GYROSCOPIC PRECESSION:
NATURE'S POWER STEERING

Discovery consists of seeing what everybody has seen and thinking what nobody has thought.
—Albert Szent-Gyorgyi von Nagyrapolt

The second component of countersteering is the gyroscopic effect of the rotating wheels. It comes into play mainly above 30 miles per hour on a heavy motorcycle in normal turns. This gyroscopic force is amplified by a motorcycle's requirement of leaning through a turn. Gyroscopes are inherently stable, so long as no other forces act upon them. "Gyroscopic precession" is the tendency of a rapidly spinning object to resist being tilted. Webster's New World Dictionary describes "precession" as: "mechanical—an effect exhibited by a spinning body, as a top, when an applied torque tends to change the direction of its rotational axis, causing the axis to . . . turn at right angles to the direction of the torque." Precession is far more powerful than gyroscopic stability, when a wheel is being steered towards a new direction.

In other words, a gyro is very stable when left alone, such as a motorcycle traveling down a straightaway, or gyroscopic navigation instruments used in aircraft, or the planet Earth spinning at 1,000 miles per hour for billions of years and maintaining its orientation to the sun without tumbling out of control. (Unless the Earth is hit by a bombardment of giant asteroids in which case it will precess. Such events often occurred early in the planet's history. Perhaps that is why the earth's axis is tilted 25 degrees giving us the four seasons.) The higher the speed, the more gyroscopically stable a motorcycle gets (so long as the handlebars are not turned).

Yet when a spinning gyro is tilted sideways, it reacts contrary to what so-called common sense would suggest—it doesn't just turn sideways and keep spinning. Nor does it only resist steering efforts by increasing steering effort (like springs that stiffen as speed increases). In a motorcycle at highway speeds, this means that turning the handlebars left will force the bike to lean to the right—as if pushed by an invisible force—due to gyroscopic precession of the rapidly spinning front wheel. The faster the wheels are spinning—or the quicker the handlebars are turned—the greater is this invisible force trying to push over the machine. This phenomenon actually helps a rider control his machine with less effort, provided he understands what is happening. Conversely, lack of comprehension can make his bike difficult to control. Gyroscopic precession adds to power of countersteering to lean the bike over quickly. Gyroscopic stability makes countersteering easier, since its stiffens the bike's steering (countersteering below 30 miles per hour is very sensitive, but just as necessary).

(diagram motorcycle gyro force with turning handlebar—looking from front view—vertical bike; turned wheel; gyro tilt force) # 4

Automobiles are also affected by gyroscopic precession while the front wheels are changing steering angle. However, this powerful force is effectively disguised by the fact that automobiles lean in the opposite direction of motorcycles. The further a bike leans over, the less effect that in-track/out-track steering has upon lean angle. This is because centrifugal force, which works in a sideways direction, cannot get as much leverage as when the bike was vertical. Fortunately, gyroscopic precession still works. Precession is not affected by gravity's extra leverage, since the bike's weight is balanced by an equal amount of centrifugal force. Paradoxically, weight-shifting can have a significant influence on steering at high lean angles, since gravity is then able to finally get leverage.

"Hanging off" is an impressive technique utilized by racers, and "leaning in" can also be used on the street to help steer the bike in high-lean-angle situations. It is especially useful when ground clearance is at a premium such when a rider needs to tighten his turn suddenly, but finds he cannot lean it any further since the footpeg (or some other part) is dragging. Note that when a footpeg suddenly drags the pavement, a rider must instantly lift his weight off that footpeg so that it may fold up. Otherwise, the bike can skate sideways as if on a metal skid, which is what the footpeg becomes if a rider does not know what to do.

Leaning in can prevent a crash in such an "emergency" situation (counter-weightshifting raises the bike towards the vertical). Using the legs to move off the seat keeps the rider in control, since any pulling on the handlebars can destabilize the bike. (Racers do all their positioning prior to entering a corner.) Hanging off is also a useful all-round safety technique for street riding on curvey roads, and is good aerobic exercise. The Motorcycle Safety Foundation teaches the safety benefits of hanging off for street riders. For government police to arrest riders simply for using a proven safety technique is insanity, especially when it is used in curves at speeds within the posted arbitrary limits.

The only exceptions to countersteering occur when pushing a vertical motorbike around a parking lot and when riding at a low speed when emergency avoidance is not required and before gyroscopic precession really kicks in. Slow (counter)weightshifting with slow turning does appear to work (sometimes)--trouble begins when the rider suddenly decides he needs to increase his turn rate and weightshifting alone is not powerful enough to quickly override the heavier weight of the machine. It is possible to ride around hands-free with a cruise control set, simply by weightshifting and making use of steering trail and castor effect—the bike will countersteer all by itself—but rapid emergency maneuvers are impossible. These exceptions make it more difficult to notice when countersteering becomes mandatory, especially at cruising speeds, and confuse the unwary rider.

The speed at which the transition occurs for gyroscopic precession to become noticable is a mathematical function of the size and weight of the wheel-tire assemblies and of the total mass in the rest of the machine. Basically, the spinning wheels must store up energy first. How quickly the rider needs to turn the handlebars and to make his maneuver tends to lower the speed at which gyroscopic precession kicks in. It is different for every type of two-wheeled vehicle, further complicating the mental equation for the rider. In other words, on a particular 500 pound motorcycle, below 30 miles per hour the rider could get away with weightshifting right to turn slowly right (the bike will momentarily countersteer all by itself in order to lean), but above 40 miles per hour gyroscopic precession will work against attempts at weight-shifting left. However, in an emergency situation at 30 miles per hour or less, requiring sudden steering input, firm countersteering would be required, in order to lean the bike as quickly as possible. Riding through a normal curve above 40 miles per hour, if a rider attempted to turn right by forcibly steering right, the gyroscopic precession (combined with centrifugal force) would try to tilt the bike to the left instead of the right, resisting any effort to turn right and causing a highside crash if the rider did not correct his mistake. This gyroscopic force can total XXX lbs. of force. Fighting this powerful force on a 500 lb. machine is a recipe for disaster, while the intelligent use of science can make 500 lbs. feel as light as a feather. On a pedal bike weighing only XX lbs., this transition speed for intentional counter steering might occur at only XX miles per hour, since there is less weight to be overcome (a rare speed???). Bizarre, isn't it? That's what makes science so fascinating. (Just be glad I'm not attempting to explain how a unicycle works.)

To repeat, once the bike has reached its maximum-desired lean angle, countersteering is no longer needed, and the rider steers straight ahead through the middle of the turn. In fact, if the motorcycle had cruise control, a rider could take his hands off the handlebars completely. Bicycle riders can easily do this since they can pedal or coast without needing to operate handlebar-mounted controls. When it is time to exit the corner and return the bike to a vertical position, the rider must countersteer in reverse. That is to say, steer in the direction of the turn so that gyroscopic precession will lift the heavy bike back to a verticle position (sort of like speed-sensitive "power steering").

By watching a motorcycle race on television or in person, one can witness the incredible maneuverability a competent rider can achieve, thanks to centrifugal force and gyroscopic precession at work. It looks unnatural. A racer can reach a lean angle of over 60 degrees in only 0.5 seconds. How could a feather-weight motorcycle jockey possibly weight-shift or "lift" 500 lbs. so incredibly fast? A rider can put his weight on the outer footpeg for support, instead of sliding around on the seat when "fighting" high-speed gyroscopic stability and resistance (he doesn't fight precession which works for him). For example, in a left turn, a rider would countersteer to the right by pushing the left handlebar (and/or pulling on the right) and supporting his weight on the right footpeg. Countersteering rates that fast can actually bend a handlebar when a rider knows how to support himself solidly on the bike. That's one reason why modern sport bikes have footpegs mounted high and to the rear—this allows the rider to get maximum countersteer leverage on the reinforced handlebars by pushing from the footpegs. Large gas tanks give a rider something to hold onto besides the handlebars. A rider who can learn to do this has graduated from the archaic "lean to turn" theory of steering. This is the same maneuverability a highway rider wishes he has when confronted by a sudden emergency.

(gyro bike wheel illustration—Euler's equations) # 5

German Euler (pronounced "oiler") was the founder of analytic geometry and was the world's most prolific mathematician, whose formulas comprise a twenty-foot-thick set of volumes. While visiting Russia, he once publically debated for proof of the existance of God, XXX years before America's Scopes' Monkey Trial. (As a joke he gave a phony equation for God—X=42--simply to confuse his athiestic and unscientific opponent.) His x-year-old equation for gyroscopic precession is relatively straightforward, just simple algebraic multiplication:

(photos discovery center bike experiment display) # 8-9

This experiment may be duplicated at home by lifting a bicycle with one hand, spinning the front wheel with the other, then experimenting with gyroscopic precession as the handlebars are turned. Turning the handlebars slowly produces only a little precession force. Turning the handlebars quickly makes the bicycle frame want to tip over in the opposite direction. (It would be a great idea for grade school teachers to make use of this experiment with their kids, so that everyone could have an opportunity to learn about the fascinating science of (safely) riding a bicycle. Thirty seconds of hands-on experimentation would make an impression that will last a lifetime.)

(bicycle precession equation examples = slow and fast steering) x2 (25 lb bike, 2.5 lb wheel, 150 lb rider)

(motorcycle precession equation example = slow and fast steering) x2 (500 lb bike, xx lb wheel, 150 lb rider)

Return to Home Page

Chapter 1. Let's Look at Some Data

Chapter 2. Risk Management

Chapter 3. Two Wheeled Physics

Chapter 4. Countersteering: Cornering Techniques

Chapter 5. Gravity Is a Good Thing

Chapter 6. Gyroscopic Precession: Nature's Power Steering

Chapter 7. Braking: Weight Transfer and Maximum Performance

Chapter 8. Controlling Slides and Tank Slappers: Mind Over Matter

Chapter 9. Group Riding

Chapter 10. Riding Etiquette

Chapter 11. MSF Courses- Editorial