Longitudinal acceleration

A typical GPS longitudinal acceleration trace (red) is displayed here with GPS speed (black)

Figure 1: A typical GPS longitudinal acceleration trace (red) is displayed here with GPS speed (black). Look for crisp transitions from acceleration to braking at the end of each straight, and smooth, gradual transitions from negative to positive g in each corner.

Figure 1 shows a typical graph of speed and longitudinal acceleration for a middleweight machine at Willow Springs. The scale of the chart measures in units of g, with positive g corresponding to acceleration and negative g indicating braking. With GPS-based systems, you will have the option of displaying either acceleration as measured by an accelerometer inside the unit or based on the GPS signal. Different software packages name the two channels in a variety of ways, and it’s important to distinguish between the two; we will always deal with GPS acceleration if possible, as it’s a much more accurate data stream. Look for a channel called GPS longitudinal acceleration, GPS G or some combination of those terms.

Characteristics of a good acceleration chart include a quick transition from positive g to maximum negative g at the end of each straight, indicating that the rider is making the throttle-to-brake transition quickly as well as using maximum braking immediately in the braking zone. Note that acceleration decreases as speed increases and aerodynamics take over; as the motorcycle reaches maximum speed, acceleration will approach zero even though the rider is still at full throttle. Likewise, at the end of each braking zone look for a gradual transition from negative g (braking) to positive g as the rider gradually lets off the brakes in the corner and feeds in the throttle.

Most motorcycles will brake at a rate of approximately -1 g. Check that this is the case in each braking zone, and if one braking area shows significantly more or less that the other braking areas, quiz the rider as to why the difference. It may be because the track goes uphill or downhill, the presence of a slippery section, or bumps in the braking zone that are the cause. A typical 600cc machine will accelerate at approximately .5 g in the lower gears, less as speeds rise. Larger machines will approach as much as 1g in the lower gears. Ensure that the rider is exiting each turn with the maximum acceleration possible, and again question any oddities.

In the math channels section, we will look at ways to dissect the longitudinal acceleration channel in various ways that can make it much easier to see these – and more – aspects.

Figure 2: The trace for acceleration (blue) as measured by the unit's internals is added to the same data shown in Figure 1.

Figure 2: The trace for acceleration (blue) as measured by the unit’s internals is added to the same data shown in Figure 1. Note that the non-GPS signal is noisier than the GPS signal, and diverges in certain areas corresponding to altitude changes.

The unit’s internal acceleration channel for the same lap is displayed along with the GPS channel in Figure 2. The signals are almost identical, although there are instances where they diverge significantly. Remember that for the internal accelerometer to work correctly, the unit must be perfectly level and square on the motorcycle; any deviation will affect the acceleration readings. Even if the unit is correctly mounted, it will not be level-generating a corresponding margin of error-as the motorcycle pitches under acceleration and braking. As well, the internal accelerometer is affected adversely as the motorcycle goes up and down hills; in this graph, as the motorcycle goes uphill in sector 3 and downhill in sector 4, the internal accelerometer reads higher and lower respectively than the GPS channel, which more reflects reality. Finally, note that the GPS acceleration graph is much smoother than the non-GPS graph; this is with minimal smoothing for the GPS trace and maximum smoothing for the non-GPS trace.

Discerning more information from the acceleration trace on its own is difficult. For further analysis, we must look to lateral acceleration and combinations of the two channels.

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