Gyroscopic Effect

The Gyroscopic Effect is a very important physical effect in a motorcycle. Because of effect of the law of angular momentum conservation, a body rotating around its own axis tends to maintain its own direction. That means that it could freely shifts over a plane perpendicular to the rotation axis, just like a motorcycle’s wheel that moves forwards (the axis shifts forwards, but retains its direction) but tends to remain vertical: or, if you prefer, like a spinning top that moves by staying upright until its rotating speed allows it. So far it’s all right, because the gyroscopic effect will prevent us from crashing because the wheel’s rotation axis will tend to stay horizontal.

The bad news is that this stabilization effect on a motorcycle is negligible compared to its undesired collateral effect: when the axis on which the body is rotating around is stimulated to change its orientation, for example during a corner in the case of a motorcycle, by the effect of the law of angular momentum conservation, the same axis reacts by generating a torque on the perpendicular axis towards the rotation’s one.

effetto giroscopico

Demonstration of the gyroscopic effect: the wheel spins around the red axis. By applying a torque on the blue axis (for example the action of the handlebar on the steer), a torque is obtained over the green axis, perpendicular to both the red and the blue axes.

In the specific case of a motorcycle, the action on the steer does apply a torque on the vertical axis that generates as a reaction a torque on the longitudinal axis; that tilts the vehicle laterally. The downside is that such torque works in the exact opposite way compared to what is needed: to be clear, if we steer on the right, the gyroscopic effect does tilt the motorcycle to the left and not towards the inner side of the corner like a rider would do. If, on the contrary, we tilt the motorcycle on the right, so applying a torque over the longitudinal axis, we obtain a torque on the vertical axis that makes the wheel steering (but on the left, and not where we expect).

It’s to be underlined that the major of riders, without knowing on details this complex physics phenomena, they know by experience these effects and, more or less unknowingly, they learnt to use them at their advantage. A classic example is the opposite lock, that is when the rider steers imperceptibly to the right before facing a left corner, to help himself to tilt the motorcycle on the proper side.

Although being perfectly manageable by experience, remain the fact that an excessive gyroscopic effect holds a series of noticeable fallouts on the riding that afflict the handling and the readiness, especially during the corners. So it is to be preferred to reduce this effect as much as possible, and to achieve this there’s only one way: by reducing the mass of the rotating body, in our case the wheel.

More specifically, it is necessary to reduce the mass away from the hub, which means the mass with a major inertial momentum. Given a fixed angular speed, the mass that are farer to the centre of the wheel spin at a higher tangential speed, holding a higher energy compared to the hub (which holds a negligible inertial momentum). That’s why the most external mass on a wheel (like the tyre, the rim and the tube) are the main causes of the gyroscopic effect. The sole removal of the tube allows a weight reduction starting from 800g (1,76 lbs.) to 2Kg (4,41 lbs.) per wheel: this is one of the advantages of the tubeless tyre.