Mounting the GPS antenna on the tailsection provides good results in general for a motorcycle.
When dealing with GPS-based systems, the quality of reception from the GPS satellites is all-important as position provides a reference for all other data. Unfortunately, a moving motorcycle is not the most suitable platform for a GPS antenna. In a car the antenna moves precisely along the path of the car and remains stationary with respect to the car. On a bike, however, the antenna is both moving off the track of the wheels and leaning as the bike moves. Both provide a certain amount of error and difficulty in reception.
As with any electronic devices, a good, clean installation pays off in performance as well as reducing problems with stray wiring. With motorcycles it’s often difficult to simply find enough room for a data logger and all the associated wiring, and a rat’s nest usually results. In general, some planning beforehand with placing the unit, routing the wiring correctly and clearly marking everything goes a long way to making troubleshooting easier in the event of a problem.
This plot shows how speed measured by GPS (red trace) differs from speed measured by a rear wheel speed sensor (blue). As expected, the error is magnified when the bike is leaned over, but note also the discrepancy under braking, where the rear wheel slows significantly compared to GPS speed.
Velocity, as measured by a wheel speed sensor, is affected by several factors that must be taken into account when analyzing data. Speed and distance are both calculated from the rotational speed of the wheel (or countershaft) and take into account the circumference of the tire. This means that changing the tire to one with a different circumference will affect the speed data. For example, if you replace a tire that measures 2050mm in circumference with one that measures 1980mm (roughly equivalent to swapping from a 190/55 rear tire to a 180/55), your speed data will decrease by a factor of approximately 3.5 percent (2050/1980) – just as your speedometer would now read lower by the same factor.
Non-GPS-based data acquisition systems rely totally on a wheel speed sensor as the basis for data collection. The more care and thought you put into this aspect of setting up your system, the more reliable and accurate your data will be, especially when it comes to comparing data from different bikes or different sessions where the bike’s setup may have changed.
When purchasing a data acquisition system, there are several factors to keep in mind. Practically any system – GPS-based or not – will record basic speed data and generate a track map, which will get you started and provide plenty of data for the novice rider to learn from. Most systems will, either as standard or through expansion, allow the addition of more sensors to allow you to collect more data. Continue reading