Tire Rolling Radius and Correcting Wheel Speeds

In the introductory pages of Data for Motorcycles, we briefly discussed tire rolling radius and GPS speed vs. wheel speed, and the potential errors that can arise when using a wheel-speed-based data acquisition system. Here we will address this issue and how to compensate for that error when using tire rolling radius in other channels.

GPS speed vs. wheel speed with no correction

Figure 1: GPS speed is shown in black and wheel speed in red. Whenever the motorcycle is leaned over, wheel speed reads significantly higher than GPS speed due to the change in rolling radius of the tire.

To recap, when a motorcycle leans into a corner, the rolling radius of the tires decrease, and this alters the relationship between measured wheel speed and actual ground speed. The most common evidence of this is in the speedometer, which will show a higher speed the more the motorcycle leans over, even though actual speed does not change. Wheel speed is typically measured by using a sensor that counts pulses from a transmission gear, a sensor ring, or some other set of objects. This count gives a number of wheel revolutions per minute or second, which is then converted to speed by multiplying by the tire’s circumference.

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