Putting gyroscopes under the microscope

A gyroscope is a rigid circular or spherical mass rotating around an axis and for over 100 years motorcycles have come with three of them: a front wheel, a rear wheel and a crank (if you're being pedantic you could include cams, balancer shafts, gearbox, clutch etc).

The classic mechanical model of a gyroscope ranges from the very small – electrons spinning around an atom's nucleus – to the very large – the Earth. They possess a number of interesting properties, but the one that matters to us is called precession. Its effects aren't always intuitive (to say the least) but they can be helpful in steering a bike. So they're worth having and, if you're designing a bike, worth knowing about.

Gyroscopic precession is, without getting snared in deep physics, the ability of a spinning gyroscope to translate force from one plane to another (at this point it helps to try and think in 3D, not 2D!).

So if you have a rotating mass (say, a wheel) spinning around an axis (the wheel axle), and you apply a torque to that axis in another plane (you move the axle), a force will appear at 90° to both the wheel axle and the axis you move the axle through.

The classic example is gripping one end of the axle of a forward spinning bicycle wheel in each hand, then leaning the wheel to the left, as if cornering it (tilting the wheel around a front-to-rear axis): a force appears in your hands making the wheel turn or steer, or 'precess' to the left. This is called roll-induced precession, and it's a good thing to have because if you're turning into a left-hander it augments the bike's roll movement. Similarly, if you're picking the bike up out of a corner, it helps pull the bike upright.

Back to the handheld bicycle wheel. If, instead of leaning it to the left you steer the wheel to the left (turning the wheel about a vertical axis), a force appears which makes the wheel lean to the right. The other way.

This is called a steer-induced precession (you may have to go and get a bicycle wheel to prove this to yourself. I just did) and it's a good thing because during the initiation of a left hand turn, you briefly countersteer to the right. This right steer immediately sets up a 90° precession which augments the bike rolling to the left (note the contribution of precession to countersteering is often overstated; it's not essential).

There are more precession effects: yaw-induced precession is the force generated as the wheels move along the radius of a turn – it effectively tries to right the bike against the direction of the turn. And finally there's the engine precession of forward-spinning engine internals which, like yaw precession, counter the bike's cornering direction. Its effects are small, but enough for Yamaha to reverse the MotoGP M1's crank rotation to maximise its rate of turn. MV Agusta do the same on their F3 triples, which are hardly slow steering.

Of course since BMW's S1000RR in 2009, some bikes come with an extra gyro: the sensor for traction control. But that's a whole other story.