The Helix roller coaster is unusual by starting in a high point, with a small ‘first drop’ and an inversion before reaching the first launch, as seen in the sequence of screen shots taken from a short video.
As the train rolls out, down the hill, it moves faster and faster. When the tail of the train leaves the station, the front of the train has already reached the bottom of the valley and the forward acceleration is reduced. The train then slows down as it moves up the next hill. The short movie clip can be used in an application, such as Tracker.
See two screen shots from tracking the motion of the front as well as the axis below rows 6 and 10. The front of the train moves slowly out from the station, as reflected by the short distance between the time stamps, compared to the much longer distance for the axes at the track of the train. The length scale is obtained from the distance 2.5 m between successive wheel axes. The acceleration is quite small in the beginning, since part of the train is still inside, on a horizontal track.
The figure below shows the horizontal (x) and vertical (h) coordinates from the video for each of the wheel axes as the train rolls out from the station.
Small or negative G forces are normally expected in airtime hills. However, the sensor data for the start of Helix, shown above together with an excerpt from the drawing, reveals that riders in the back of the train experience ‘negative G forces’ not in the highest point, but just before entering the valley, related to the shape of the track.