# X-Y plots: Speed and engine rpm

This plot shows rear wheel speed (black) and rpm (red) for a typical lap

A useful way of displaying data that can make it easier to see certain aspects is by using an X-Y plot rather than plotting the data versus time or distance. That is, two channels are plotted against each other and time or distance does not factor at all. For example, we can plot apple and orange sales for a market versus time: 5 apples and 4 oranges in the first hour, 3 apples and 6 oranges the second and so on. But we can also plot the number of apples versus oranges in each sale. This would allow a quicker correlation of the ratios involved; we would easily see if people were buying twice as many apples as oranges, for instance.

# Case study: Engine rpm, throttle position and tire circumference

Figure 1: The initial data plot shows GPS speed (black), engine rpm (red) and throttle position (blue). The area of concern is at 400m, where the engine rpm spikes as the rider leans onto the smaller circumference of the tire, effectively shortening gearing as he enters the turn.

In this situation, the rider is on a 600cc machine at Willow Springs. Figure 1 shows data for GPS speed (black), engine rpm (red) and throttle position (blue). In the area between turns 1 and 2, the rider must cross from one side of the track to the other, then arc into fast turn 2. At the 400m mark, the rider is just arcing into turn 2; as the bike leans onto the side of the tire, the reduced circumference causes the rpm to spike to the rev limiter momentarily and the rider must close the throttle slightly to account for the increase, before closing the throttle for the entrance to the turn.

# Throttle position

A typical throttle position trace for a 600cc machine at a fast track shows the throttle is wide open for a good portion of the lap. Expect much less time at full throttle on slower tracks or with larger bikes. In general, check that the throttle is wide open on every straight, and that the gearshifts are short, downward spikes.

Throttle position is the most basic and important rider input. Luckily, almost all motorcycles are now equipped with throttle position sensors (TPS) that make it an easy channel to add to your data acquisition system. This should be the first additional channel to your system, after speed and rpm. The TPS is usually found at one end of the throttle bodies, and has a three-wire connector; you can use a multimeter to find which wire has active voltage when the throttle is opened, or check in your service manual for the wiring diagram. Be sure that you connect to the correct sensor; you want to measure the rider’s throttle input, not the secondary butterflies or – in the case of Yamaha R6 and R1 models – the main butterflies.

# Engine rpm

This channel monitors the speed of the engine, and is usually tapped into the tachometer signal from the ECU. Most loggers provide a dedicated input for this channel that senses a typical tach signal, so you don’t have to use one of the system’s additional channels, nor will you (in most cases) have to provide any special configuration information. The wire to use into can be found by checking the service manual or wiring diagram for your bike; use a tap-in squeeze connector in the harness as close to your logger box as possible, to avoid running a long, unshielded wire that can be susceptible to interference.