Countersteering

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Graphs showing the lean and steer angle response of an otherwise uncontrolled simplified model of a typical bike, traveling at a forward speed in its stable range (in this case 6 m/s), to a positive steer torque (to the right) that begins as an impulse and then remains constant. It causes an initial steer angle to the right, a lean to the left, and eventually a steady-state lean to the left, steer angle to the left, and thus a turn to the left.
Countersteering is required to turn any tandem 2-wheeled vehicle
A hypothetical curve on dry asphalt

Countersteering is used by single-track vehicle operators, such as cyclists and motorcyclists, to initiate a turn toward a given direction by momentarily steering counter to the desired direction ("steer left to turn right"). To negotiate a turn successfully, the combined center of mass of the rider and the single-track vehicle must first be leaned in the direction of the turn, and steering briefly in the opposite direction causes that lean.[1]

Definition

The scientific literature does not provide a clear and comprehensive definition of countersteering. In fact, "a proper distinction between steer torque and steer angle ... is not always made."[2]

It is important to distinguish the steering torque and steering angle necessary to establish the lean required for a given turn from the sustained steer torque and steer angle necessary to maintain a constant turn radius and lean angle until it is time to exit the turn. The initial steer torque and steer angle are both opposite the desired turn direction. The sustained steer angle is usually in the same direction as the turn, but may remain opposite to the direction of the turn, especially at high speeds.[3] The sustained steer torque required to maintain that steer angle is usually opposite the turn direction.[4] (See the graphs to the right.) The actual magnitude and orientation of both the sustained steer angle and sustained steer torque of a particular bike in a particular turn depend on forward speed, bike geometry, tire properties, and combined bike and rider mass distribution.

Another way to cause the bike and rider to lean is by applying appropriate torques between the bike and rider similar to the way a gymnast can swing up from hanging straight down on uneven parallel bars, a person can start swinging on a swing from rest by pumping their legs, or a double inverted pendulum can be controlled with an actuator only at the elbow.[5]

Bicycles

For bicyclists, Countersteering can be very useful, even potentially lifesaving, in emergency maneuvers. It is taught as the instant turn (a term coined by John Forester) in traffic cycling courses offered through the League of American Bicyclists.

League certified instructors teach students to be prepared to make a sudden sharp turn to avoid, for example, being hit by a motorist who just overtaken them and then turned across their path, cutting the cyclist off. The instant turn is initiated by quickly jerking the bars to the left (the countersteer), which initiates the necessary lean to the right, and then turning sharply into the required turn to the right.[6][7] Racing cyclists and cyclists performing fast hill descents may also use deliberate countersteering in order to initiate and manage the fast, precise turns necessary.

Motorcycles

Even more so than on a bicycle, deliberately countersteering is essential for safe motorcycle riding, and as a result is a part of the safe riding courses run by the Motorcycle Safety Foundation and the Canada Safety Council. At the higher speeds that motorcycles commonly attain, it becomes increasingly impractical to steer by taking advantage of the minute and random corrections needed to maintain balance.

Much of the art of motorcycle cornering is learning how to effectively "push" the grips into corners and how to maintain proper lean angles through the turn. When the need for a quick swerve to one side suddenly arises in an emergency, it is essential to know, through prior practice, that the handlebars must be deliberately pressed away on that side instead of being pulled. Many accidents result when otherwise experienced riders who have never carefully developed this skill encounter an unexpected obstacle.

To encourage an understanding of the phenomena around countersteering, the phrase positive steering is sometimes used,[8][9] and is summed up in a simplified way as "Push the right-hand bar to steer right; push the left-hand bar to steer left".

Three wheelers

Typically, three wheelers like trikes and sidecar rigs cannot be countersteered. There are unconventional multi-tracked motorcycles like the three wheel Piaggio MP3 which use complex mechanical linkages to lean the two front wheels in parallel, so that it is countersteered in the same manner as a two wheeled motorcycle.[10] In contrast, the three wheeled BRP Can-Am Spyder Roadster uses two front wheels which do not lean, and so it steers like a car.[10]

How it works

A single-track vehicle such as a bicycle or a motorcycle is an inverted pendulum—it will fall over unless balanced.

Countersteering is used by cyclists and motorcyclists to initiate turning in a given direction is to first apply a steering torque in the opposite direction. For example, if a turn to the left is desired, it is started by applying a torque on the handlebars to the right. This causes the front wheel to rotate about the steering axis to the right and the front tire will generate forces in the contact patch to the right. The machine as a whole steers to the right briefly, and because the forces in the contact patch are at ground level, this pulls the wheels "out from under" the bike to the right and causes it to lean to the left. Then the rider, or in most cases, the inherent stability of the bike provides the steering torque necessary to rotate the front wheel back to the left and in the direction of the desired turn. Finally, the bike begins a turn to the left.[11] It is often boiled down to "push left to go left".

While this appears to be a complex sequence of motions, it is performed by every child who rides a bicycle. The entire sequence goes largely unnoticed by most riders, which is why some assert that they do not do it.

It is also important to distinguish the steering torque necessary to initiate the lean required for a given turn from the sustained steering torque and steering angle necessary to maintain a constant radius and lean angle until it is time to exit the turn. The initial steer torque and angle are both opposite the desired turn direction. The sustained steer angle is in the same direction as the turn. The sustained steer torque required to maintain that steer angle is either with or opposite the turn direction depending on forward speed, bike geometry, and combined bike and rider mass distribution[citation needed].

Need to lean to turn

A bike can negotiate a curve only when the combined center of mass of bike and rider leans toward the inside of the turn at an angle appropriate for the velocity and the radius of the turn:

$\theta = \arctan \left (\frac{v^2}{gr}\right )$

where $v$ is the forward speed, $r$ is the radius of the turn and $g$ is the acceleration of gravity.[12]

Higher speeds and tighter turns require greater lean angles. If the mass is not first leaned into the turn, the inertia of the rider and bike will cause them to continue in a straight line as the tires track out from under them along the curve. The transition of riding in a straight line to negotiating a turn is a process of leaning the bike into the turn, and the most practical way to cause that lean (of the combined center of mass of bike and rider) is to move the support points in the opposite direction first.[13] The rider can shift his weight of course, but any force used to move one way laterally pushes the bike laterally the opposite direction with equal force. That makes the bike lean (and can affect the steering), but it does not change the combined center of mass of bike and rider.

Stable lean

As the desired angle is approached, the front wheel must usually be steered into the turn to maintain that angle or the bike will continue to lean with gravity, increasing in rate, until the side contacts the ground. This process often requires little or no physical effort, because the geometry of the steering system of most bikes is designed in such a way that the front wheel has a strong tendency to steer in the direction of a lean.

The actual torque the rider must apply to the handlebars to maintain a steady-state turn is a complex function of bike geometry, mass distribution, rider position, tire properties, turn radius, and forward speed. At low speeds, the steering torque necessary from the rider is usually negative, that is opposite the direction of the turn, even when the steering angle is in the direction of the turn. At higher speeds, the direction of the necessary input torque often becomes positive, that is in the same direction as the turn.[14]

At low speeds

At low speeds countersteering is equally necessary, but the countersteering is then so subtle that it is hidden by the continuous corrections that are made in balancing the bike, often falling below a just noticeable difference or threshold of perception of the rider. Countersteering at low speed may be further concealed by the ensuing much larger steering angle possible in the direction of the turn[citation needed].

Unthinking behavior

Countersteering is indispensable for bike steering. Most people are not aware that they employ countersteering when riding their bike any more than they are aware of the physics of walking[citation needed]. They have learned to apply the required countersteering without thinking.

As is well known in bicycle racing, the countersteering becomes evident when there is an obstacle preventing the wheel from countersteering (e.g., when closely overlapping wheels or riding very close to a curb). In these situations, the way to initiate a turn with the handlebars away from the obstacle is to countersteer towards obstacle to avoid crashing into it.[13] Lack of understanding of this principle leads to accidents in novice bicycle races[citation needed].

Gyroscopic effects

One effect of turning the front wheel is a roll moment caused by gyroscopic precession. The magnitude of this moment is proportional to the moment of inertia of the front wheel, its spin rate (forward motion), the rate that the rider turns the front wheel by applying a torque to the handlebars, and the cosine of the angle between the steering axis and the vertical.[14]

For a sample motorcycle moving at 22 m/s (50 mph) that has a front wheel with a moment of inertia of 0.6 kgm2, turning the front wheel one degree in half a second generates a roll moment of 3.5 Nm. In comparison, the lateral force on the front tire as it tracks out from under the motorcycle reaches a maximum of 50 N. This, acting on the 0.6 m (2 ft) height of the center of mass, generates a roll moment of 30 Nm.[14]

While the moment from gyroscopic forces is only 12% of this, it can play a significant part because it begins to act as soon as the rider applies the torque, instead of building up more slowly as the wheel out-tracks. This can be especially helpful in motorcycle racing.[14]

Countersteering by weight shifting

With a sufficiently light bike (especially a bicycle), the rider can initiate a lean and turn without using the handlebars by shifting body weight, called counter-lean by some authors.[15][12][16][17] Documented physical experimentation shows that on heavy bikes (many motorcycles) shifting body weight is less effective at initiating leans.[18]

To turn left, a rider applies a momentary torque, either at the seat via the legs or in the torso that causes the bike itself to lean to the right, called counter lean by some authors.[17] The combined center of mass of the bike and rider is only lowered, of course. However, if the front of the bike is free to swivel about its steering axis, the lean to the right will cause it to steer to the right by some combination of gyroscopic precession (as mentioned above), ground reaction forces, gravitational force on an off-axis center of mass, or simply the inertia of an off-axis center of mass, depending on the exact geometry and mass distribution of the particular bike, and the amount of torque and the speed at which it is applied.[12][19]

This countersteering to the right causes the ground contact to move to the right of the center of mass, as the bike moves forward, thus generating a leftward lean. Finally the front end steers to the left and the bike enters the left turn. The amount of leftward steering necessary to balance the leftward lean appropriate for the forward speed and radius of the turn is controlled by the torque generated by the rider, again either at the seat or in the torso.

To straighten back out of the turn, the rider simply reverses the procedure for entering it: cause the bike to lean farther to the left; this causes it to steer farther to the left, which moves the wheel contact patches farther to the left, eventually reducing the leftward lean and exiting the turn.

The reason this no-hands steering is less effective on heavy bikes, such as motorcycles, is that the rider weighs so much less than the bike that leaning the torso with respect to the bike does not cause the bike to lean far enough to generate anything but the shallowest turns. No-hands riders may be able to keep a heavy bike centered in a lane and negotiate shallow highway turns, but not much else.

Complex maneuvers are not possible using weightshifting alone because even for a light machine there is insufficient control authority.[15] Although on a sufficiently light bike (especially a bicycle), the rider can initiate a lean and turn by shifting body weight,[12] there is no evidence that complex maneuvers can be performed by bodyweight alone.[18]

Motorcycle speedway racing

Other uses

The term countersteering is also used by some authors to refer to the need on bikes to steer in the opposite direction of the turn (negative steering angle) to maintain control in response to significant rear wheel slippage.[14] Motorcycle speedway racing takes place on an oval track with a loose surface of dirt, cinders or shale. Riders slide their machines sideways, powersliding or broadsiding into the turns, using an extreme form of this type of countersteering that is maintained throughout the turn.

The term is also used in the discussion of the automobile driving technique called drifting.

The Wright Brothers

Wilbur Wright explained countersteering this way:

I have asked dozens of bicycle riders how they turn to the left. I have never found a single person who stated all the facts correctly when first asked. They almost invariably said that to turn to the left, they turned the handlebar to the left and as a result made a turn to the left. But on further questioning them, some would agree that they first turned the handlebar a little to the right, and then as the machine inclined to the left, they turned the handlebar to the left and as a result made the circle, inclining inward.[20][21]

References

1. ^ Sheldon Brown. "Countersteering". Retrieved 2012-11-09. ""Countersteering" refers to the momentary motion of the handlebars in the opposite direction of the desired turn."
2. ^ Sharp, R. S. (2008). "On the stability and control of the bicycle". Applied Mechanics Reviews 61 (6): 1–24. doi:10.1115/1.2983014. "A positive right-hand torque leads to negative steer and roll angles, corresponding to a left turn. This behavior is often called countersteering, and it appears that it was known in the very early days of cycling, although a proper distinction between steer torque and steer angle forcing is not always made."
3. ^ V Cossalter, R Lot, and M Peretto (2007). "Steady turning of motorcycles". Journal of Automobile Engineering. 221 Part D: 1343–1356. doi:10.1243/09544070jauto322. "As concerns the first street vehicle, notable over-steering behaviour is evident; ..., and hence driving is carried on using some counter-steering angle."
4. ^ V Cossalter, R Lot, and M Peretto (2007). "Steady turning of motorcycles". Journal of Automobile Engineering. 221 Part D: 1343–1356. doi:10.1243/09544070jauto322. "Correlations with the subjective opinions of expert test riders have shown that a low torque effort should be applied to the handlebar in order to have a good feeling, and preferably in a sense opposite to the turning direction."
5. ^ Russ Tedrake (2009). "Underactuated Robotics: Learning, Planning, and Control for Efficient and Agile Machines Course Notes for MIT 6.832". Retrieved 2012-05-31.
6. ^ Forester, John (1993). Effective Cycling (Sixth ed.). The MIT Press. pp. 204–205. ISBN 0-262-56070-4.
7. ^ "Emergency: Instant Turn". League of American Bicyclists. Retrieved 7 August 2008.
8. ^ Jon Taylor & Stefan Bartlett (2009). How to be a Better Rider. Institute of Advanced Motorists. ISBN 978-0-9562239-1-3.
9. ^ "Novice Motorcycle Riders to Learn Positive Steering". Biker 24/7 News. 29 June 2009. Retrieved 31 December 2009.
10. ^ a b The Editors of Motorcyclist Magazine; Stein, John L. (2011). The Complete Idiot's Guide to Motorcycles (5 ed.). Penguin. p. 339. ISBN 1-61564-070-3. Retrieved 15 January 2011.
11. ^ Jones, David (1970). "The Stability of the Bicycle" (PDF). Retrieved 31 March 2009.
12. ^ a b c d Fajans, Joel (July 2000). "Steering in bicycles and motorcycles" (PDF). American Journal of Physics 68 (7): 654–659. Bibcode:2000AmJPh..68..654F. doi:10.1119/1.19504. Retrieved 4 August 2006.
13. ^ a b Wilson, David Gordon; Jim Papadopoulos (2004). Bicycling Science (Third ed.). The MIT Press. pp. 270–272. ISBN 0-262-73154-1.
14. Cossalter, Vittore (2006). Motorcycle Dynamics (Second ed.). Lulu.com. pp. 241–342. ISBN 978-1-4303-0861-4.
15. ^ a b Evangelou, S, 2004 "The Control and Stability Analysis of Two-Wheeled Road Vehicles", PhD Thesis, Imperial College London
16. ^ Cocco, Gaetano (2004). Motorcycle Design and Technology. Motorbooks. p. 25. ISBN 978-0-7603-1990-1.
17. ^ a b Foale, Tony (2006). Motorcycle Handling and Chassis Design, the Art and Science (2nd ed.). Tony Foale Designs. pp. 4–7. ISBN 978-84-933286-3-4.
18. ^ a b Gromer, Cliff (1 February 2001). "STEER GEAR So how do you actually turn a motorcycle?". Popular Mechanics. Retrieved 7 August 2006.
19. ^ Brandt, Jobst (16 September 1997). "What keeps the bicycle upright?". sheldonbrown.com. Retrieved 17 October 2007.
20. ^ Crouch, Tom D. (1989). The Bishop's Boys. New York: W. W. Norton. p. 170. ISBN 0-393-30695-X.
21. ^ Kelly, Fred C. (1989). The Wright Brothers. Courier Dover Publications. pp. 297–299. ISBN 978-0-486-26056-3.