This G-G plot combines lateral and longitudinal acceleration and gives an idea of how effectively the rider utilizes the traction circle. The top half of the chart shows acceleration, the bottom half shows braking; the right and left halves show right-hand and left-hand cornering. This G-G plot combines lateral and longitudinal acceleration and gives an idea of how effectively the rider utilizes the traction circle. The top half of the chart shows acceleration, the bottom half shows braking; the right and left halves show right-hand and left-hand cornering.

Analyzing GPS data - lateral and longitudinal acceleration

Written by  on Thursday, 05 April 2012 16:32

In the current issue of the magazine and in my last blog, I talk about GPS-based data acquisition systems and touch on some of the GPS channels available. As a tool to improve your riding, the accurate position data and segment times available from a GPS-based system are the most useful information, but the lateral and longitudinal acceleration channels are also very helpful. Longitudinal acceleration refers to acceleration in a straight line, with a positive value to indicate what we normally call acceleration and a negative value for braking. Lateral acceleration is an effective measure of cornering performance and what automotive people test on a skidpad.

One common misconception that many riders have is that "faster" is better, and that speed is the most important variable. But in reality, on the racetrack we are trying to minimize the lap time; more speed does not always equate to less time. For example, you can go faster in a given corner by taking a wider line, but because you are going further it may actually take you longer to complete the same arc. Lateral and longitudinal acceleration data is irrespective of speed or distance, and gives a much more accurate representation of performance than does speed alone. In general, more lateral acceleration through a given corner or more longitudinal acceleration on a straight corresponds to quicker lap times.

There are limits to lateral and longitudinal acceleration based on physics, the motorcycle and the tires used, but mostly it comes down to tires and traction. For example, most motorcycles are capable of braking at just over 1 G of longitudinal acceleration, limited partly by the rear tire coming off the ground but also the front tire locking up. Acceleration in a straight line is limited by the engine's performance, but - again - also by the rear tire's traction. And lateral acceleration is determined almost completely by the tires. How close a rider gets to these maximum values is one indication of effort and skill, and is easily seen in the raw lateral and longitudinal acceleration data.

For experienced riders that can regularly reach those limits of lateral and longitudinal acceleration, the next aspect to consider is how quickly the rider can get to, and stay at, those limits. Expert-level riders can get to maximum braking of 1 G at the end of a straight in about 1 second, and hold that level of braking through the entire braking zone. Likewise, expert-level riders quickly and smoothly transition the motorcycle to maximum lean (and maximum lateral acceleration) and hold that value all the way through the corner. Manipulation of the raw GPS data helps here; math channels can show braking G or cornering G separately, and a derivative channel will show the rate of change.

Another measure of riding skill is how well the rider combines lateral and longitudinal acceleration by trail braking into a corner or accelerating out of a corner while leaned over. This can be shown graphically in several ways, but the most interesting method is to use an X-Y plot of the two acceleration channels. This plot, commonly referred to as a G-G plot, effectively shows the traction circle and how close the rider is to utilizing the maximum available traction (shown by acceleration) at any given time. Some data acquisition software allows you to create such graphs; with others you may have to export the data to Excel and make your own.

The G-G plot shown here displays data from an entire practice session at Las Vegas Motor Speedway's Classic Course, with Javelin Broderick aboard his Yamaha YZF-R6. You can quickly and easily see how hard Javelin is braking and cornering, as well as how much trail-braking he uses and how much acceleration while leaned over. Any points outside the general cluster of the graph require further investigation from both safety and performance aspects. This is just one way to combine and display the lateral and longitudinal acceleration data, but is generally considered the best for showing the most information in a simple format.

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Last modified on Thursday, 05 April 2012 16:37
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