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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Anti-Squat Geometry


The typical sportbike has the countershaft in front of and slightly above the swingarm pivot, giving the motorcycle anti-squat geometry. (Courtesy of Kawasak)

As we have discussed, when a motorcycle accelerates, load is transferred to the rear and increases the weight or load on the rear suspension. This acts to compress the rear suspension just as forward weight transfer under braking acts to compress the front suspension, causing the motorcycle to squat under acceleration. By using characteristics of the rear suspension design and the layout of a typical chain drive, however, we can give the motorcycle anti-squat geometry tendencies that offset the compression from load transfer, in turn improving handling performance on corner exits as the motorcycle accelerates.

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Histograms and Suspension Velocity Analysis

Ideal suspension velocity histogram

A theoretically ideal histogram for suspension velocities, showing a bell-shaped distribution of velocity over time. Positive values represent compression, while negative values represent rebound.

One very useful technique for suspension setup is to use histograms and suspension velocity analysis to evaluate spring and damping settings. Histograms put values to how much time the suspension is active in each damping range (high-speed and low-speed compression and rebound damping), and by using some basic statistical analysis techniques, that information can be used to make spring and/or damping adjustments to the motorcycle and improve its performance.

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Data Acquisition Math Channels Reference

This page is a summary of useful data acquisition math channels that are referenced on Data For Motorcycles. Because every data software package deals with operators and channel names differently, you may have to make changes specific to your software. For example, some packages have a built-in derivative function that can be used, while others use a completely separate derivative channel and a second math channel must be built to accommodate the entire formula.

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