Dave Moulton’s Blog: Head Angles and Steering

In an older post, in which he get’s into stability and handling, Dave helps perpetuate two old misconceptions:

Because you are riding straight the gyroscopic action of the spinning wheels, plus your own weight and momentum, is holding you vertical just as surely as if you were physically holding the front wheel.

1. Gyroscopic action simply does not work like this. Instead, either a spinning object precesses in response to an applied torque, as is approximately the case with the front wheel of a bike, or it is prevented from precessing and so reacts to an applied torque exactly as if it were not spinning at all, as is approximately the case with the rear wheel of a bike. Neither holds anything vertical. The front wheel steers to accelerate the contact patches laterally and create a roll moment to counter the roll moment of gravity on the leaning bike, and the rear wheel simply leans with the rear frame.

2. The forward moment of the bike and rider contribute zero resistance to tipping. Linear momentum is a vector quantity and so linear momentum in the forward direction is completely independent of linear momentum in the lateral direction.

3. The mass of the bike and rider resist acceleration due to the force of gravity exactly the same when moving forward as they would when stationary. Whatever velocity they may have makes no difference.

Dave is no longer accepting comments on this post, so I cannot point out these mistakes there. Perhaps he will see them here and make a correction.


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