest. For further clarity, imagine that as the vehicle braked it traced a line of chalk on the road. The path distance is the length of this line.
- Direct distance. This is the straight line distance between the point at which the braking test starts and where the vehicle comes to a rest. This is the distance that would be measured by a tape measure stretched between the start and end points of the brake test.
- Forward distance. This is the distance that the car moves forward in the direction of the initial heading during the test. The direction that is defined as forward in this instance is taken from the 0.5 second period before the brake test starts, so for this result to be meaningful care must be taken to make sure that the car is moving in a straight line immediately before the test starts.
- Deviation Distance. This is the distance that the vehicle deviated from a straight line during the test. As with the "forward distance" forward is defined as the direction of travel for the 0.5 second period before the brake test starts, so for this result to be meaningful care must be taken to make sure that the car is moving in a straight line immediately before the test starts. The sign convention is that deviation to the right is positive, and deviation to the left is negative.
The 4 distance measurements are not independent, in particular by simple Pythagoras:
Direct distance2 = Forward distance2 + Deviation Distance2
It should also be clear from the diagram that if the vehicle travels in a straight line whilst braking then
Direct distance = Forward distance = Path distance
Deviation Distance = 0
Figure 4: Distance measurements