Rider Education Of New Jersey: Stability And Cornering

While there are several good details in this article, including the observation that the best way to

restore the situation where the contact patch line is under the Center of Gravity

is to

Move the tire contact patch line back UNDER the new Center of Gravity

it does invoke not one, but two common misconceptions:

1. Forward inertia of the motorcycle will tend to keep the bike moving in a straight line.

2. Gyroscopic forces, primarily of the two wheels, but also other components whose axis of rotation is in the same direction as the wheel spindles, tend to resist any change in the angle of lean of the motorcycle.

To understand the problem with the first point, it is helpful to remember that inertia is a vector quantity, and as such, the inertia in one direction, such as straight ahead, is completely independent of the inertia in an orthogonal direction, such as to the side. Therefore increasing inertia in the direction of travel can have no effect on inertia to the side.

Instead, by moving forward faster, a smaller steering angle is necessary to accelerate the contact patches in the direction of lean

The problem with the second point is that gyroscopic forces of the wheels, and any other spinning parts simply do not resist changes in the lean angle of the motorcycle.

Instead, those spinning parts simply move, in response to an applied torque, in a way unlike that of non spinning parts. This is called precession. If a spinning part is prevented from precessing, as the rear wheel and most engine parts are, by the friction of the front and rear tire contact patches, then they roll in response to applied roll torques exactly as they would if they were not spinning.

The two roles that gyroscopic effect can have on the front tire are:

1. to generate a steering torque towards the direction of lean

2. to generate a roll torque in response to apply steering torque.

The first can contribute to the overall tendency of the bike to steer in the direction of a lean, but only if the front wheel is free to steer.

The second can contribute to the desired leaning of the bike in response to an applied coutersteering torque on the handlebars.


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