We've used data acquisition for comparison tests before, but this was our first chance to try Racepak's G2X, a GPS-based system that uses satellite information rather than speed sensors, gyroscopes and accelerometers.
The Global Positioning System utilizes 24 satellites in geosynchronous orbits around the earth and was originally developed for military use. Using trilateration (think triangulation but in three dimensions), a GPS receiver can calculate its exact position and altitude on the Earth. The G2X saves this position data to a compact flash card, which can then be downloaded and analyzed using Racepak's software.Installation of the system is simple enough; for basic data recording all that is required is to power the unit, secure it in position and run the GPS antenna so that it has a clear view of the sky. We used a tankbag strapped to the passenger seat, with power coming from each bike's battery and the antenna slipped inside the bag's map pocket. We drove around the track once on the racing line for a basic track map, then around the inside and outside of the whole course for a complete and real map--the system is that accurate.
With the data downloaded, it's a simple matter of marking the start-finish line on the track, and lap times are calculated from the data--no beacon is necessary. Furthermore, the track can be split into as many segments as desired and split lap times shown. The company claims a speed accuracy of 0.1 mph and a lap-time accuracy of 0.05 seconds. One nice aspect of the G2X is that recorded speed is not subject to the variable of changing tire circumference, which can impact data significantly in a system that uses a wheel-speed sensor. While tracks can be broken down into corners or segments with a standard system, the segments are all based on distance from a beacon, which is variable because of the tire's changing circumference. We have a lot more confidence in the G2X's segment times and corner-speed numbers, as the mapping and distances are more accurate and repeatable.
The basic $899 G2X system includes an input for recording rpm and internal accelerometers. A dashboard, also included, can be used to show lap times, speed, gear position or any of the recorded data. For more inputs, such as suspension travel or throttle position, additional modules and sensors are required at a cost of $100 each.