# Gear Position

Figure 1: GPS speed (black) and the gear position math channel (red) are displayed here. The lowest level of the gear position trace represents first gear here; each successive upward step is one higher gear, to a maximum of fourth on this particular lap.

While most motorcycles are equipped with a gear position sensor and this can be fed directly to the data acquisition system to determine gear, using a math channel to calculate gear position has a number of advantages. The ratio of rpm and rear wheel speed is directly proportional to the gear selected, and we can create a math channel as such. A gear ratio can be calculated as follows:

$\text{gear ratio} =\frac{\text{engine rpm}}{\text{countershaft rpm}}$

Alternatively, rear wheel speed can be used and the secondary gearing factored into the equation.  From here, you can use a lookup table to relate the gear ratio to the actual gear position. You will need to know the primary gear ratio and internal transmission ratios for your motorcycle; incorporate the primary ratio either in the gear ratio calculation or the lookup table. In most cases, however, it is easy enough to decipher the graph without the use of the lookup table – the gear position is fairly obvious from the chart.

Remember that changing secondary gearing will affect the gear position calculation if you are using rear wheel speed and a lookup table. Figure 1 shows a lap of Laguna Seca with GPS speed in black and the calculated gear position in red and here we can see one of the advantages of using a calculated gear position rather than simply logging the gear position sensor – the trailing edges in the graph represent each downshift, with the small upward spikes showing how well the rider matches rpm to wheel speed. These tails should be minimal, and the rest of the downward trace should be smooth and clean. A sloped line – not exactly vertical – represents the rider feathering the clutch out after the downshift, while the vertical spikes show a quick release of the clutch and mismatch of rpm and wheel speed. In addition to displaying the rider’s shifting behaviour, this graph will also show potential troubles such as a slipping clutch.

Figure 2: Using GPS speed as opposed to rear wheel speed for the calculation makes it much more difficult to decipher the gear position trace. Using wheel speed is the optimum solution, but in the absence of that channel additional data can be incorporated into the calculation to make the gear position channel easier to understand.

Note that rear wheel speed or countershaft speed must be used for this channel, as the direct relationship between that and engine rpm makes this calculation exact. Figure 2 shows the same lap of the track but with GPS speed used instead of countershaft speed for the calculated gear position channel; here, the changing circumference of the rear wheel disrupts the relationship, and the gear channel is almost incomprehensible. It is somewhat possible to discern the gear position on the straight sections of the track, but in turns and sections where the motorcycle is leaned, the gear trace is difficult to decipher.