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.
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.
The fork potentiometer on this Ducati superbike is mounted in front of the fork tube with the exposed portion nicely protected by the front fender.
For accurate, usable data it’s important to use good-quality suspension potentiometers intended specifically for data acquisition. These suspension potentiometers live a hard life exposed to the elements, and are subject to extremely high shaft speeds; look for units that have good specifications for resolution, repeatability and operating speed. We have had good success with the units offered from AiM, but there are others available.
Another important consideration for good suspension data is the sample rate of your base unit. While it’s possible to obtain useful suspension travel data logged at 100 Hz or even 10 Hz, if you wish to perform frequency or velocity analysis you will need to log potentiometer data at 200 Hz or higher. Wheel travel often exceeds velocities of 300mm/s, and a slow sampling rate can cause plenty of information to be lost.