Anti-Squat Geometry

anti-squat-geometry-kawasaki

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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Suspension Analysis – Squat

suspension squat data

Suspension data for a typical lap showing speed (black), front fork travel (blue), rear wheel travel (red) and squat (green).

One important aspect of motorcycle setup, especially as bikes become more powerful, is how the suspension reacts when the motorcycle accelerates. As we have discussed previously in the section covering front and rear weight, load transfers during acceleration and braking and acts to compress or extend the suspension. This load transfer causes the bike to “squat” to the rear under acceleration, and in an extreme example, all the weight can transfer to the rear wheel in a wheelie.

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Front and Rear Weight

An approximation for front and rear dynamic weight can be determined from longitudinal acceleration and values for the wheelbase and center of gravity position.

An approximation for front and rear dynamic weight can be determined from longitudinal acceleration and values for the wheelbase and center of gravity position.

Now that we have looked at static weight distribution and also found how cornering forces add to the total weight of the motorcycle and rider, in this post we will show how total weight is distributed between the front and rear wheels. As the motorcycle accelerates, weight is transferred from the front wheel to the rear wheel; under braking, weight transfers from the rear wheel to the front wheel. Note that the acceleration and braking forces act on what we have designated the total weight of the bike and rider, which includes cornering forces.

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