Earlier we discussed the G-G plot (an X-Y plot of lateral and longitudinal acceleration) and the traction circle and how it combines the two acceleration values into an easy-to-read format. The G-G plot, shown here in figure 1, graphs the two forces combined for every point on the racetrack and is useful for a quick evaluation of the overall lap. Ideally, the plot would consists of points only on the outer edge of the traction circle, with no points filling the graph – in other words, the rider is always using the maximum traction available by accelerating, braking or turning at any given point on the racetrack. On the G-G plot, this measure of how much traction is being utilized – usually called Total G – is viewed as how far the point is from the center of the plot and can be calculated by summing the lateral and longitudinal accelerations using vector addition and the Pythagorean theorem:
GPS_LatAcc=GPS lateral acceleration
GPS_LonAcc=GPS longitudinal acceleration
To picture vector addition, imagine going from your home to the nearest Starbucks. You can walk three blocks north and four blocks west to get there, but the total distance is five blocks as the crow flies – do the math yourself, substituting the distances for the accelerations in the formula above. In terms of acceleration, we can add .3 g of lateral acceleration and .4 g of longitudinal acceleration for a total of .5 g. Note that the equation is similar to that of a circle with a radius of the maximum total acceleration (just as you can picture a five-block radius from your house).
The Total G math channel shows just how close the rider is to the edge of the traction circle at any given point over the course of a lap. Figure 2 shows a lap at Auto Club Speedway with GPS speed and total G displayed.
It may appear jumbled at first, but the total G trace can show at a glance some areas that need attention. To better understand what is happening, figure 3 shows a portion of the lap with some other data added. When the rider is cornering only, the total G trace follows the corner G trace, and when the rider is braking or accelerating only, the total G trace follows the braking G and acceleration G traces.
However, when the rider is combining lateral and longitudinal acceleration, the total acceleration channel represents the vector sum of the two GPS channels. Note that there are only a few instances where the total acceleration should dip to zero: When the rider transitions from accelerating to braking in a straight line, total acceleration will momentarily be equal to zero; as the motorcycle speeds up to reaches top speed, total acceleration will gradually decrease until it is zero at top speed; finally, when the rider transitions from side to side in some chicanes, total acceleration will be close to zero. At all other times, the total acceleration chart should be close to its maximum value. Any other instances indicate the rider is coasting, and closer examination of other data is necessary to find out why. In this case, total G is between 1.0 and 1.2 g fairly consistently throughout the graph aside from the transitions mentioned, and for the longer straights on the circuit.
Areas of concern here are the dips that are not related to a transition or straight, such as at 13 seconds or 30 seconds, where total G dips below 1.0 g for an extended time under braking.