Tag Archives: gyroscopic forces

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.

PhysLink.com: Do gyroscopic forces from the wheels make any significant contribution to the rideability of a bicycle?

PhysLink.com tries to answer the question, and the author of the reply does not beat around the bush:

Yes, the gyroscopic forces, better known as the angular momentum, of the wheels on a bike allow us to ride a bike. … The reason that you stay up on a bike is that angular momentum, like regular momentum, must be conserved if no external torques act on the object. … When you are riding a bike forward, the right hand rule gives the direction of angular momentum to be to the left, perpendicular to the wheel. This direction does not want to change, therefore the wheel wants to stay upright and it makes the bike very ridable.

Wow. Flat out wrong in many ways and no mention of steering at all.

1. The  David Jones Physics Today article from 1970,  demonstrates that a bike is quite ridable if the gyroscopic forces are canceled.

2. Gravity acting on the center of mass and the ground reaction forces create a couple that acts as a large external torque. If there were no external torques, bikes wouldn’t tip over at all and there would be not need for a way to balance them

3. The direction of the angular momentum vector can easily be changed by the application of an external torque, such as the one described in the previous point.

The author even goes so far as to suggest that he has seen Jones’ article but comes to the opposite conclusion that Jones does.

However, when the extra wheel was spun backwards, the bike became almost impossible to ride because the vectors for angular momentum cancelled each other out. It was like trying to balance a bike that was not moving.

Instead, a main point of Jones’ article is how hard it was for him to make an unridable bike.