What does the VBOX use as zero velocity?
By default the VBOX starts logging when the speed goes above 0.5 km/h. There is an option to change this to log all the time, but what difference can this make to the accuracy of the measurements?
In a brake stop, the distance travelled between 0.5 km/h and 0.0 km/h is negligible. In order to illustrate this, look at the following table where we analysed the data from a brake stop at 100 km/h to 0.5kmh and from 100 km/h to 2 km/h to exaggerate the effect:
| Run Num | Distance to 2 km/h | Distance to 0.5 km/h | Difference (m) |
|---|---|---|---|
| 1 | 42.02 | 42.04 | 0.02 |
| 2 | 40.27 | 40.29 | 0.02 |
| 3 | 52.29 | 52.30 | 0.01 |
| 4 | 39.22 | 39.24 | 0.02 |
| 5 | 40.47 | 40.48 | 0.01 |
| 6 | 40.87 | 40.89 | 0.02 |
You can see that even changing the end point to as high as 2 km/h only makes a 1 – 2 cm difference. In fact, any braking distance measurement is taken down to 0.8 km/h, as this eliminates the effect of Rock Back.
All acceleration runs use 0.5 km/h as a starting point. On an acceleration, setting the start speed to 0.5 km/h gives the system some tolerance to velocity noise when stationary, this stops the system triggering a false start. Again, this low threshold makes very little difference to the acceleration time from rest, usually less than 0.02 s.
See also