We all use counter-steering to turn, whether we are conscious of doing so or not. The physics of steering a motorcycle are quite complicated yet the human brain manages to cope with all the forces involved in an almost effortless manner and learns from experience.
The most basic description of counter-steering is "push right, go right" or "push left, go left" where the rider exerts a forward (not downwards) pressure on the handlebar in the direction they wish to turn. This is very much counter-intuitive as you are effectively trying to steer the bike the wrong way.
To turn a motorcycle at speeds above about 10 mph requires the machine to lean in the direction of the turn and most of us are quite happy with this state of affairs. The motorcycle, on the other hand, really wants to sit up and continue in a straight line. This can be demonstrated when the rider is removed from a moving machine; the bike will flap about a bit before stabilising itself and will continue, upright, in a straight line until some other physical factor brings it to a halt. This is because the various forces involved firstly fight each other and then find a point of equilibrium and stabilise the bike, upright and in a straight line. This self-correcting design is built in to the steering geometry of all motorcycles (or push bikes for that matter).
So what are the forces involved? Centrifugal (or centripetal) force, gyroscopic precession and camber thrust.
Many people think that gyroscopic precession is the main force involved in counter-steering but it only initiates the lean. It is centrifugal force that initiates counter-steering and camber thrust which actually turns the bike.
Gyroscopic Precession: This is what happens when a lateral force is applied to the axis of a spinning gyroscope (the front wheel in the case of a motorcycle). The spinning gyroscope translates this force vector ninety degrees off the direction of spin. So, if the rider applies a force to the right side of the wheel axle this force tries to lean the wheel to the right. As the steering is connected to the rest of the bike it follows that the whole bike will lean.
Camber Thrust: Consider the profile of a motorcycle tyre. It is rounded, unlike a car tyre which is flat. When you ride a motorcycle upright the contact patch is in the middle of the tyre at its furthest point from the hub of the wheel. As you lean the bike the contact patch becomes closer to the hub and the farthest parts of the tyre from the hub are turn faster than than any part that is closer to the hub. This camber thrust causes your wheels (front AND rear) to turn the bike in response to the lean.
Centrifugal Force: With the moving motorcycle leaning over, the rider needs to counteract the force, gravity, that is trying to lay the machine on the floor. If the bike were stationary then gravity would obviously win. Centrifugal force in the turn attempts to stand the bike upright so the rider has to balance these two forces to maintain the lean angle.
To simplify; gyroscopic precession initiated by force on the handlebar starts the lean, camber thrust turns the bike and centrifugal force stops it falling over.
- Counter-steering by James R. Davis
- Centrifugal Force Causes Causes Counter-Steering, not Gyroscopics! by James R. Davis
- Gyroscope - Wikipedia
- Countersteering 101 by Ian Johnston (YouTube)