Hear that? It's a great big can of worms being opened, with the worms all asking, "how does a motorcycle steer?" Ask a sport rider and you'll get one of any number of answers, ranging from a vague, "I just lean it," to a complete discourse on body steering or counter steering-and some proponents of one will vehemently deny the existence of the other. But there is a good basis in physics for exactly how a motorcycle goes around a corner, and in this first part of a four-part series we will explain the science involved. In part two, we will ask the experts for their opinions: top racers will comment on how they steer their bikes. In part three we will attend a selection of riding schools and report what each teaches on the subject. Finally, in part four we will physically measure both body steering and counter steering inputs to determine the exact rider/motorcycle interface.
First, some definitions:
? Counter steering is steering a bike by turning the handlebars counter to the direction of the turn. For instance, to enter a right-hand corner, you turn the bars to the left, and vice-versa.
? Body Steering is physically leaning the bike into a turn with other parts of your body. For example, to enter a right-hand turn, you push down on the right footpeg, or push your left knee into the tank and literally force the bike onto its side.
For a motorcycle to go around a corner, it must be leaned over so that the weight of the bike (acting downward) balances the centrifugal force (acting to the outside of the turn) and a state of equilibrium is reached. Too much or not enough of one of these forces will cause the bike to tip over-for example, if you lean a bike over at a stop where there are no centrifugal cornering forces, it will simply fall over on its side. Likewise, if you turn a bike without leaning, it will tip to the outside of the turn-much as an ATV will lift its inside wheels when cornering too hard.
What we are concerned with is how to change the lean angle of the bike, either from straight up to full lean in one direction or from side to side, and either body steering or counter steering can accomplish this. The balancing centrifugal force, which depends on the bike's speed and the radius of the corner, will naturally follow.
Counter steering is made obvious when racers turn the bars hard enough to make the front wheel skid sideways.
Counter steering
Counter steering is generally taught in a Motorcycle Safety Foundation course as "push left, go left," meaning to go around a left-hand turn, you push on the left handlebar. There are two forces at work that can subdivide the counter steering category: one is the gyroscopic reaction of the bike's wheels, the other is the centrifugal effect of the front wheel steering out from under the bike-much like the ATV example. You no doubt played with bicycle wheels in your youth, but it's worth revisiting to explain the gyroscopic effect.
You can experiment with a bicycle wheel to feel the effects of gyroscopic precession. This effect forces the bike to lean opposite to the direction the bars are steered.
Experiment #1: The Bicycle Wheel
Hold a wheel by the axle as shown in Figure 1, and have a friend give it a good spin so that the top of the wheel is going away from you. Keeping a firm grip, turn the wheel sharply to the left, as if it were the front wheel of a motorcycle. What happens? The wheel twists in your arms, with the top of the wheel going to the right, the bottom to the left-if the wheel were attached to a motorcycle, it would force the bike to lean to the right. Experiment with different forces and wheel speeds and you will find the following: The faster the wheel is spinning, the harder it is to turn to the side and the reacting twist of the wheel depends more on how fast you turn it rather than how far.
The equation that defines the gyroscopic reaction depends on conservation of the wheel's angular momentum, and is as follows:
T=IwW
When you counter steer, the front wheel momentarily steers out from underneath the motorcycle, and follows the path shown. At A, the right hand turn is initiated by turning the bars to the left. The bike begins to turn left before leaning right and turning right at B.
Where T is the resulting moment, or torque, that twists the wheel, I is the moment of inertia of the spinning wheel, w the wheel speed, and W the angular velocity of the turn. Note also that the greater the wheel's moment of inertia (a measure of the wheel's weight and how far that weight is from the axle) the greater the torque reaction-a motorcycle wheel will have a greater effect than a light bicycle wheel.
While the scientific explanation for steering the motorcycle's front wheel out from under itself is less easily detailed, the process can be described. A motorcycle traveling in a straight line has momentum, and wants to maintain that trajectory if possible. Turning the bar to the left will cause the bike to momentarily arc slightly to the left-the movement may even be tiny enough to be virtually unnoticeable, but nonetheless the initial turn of the bike is to the left. Centrifugal force from this turn however, will force the bike to tip over and lean to the right-initiating a right-hand turn. Figure 2 shows the front wheel's exaggerated path for this type of maneuver.
Experiment #2: Counter Steering
For this riding experiment it's best to find an empty parking lot or suitable open space with no distractions. Ride in a straight line with a constant throttle at approximately 35 mph, and with no other body movements push forward on the left handlebar. A sudden jerk is not required, but rather start with a forceful push of about 10 pounds. You'll find that your bike will lean to the left and arc in that direction until you release the force on the bar. Once you become comfortable with counter steering, experiment with various steering inputs, using both arms and more force.
To effectively use body steering, your body must move momentarily in the opposite direction of the motorcycle in order for momentum to be conserved.
Body steering
Turning a motorcycle using your body generally refers to pushing on the appropriate footpeg to rotate the bike around its roll axis, but can also include exerting force on the side of the bike or simply leaning your body off one side or the other. To understand body steering, it's important to understand and recognize that the rule of conservation of momentum must apply. Momentum quantifies the motion or impetus of an object, and is defined as the product of its mass and velocity:
M=mV
Where M is momentum, m is the object's mass and V its velocity. Conservation of momentum means that in a closed system there can be no change in the overall momentum.
When you jump off a wagon and push it away from you, an empty wagon will shoot away quickly while you move very little. Push off a loaded wagon and it will move away much more slowly while you move in the opposite direction faster than previously. This demonstrates the conservation of momentum, a key to body steering.
Experiment #3: Falling Off The Wagon
Commandeer a small wagon from the neighborhood kids and stand on top of it at a stop. The total momentum in the system, because you are not moving, is zero. Now, jump off the wagon, pushing it away with your feet. While the wagon shoots off in one direction, you move very little in the opposite direction. However, the combined momentum of you and the wagon still equals zero-your momentum in one direction cancels out the wagon's momentum in the other. Note that the wagon, being significantly lighter than you, ends up with a much higher velocity.
Now, repeat the experiment with the wagon loaded (children make excellent ballast and will be more than happy to assist a crazy grownup). Jumping off, you will end up with more velocity than before, and the wagon less; all because of the change in the respective weights. Imagine the same scenario, taken to an extreme, such as jumping off the bumper of a car (in neutral, of course). The car would barely move, and you would end up with all the velocity. This experiment shows two things. First, the speed of your body movement can have as much effect as your weight-a quick and forceful jump will have a much greater effect than slowly stepping off. Second, if you are going to body steer your bike in one direction, your body must move in the opposite direction relative to the bike. Pushing on a footpeg or applying force without moving will have little effect-like trying to make the wagon move by sitting in it and pushing on the side.
The teeter-totter. Imagine the board of the teeter-totter to be the footpegs of your motorcycle to get a feel for how body steering works.
Experiment #4: The Teeter-totter
Stand on a teeter-totter near the center, and balance the bar horizontally. Imagine the teeter-totter to be the footpegs of your bike, and experiment with making the bar go one way or the other. Try jerking your body quickly to one side or the other, and note which way the teeter-totter goes. Because the teeter-totter has so little mass, you will remain essentially motionless but can move the bar in either direction.
Experiment #5: Body Steering
As before, ride in an open area with no distractions, at approximately 35 mph. Try to not affect the steering of your bike using the handlebars or, if you are comfortable doing so, ride with no hands. In one fluid motion, lift your butt slightly off the seat and move your upper body to the right of the bike. You will find that the bike will initially lean to the left and turn left, then lean right and turn right. Surprised? When you moved to the right, your bike (like the wagon) moved in the opposite direction to conserve momentum, and counter steered into a right-hand turn.
Now hang slightly off to the left of your bike, and then use your right knee on the tank and left foot on the footpeg to force the bike to quickly lean to the left. What happens? The bike will lean to the left, but because momentum must be conserved, you ended up in line with the bike or maybe even hanging off a bit to the right. Consider how this affects you when changing direction in a chicane-you must quickly move the bike in one direction, then bring your body slowly after. Or, move your body slowly first, then bring the bike along after. Continue to experiment with different inputs and amounts of force, always letting your body move in relation to the motorcycle.
Body steering or counter steering then, which is correct? Both will cause a motorcycle to change direction, but the advantages and disadvantages of each need to be explored further. In next issue's part two, we will ask a variety of racers how much of each they use and what they think is more effective, and in part three we investigate some riding schools and find out what students are being taught on the subject. The can is open, the worms are everywhere.
Resources
Motorcycle Design and Technology
Gaetano Cocco
(800) 531-1133
www.whitehorsepress.com
Motorcycle Chassis Design: The Theory and Practice
Tony Foale and Vic Willoughby
www.tonyfoale.com