# What does the VBOX use as zero velocity?

By default the VBOX starts logging when the speed goes above 0.5kmh. 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 0.5kmh and 0.0kmh is negligible. In order to illustrate this, look at the following table where we analysed the data from a brake stop at 100kmh to 0.5kmh and from 100kmh to 2kmh to exaggerate the effect:

 Run Num Dist to 2km/h Distance to 0.5km/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 2kmh only makes a 1-2cm difference. In fact, any braking distance measurement is taken down to 0.8km/h, as this eliminates the effect of Rock Back.

All acceleration runs use 0.5km/h as a starting point. On an acceleration, setting the start speed to 0.5kmh 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 0.02s.

11:48, 12 Dec 2014