# 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.