GRAVITY GAMES

I do not know how I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or prettier shell than ordinary, whilst the great ocean of truth lay undiscovered before me. --Sir Isaac Newton

Webster's Dictionary defines "centripetal force" as: "a force tending to pull a thing toward the center of rotation when it is rotating around a center." Inertia wants the object to go straight, but some mechanical device keeps the object turning. Centripetal force is often confused with centrifugal force, which is the exact opposite. Webster's defines "centrifugal force" as: "an apparent force tending to pull a thing outward when it is rotating around a center." Many carnival rides make fun use of this invisible scientific force. In fact, centrifugal force is what makes a countersteering work, since it is the force that tips the bike over in anticipation of a turn. While centrifugal force does balance out centripetal force when a motorcycle is cornering, a mistake made by a rider can allow the centrifugal force to take over, sending the bike and rider crashing to the outside of the turn.

Weight-shifting verses countersteering? That is the question. Many people require more than advice and photographs in order to expand their consciousness. They need expert scientific analysis from an engineering perspective.

The scientific "theory" of countersteering was reconfirmed as scientific fact by Dr. Hugh H. Hurt of the University of Southern California Traffic Safety Center, and a group of scientists from Honda Motor Company. They presented their findings at the Second International Congress on Automotive Safety held in San Francisco in 1973.

A quarter of a century ago, in "Motorcycle Handling and Collision Avoidance: Anatomy of a Turn," Dr. Hurt wrote: "The path between straight-line motion and free equilibrium turn requires an initial steering motion opposite that of the steady turn. To achieve the left turn, an initial steering displacement is made to the right causing the front wheel to track out to the right with the rear wheel following in track. As the desired angle of lean to the left is reached, the second steering displacement is made into the turn to match the true track of the equilibrium turn conditions. Figure 1 illustrates the vehicle tracks under such conditions."

"This process of initial out-track to turn is peculiar to the single-track vehicle and describes the fundamental steering behavior of the motorcycle (or any bicycle). In order for the vehicle to recover to a straight path, the vehicle must in-track to reduce the lean angle and bring the vehicle upright. The recovery requires a steering input into the existing turn, causing the front wheel to track in with the rear wheel following in its track. As the vehicle reaches the upright condition, the second steering input is away from the previous turn towards the straight-ahead path.

In other words, countersteering allows the rider to "scoot" the bike's wheels sideways out from under him, causing the bike to initially fall over into the curve thanks to centrifugal force. Since the bike is vertical, centrifugal force has a maximum effect. He does not have to throw himself over the side to lean the heavy bike, or to shift his weight in any way in an attempt to "balance." Since the bike is vertical, weight-shifting has a minimal effect. This basic component of countersteering works at all speeds on every type of motorcycle. This is the first of the two components that makes countersteering work.

Note that countersteering only begins the turn, by leaning the bike over to its correct lean angle. If the rider were to continue to countersteer, centrifugal force and gravity would tip the bike over until it hit the ground. Releasing the handlebars instantly stabilizes the lean angle. Once the bike has reached its maximum lean angle and has settled down in the turn, the handlebars automatically turn themselves to track the arc of the turn depending upon the lean angle and the speed, due to "steering trail" (the castor effect). Unlike a car, a motorcycle will actually steer itself around a curve (provided the rider does not interfere).

In the 1976 scientific study, "The Photographic Analysis of Motorcycle Operator Control Responses," performed by the National Public Services Research Institute, initial countersteering in "normal" turns was measured to last only 0.500 seconds. Gentle turns might require only 0.125 seconds of countersteer, while sharper turns might require 1.000 seconds. This is how long it takes for the bike to begin to lean. A rider may only need to apply 5 degrees of countersteer (2 degrees of lean) for a corner that requires 25 degrees of regular "steering" in the middle of the turn (35 degrees of lean). Obviously, this short duration of slight steering could easily be overlooked by a typical rider unfamiliar with countersteer.

This study, which only tested low-speed turning at a maximum speed of 25 miles per hour—simulating a typical intersection turn—found that only half of the turns produced an initial countersteer, but all turns required "reverse countersteer" to exit the turn (turn left to end a left-hand turn and start leaning back to the right). In the 50% of slow turns that the rider did not countersteer, he did hold the steering wheel straight ahead a fraction longer than "required," giving what I call a "pseudo-countersteer" input, allowing centrifugal force to tip the bike (but at a slower rate).

None of the turns were made by initially steering towards the curve. All turns required "counter" steering inputs to "balance" the bike throughout the turn, rather than weightshifting. In fact, after the first half-second of leaning toward the turn, the rider always maintained about 1/3rd less lean angle than the bike, proving that weight-shifting was not a factor. A second countersteer was needed to raise the bike vertical.

Several movie cameras recorded the rider's performance in 1/18th-second photographs, and 14 different measurements were taken of control movements. The study concluded that: "The motorcycle operator's body lean was not a significant factor in leaning the motorcycle."

Ninety-five percent of riders presume that weightshifting is how a motorcycle is forced to lean into a curve (and they have never heard the term countersteering, nor figured out for themselves how it works). Yet these riders do not even comprehend what weight shifting really is. This is truly a sad reflection on the (wasted) analytic abilities that most people apply to intellectual understanding of "sport" (and a sad reflection on the government's ability to educate citizens).

When a (non-countersteering) rider wants to lean his motorcycle into a curve, he leans slightly in the same direction of the curve. This lasts approximately one-half second—coincidentally the same time that countersteering takes place. In the government's report on the photographic analysis of countersteering, this "weightshift" was discussed: "The role of this lean is difficult to determine. It is hard to believe that such a small lean angle [2.5 degrees] could have much effect upon the motorcycle. It may simply reflect the operator's anticipation of a [35 degree] lean to the left." After this initial half-second lean into the curve, the rider actually leans away from the motorcycle's lean angle. The report states: "Body lean lags behind that of the motorcycle and reaches a maximum of approximately two-thirds that of the motorcycle."

A motorcycle does not have the "benefit" of four wheels to "stabilize" it during weight-shifts by a rider. This means a rider is sitting on a "tightrope," and any movement he makes will cause the tightrope to move in the opposite direction. If he moves to the left, the tightrope moves to the right, and vice versa. A motorcycle works exactly the same way. A rider can prove this to himself by leaning to the side while traveling on a straight road. When he leans left, the bike leans to the right. When he leans to the right, the bike leans to the left. Whenever a bike leans, steering trail and castor effect kick in and force the bike to turn slowly in the direction of lean. In other words, when a (non-countersteering) rider initially leans into a corner, the bike initially leans away from the corner. This has exactly the same effect as countersteering, and the bike steers itself away from the curve (steering trail and castor effect). As the bike countersteers itself away from the curve, the rider's weight then has something to "lean against," and this pulls the bike over into the curve. One might call this phenomenon "counterweightshifting."

It is true that the front wheel must follow the arc of the road, due to steering trail, but the steering angle is reduced by the angle of lean. In other words, the greater the lean angle, the longer the effective turn radius (like an inverted cone), the less the handlebars will need to turn, and the more centered the handlebars will appear. A 45-degree lean angle can reduce steering angle by approximately 30%.

The motorcycle's arc through a particular curve in the road depends on its lean angle to offset the centrifugal force trying to tip it over, which requires more angle at higher speeds. Lean angle is changed by countersteering. If he had a cruise control, and enough open space, he could take his hands off the handlebars and circle round and round in the same arc until he ran out of gas.

When countersteering, the general rule is to sit upright relative to the bike and to lean with the bike's lean angle. Novice and insecure passengers who fear motorcycles usually violate this rule by trying to stay vertical, forcing the pilot to lean the machine even more to compensate. Explaining to the passenger that you have to lean the machine even more to compensate for such behavior can help correct the problem. A possible exception occurs with heavy, poor-handling bikes. Staying vertical during corner entry while countersteering and "pushing" the bike down first in "motocross style" can help reduce the weight and lower the center of gravity, helping the bike to turn in quicker. Although light weight, motocrossers have high C.G.s and large caster angles, which make them harder to turn. Once it begins to lean, the rider can then "catch up" to it and lean with it for the remainder of the corner. This is merely a quick fix, and is not conducive when riding a modern sport bike.

When riding a pedal-driven bicycle without touching the handlebars, turning is accomplished by leaning the body (counterweightshifting). This is easier on a pedal bike since the rider's body weighs many times more than the bike, and speeds are so much lower. At cruising speed, the handlebars will initially countersteer all by themselves, then follow the arc in the middle of the curve once the bike is leaned over (thanks to steering trail and castor effect). Beginning riders will panic and grab the handlebars when it begins to countersteer. Experienced riders will ignore it without understanding it. Many riders experience difficulty when riding a "new" motorcycle for the first time. Different bikes have different steering geometries, which confuses the uneducated rider. It takes time for the subconscious mind to overrule the conscious mind. "I can't make it turn," is the common complaint made by a rider who attempts weightshifting and stiff-arming instead of countersteering. Bad riding habits are magnified by switching machines.

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