Introducing Precession and Gyroscopic Issues

Precession is a scientific word that describes how a spinning wheel reacts if the axis is tipped or rotated in space. Given a tip in the axis of spin, the spinning wheel will exhibit precession meaning that the axis will tend to want to tip in a direction 90 degrees to the initial axis tip. Moreover, the direction of tip is dictated by what physicists and experts in mechanics refer to as the right-hand-rule. This tendency to exhibit precession has to do with the principle of conservation of angular momentum. In lay terms, it means that any spinning object has gyroscopic tendencies, and that the object is more stable about its spinning axis, and yet if forced to tip or deviate, then precession will cause an action in a direction 90 degrees to the initial axis.

Scientists and experts elsewhere have made similar claims regarding how the bicycle is critically dependent upon gyroscopic principles and the right-hand rule so as to remain upright, but we’ll defer from going down that path in the interests of brevity. We will mention experiments to be described later, specifically involving Rear Steered Bicycle I. If gyroscopic arguments constituted the primary stabilization mechanism for a bicycle, then it would appear to matter little if the bicycle was steered by the front wheel, or the rear wheel. The experimental record reveals evidence quite the contrary. Rear Steered Bicycle I has a nearly 20 year history of defying being ridden successfully in spite of exhaustive attempts, in cases including some skilled would-be riders.

A predictable outcome of somebody “scientific” explaining or demonstrating precession with the aid of a weighted wheel and a rotating stool or platform, is to be prone to conclude by saying, “And this is why a bicycle works.” Unfortunately, this scenario is more common than not among high school and university level physics instructors, as many teaching schools have access to a weighted bicycle tire capable of being spun on an axis (a handle), as well as a precision stool for the demonstrator to sit on while holding the spinning wheel, or possibly a rotating platform on which to stand. We state that the wheel used in these “demonstrations” is usually (and unfairly) weighted as the tire used is constructed of solid rubber, and thus with greater mass than, say, a conventional pneumatic tire. A central problem is that these “scientists” failed to do one thing – to test the conjecture or hypothesis with an actual experiment involving a complete bicycle, and where some type of valid control was devised to test the hypothesis.

An English chemist, Dr. David Jones, in the late 1960’s, performed an array of simple bicycle experiments, published in Physics Today on April 1st (Jones, 1970). A dominant feature of one of Jones’ experimental bikes was that he mounted an auxiliary spinning wheel on a bicycle’s front fork that was able to spin clear of the ground. Jones could spin the wheel in either direction and at various speeds, and yet the bike was discovered to be still rideable and independent of the resulting angular momentum magnitudes. Aside from the publication date being April Fool’s Day, Jones went on to shock the world of bicycle and physics aficionados by making several points: (1) In spite of Jones’ efforts to cancel or alter precession and thus gyroscopic action, Jones’ experimental bikes, and in all variations tested, were quite capable of being ridden. (2) Jones sought to discover how to design and build an unrideable bike, but he failed in that quest.

Dr. Richard P. Feynman (1918-1988), Nobel laureate of physics fame made the statement (paraphrased) – “Experiment is the ultimate authority.” In keeping with this spirit, Dr. Richard Klein initiated a number of bicycle related experiments in conjunction with students at the University of Illinois at Urbana-Champaign (UIUC), Department of Mechanical and Industrial Engineering, starting in 1983. Some these UIUC experimental bikes and conclusions are described below.

Zero-Gyroscopic Bike I -- A Fundamental Bicycle Experiment

Zero-Gyroscopic Bike I is a clever and yet simple experiment that dispels once and for all the centuries old conventional wisdom that a bike stays upright primarily due to the gyroscopic action of the two rotating tires.

The two additional or upper wheels are positioned on this bike so as to rest on the regular two lower wheels. Based on simple frictional contact between the two respective pairs of wheels, the upper wheels attain essentially equal but opposite rotation when forward motion of the bicycle is initiated. In the process of being ridden, the gyroscopic torques present and associated with spinning wheels will experience a precession cancellation effect whenever the bike rotates (yaws) or leans or when the front fork is turned. That is to say, the precession torques are still present and act on the frame and front fork assembly, respectively, however the double wheel pairs rotating in reverse directions to each other cause all precession torques associated with spinning action to be cancelled. In essence, this bicycle as configured can said to be a zero precession bicycle.

Because “precession” is an action, frequently lay people without sufficient technical background tend to become confused. The gyroscope on the other hand is a physical and familiar thing, so if we speak of gyroscopic action, the lay public tends to have a sense of what is being discussed. Hence, in many of our writings, as well as here, we will refer to bikes with precession canceling or even precession altering characteristics as “zero gyroscopic,” but we admit that this is technically in error. The action of a gyroscope can’t be made to equal zero. Newton’s Laws as well as conservation of angular momentum are stalwart pillars of classical science and mechanics. Instead, we do note that two gyroscopes can be designed to counter rotate on the same shaft or on parallel shafts so as to negate or cancel each other. The result is as if the combination of gyroscopic actions was “zero” but the gyroscopic action is only canceled out by clever design of opposing gyroscopic actions.

The empirical significance of Zero Gyroscopic Bike I is that the argument advocating sole dependence upon the “gyroscopic action” for stabilization of a bicycle is smashed and thrown out the window. Not only is the Zero Gyroscopic Bike I rideable, it is in fact, easily rideable. Hundreds of average persons, and even some fairly novice bicyclists, have ridden this bike. Other than being slightly heavier and bulkier due to the two additional wheels, this bike acts almost indistinguishable in handling as compared to a conventional bike.

Jones was the first, to our knowledge, to experimentally investigate precession cancellation as related to bicycles. We note, however, that UIUC Zero Gyroscopic Bike I, circa 1986, was the first to embody a technique that would cancel the gyroscopic torques at all operating speeds. Moreover, Zero Gyroscopic Bike I cancelled not only the front fork gyroscopic component, but also that associated with the rear tire. Of course, we routinely encounter ardent believers in the right hand rule who claim that our experiments are lacking in validity as the sprockets are still turning and even the legs of the rider, when pedaling, create the equivalent of a rotational gyroscope. Such arguments of desperation of this kind are easily silenced – merely by riding the Zero Gyroscopic Bike I in a coasting mode. Leg action ceases, and the sprockets and chain aren’t moving. The Gyroscopic Bike I is easily ridden while in the coasting mode.

In our archives, for those who doubt or those who just want fun, we have the written transcript of a lengthy interview with a fire and brimstone young Ph.D. in physics, an assistant professor of physics at a major Midwestern land grant university, circa 1985. The “professor” professed and defended the exclusive role of precession and gyroscopic action down to the last minutiae – in asserting that precession/gyroscopic alone was the sole mechanism responsible for keeping bicycles and motorcycles upright. He even went on to say that he’d bet $5 that UIUC Zero Gyroscopic Bike I, as proposed on paper the time of the interview, would be impossible to ride. He added, “Of course, unless one was a Chinese acrobat.” He would bet only the $5 amount as he said that he was too impoverished as a brand new PhD to bet more.

The only significant handling difference discovered in riding trials, compared to conventional bikes, was that the Zero-Gyroscopic Bike I was not capable of being ridden “no-hands.” We hypothesized that the additional front wheel, being extended in an upward and forward position, had caused the mass of the front fork assembly to be increased and also that the center-of-mass was shifted forward (as measured relative to the location of the steering axis). As a consequence, if the Zero-Gyroscopic Bike I were to go into a tilt, we reasoned that the action of gravity caused the front fork to turn excessively into the direction of tilt, and thus the bike was not rideable in the “no-hands” sense.

The Zero-Gyroscopic Bike II

Zero-Gyroscopic Bike II was built at the University of Illinois by three students in the mid-1980's. Its purpose was to see if a Zero-Gyroscopic Bike could be designed that could be ridden with "no-hands."

Once the rideability of Zero-Gyroscopic Bike I was established; that precession wasn’t a superior godhead, another question arose – Is it possible to devise a zero gyroscopic bike that can be ridden no hands? University of Illinois undergraduate students in Mechanical Engineering proceeded to design such a bike to test this hypothesis, called Zero-Gyroscopic Bike II. Emphasis in the design was to use two smaller wheels on the front fork, 12.5 inch scooter tires, and a modified front fork as shown. Moreover, the upper front wheel was mounted onto two slider rods thus permitting adjustment of the center of gravity of the front fork assembly.

This bike, quite similar to Zero-Gyroscopic Bike I, was also found to be quite easily rideable, provided the rider kept hands on the handlebars. Students at the time reported that the Zero-Gyroscopic Bike II bike was rideable “no-hands,” but only for brief instants. With body articulation, one could cause the bike to initiate a turn in the usual manner, but a front fork oscillation (often called a “wobble”) would characteristically result. In essence, the Zero-Gyroscopic Bike II defied being ridden no hands for any appreciable distance. In short, it seemed that the Zero-Gyroscopic Bike II lacked a mechanism to dampen any front fork rotational inertia, with the result that a wobble was frequently induced in the process of riding. In the no-hands mode, it was beyond the rider’s ability to control or dampen the resultant front fork wobble.

Other students in subsequent semesters attempted to modify Zero-Gyroscopic Bike II by adding front fork torsional damping, however the results of that experimental effort had to be curtailed at the time due to cost considerations and the difficulty of fitting the bicycle with the proper hardware for torsional damping. In short, we ran out of available time and resources as this work was performed subject to a class schedule where other priorities took precedence. Until it is experimentally proven (validated) or disproved, we will stand by the following:

Conjecture: A zero precession bicycle of the form of Zero-Gyroscopic Bike II is capable of being ridden in a no-hands configuration provided that the front fork can be fitted with a properly sized and adjusted torsional damping mechanism.

The Naïve Bike

The Naïve Bike was built at the University of Illinois, also in the mid-1980's. It was (and still is!) easily rideable, and thus demonstrated that a bike can easily be ridden even when gyroscopic torques were cancelled as well as the front fork having been stupified, or made Naïve.

Bird's Eye View of Naive Bike Front Fork.

In the next quest to determine more about the role of bicycle design as well as front fork geometry, it was decided to construct a bicycle with what we’ll call a naïve front fork. The frame has been modified to move the steering head forward, and to have the head angle be vertical (and thus at 90 degrees). Next, a front fork was created that caused the trail to be zero, that is, the point of contact with the ground coincided with the axis of rotation of the front fork. As a consequence, the friction and contact forces as might ever exist between the tire and the ground will act coincident with the axis of rotation of the front fork. Thereby no moment arm exists to induce any significant ground force generated turning moments on the front fork about the steering axis. We called this the Naïve Bicycle, as the bicycle is naïve in behavior and has no predisposition for the front fork to turn in spite of what the bicycle dynamics might be or even according to how the bike tilts.

Concerning the matter of gyroscopic actions, the use of two 12.5 inch tires mounted, one atop the other, caused a cancellation of all front fork precession as well as gyroscopic effects. Other than rider exerted torques by placing hands on the handlebars, this bike has been designed to have scant front fork torques. As a sticky point, we have not as of yet attended to the detail of counter-weighting the handlebars, so admittedly there would be a slight moment in turns due to the action of gravity tugging on the mass of the handlebars. Our plan is to replace the bent back handlebars with a circular-style steering wheel centered on the front fork axis. We note that the center of mass of the front fork is presently behind the steering axis, and thus if the Naïve Bike were to lean – the tendency of gravity action would be to turn the front fork away from the lean causing destabilization. To preliminarily test this shortcoming, the Naïve Bike has been ridden with the handlebars turned 180 degrees, in essence with forward center-of-mass, and the bike is still rideable. Another possible front fork torque could be argued to be due to aerodynamic actions; however we feel that aerodynamic aspects are not significant.

The experimental findings and significances of the Naive Bicycle are many. First of all, the Naïve Bicycle is easily capable of being ridden, meaning that an array of persons of relatively average abilities can and have ridden this bike. Another way to say it is that the bike has always been capable of having been ridden, as every person trying has been successful. Of course, the rider must keep hands-on. No able-bodied adult sized person has ever failed, and hundreds of persons have ridden this bike. The first inference is that precession is not essential in order for a bicycle to be ridden. The second inference is that front fork geometry and trail, in particular, are not critically imperative in order for a bike to be rideable. Moreover, as illustrated by the Naïve Bike, the combination of zero precession and zero trail of the front fork also result in a bicycle configuration that is easily rideable.

Close examination of the front fork on the Naïve Bicycle will indicate the presence of an additional tire axle slot. In short, the front fork was designed such that the two smaller wheels can be removed, and replaced with a single conventional full-sized front wheel. Doing so then restores the precession effects of the front wheel. Engineering students reported in experimental trials that this bicycle, with zero trail but with a conventional balloon style tire on front, was capable of being ridden “no-hands” by a skilled rider. The inference is that when sufficient gyroscopic front wheel action exists that (1) a controllable front fork torque can be induced based on upper body leaning, which in turn, causes a lean reaction of the frame of the bike, and (2) that the front wheel’s gyroscopic properties are sufficient to prevent or at least make controllable any front fork wobble.

Semi-Naive Bike. The upper photo shows the Semi-Naive Bike with a conventional front tire in place (rather than the two small 12 1/2 inch counter-rotating tires), as well as the rear precession canceling tire removed. It is actually the same bike that we previously dubbed the "Naive Bike," but we call it semi-naive as it isn't quite as naive. In the lower photo Bill Becoat demonstrates riding the Semi-Naive Bike "no-hands." You will note that Bill's hands are raised and extended outward. For the critics, scoffers, and doubters, please understand that Bill is sufficiently "mature" that he qualifies for senior citizen discounts. Moreover, Bill had never previously ridden this bike. He was able to hop on and on his first ride he could easily ride this bike "no-hands."

A summary of what the three gyroscopic canceling bikes and variations is telling us. The three experimental variations on precession canceling bikes as described above, when considered in overview, provide a considerable storehouse of empirical realizations regarding the dynamics of bicycles. Once the experiments are performed including numerous public and media presentations, those who previously touted the almighty precession theory along with the “right-hand rule in physics being everything” tend to silently slip away into the background. In short, it is hard to argue with experiment – as experiment is the ultimate authority. Precession is not essential in keeping a bike upright, but it does help. Moreover, the Semi-Naive Bike tells us that precession can alone be responsible for being able to ride "no-hands," as the Semi-Naive has zero trail and a straight vertical head angle. Thus all castor-camber effects have been eliminated.

In the photograph above, we have placed the Semi-Naive Bike into a lean. Because of zero trail, as well as no castor-camber effect, the front fork does not turn, even statically, into the direction of lean.

The Los Angeles Times (1988) published a story and photo with an array of UIUC precession canceling bikes in 1988 being ridden on the university campus in unison. We thought that you would like to see the article, as it establishes that we go back almost two decades. The ability to work with so many highly qualified students in engineering at a top-level land grant institution like the University of Illinois in Urbana-Champaign represented a dream come true. Over the period of about one decade, approximately 1,000 students were involved in a wide array of bike research investigations and experiments. The students made a number of cutting edge contributions -- contributions that remain cutting edge even to this day. The adapted bike program would not have happened if had not been for these dedicated and talented students. We apologize that our riders are without helmets, but recall that a decade and a half ago, the matter of mandatory helmet use had not yet become a public issue. In the 1989 era we instituted a policy that all persons riding any of our bikes will wear suitable helmets.

One more experimental bicycle is worth discussion as related to precession and gyroscopic principles.

Students opted to test the extremes to which gyroscopic magnitudes could be varied. A bicycle was constructed with two heavier auxiliary wheels mounted to the front fork assembly.

SHOW PHOTOGRAPH OR SKETCH

The two auxiliary wheels were mounted on and connected by a solid shaft, but otherwise free to spin, in a manner somewhat analogous to Jones’ gyroscopic experimental bicycle. The difference in our case was that both wheels were secured to the axle. We could rotationally accelerate the assembly of both auxiliary wheels and axle by causing the supporting axle shaft itself to rotate. The students used a cord wound up on the shaft. Pulling on the cord to unwind would cause a rotation of the wheel assembly. By using painted pie-shaped segments on the exterior sides of the wheels, a laboratory strobe light (a precision and calibrated timed flashing light) yielded a measure of the angular rotational speed. Once the rope was pulled and auxiliary wheel rotation initiated, then the angular speed was measured using the strobe. At that point the experimental bike then could be ridden, or at least attempted. By knowing the angular speed of the auxiliary disks, along with the mass distribution properties, the angular momentum and precession properties were thereby determined.

A series of experiments indicated that the bicycle was indeed rideable, that is capable of being ridden, and for a wide range of precession values. In essence, the bicycle was found to be rideable so long as the gyroscopic values were restricted to less than about 100 times in magnitude, in either direction, as compared to when conventional bikes are ridden. Only when the gyroscopic torques, as calculated, were beyond (approximately) one-hundred times in magnitude as compared to a conventional bike, did the experimental bike become unrideable in the hands-on case. This experiment reinforced previous conclusions that gyroscopic contributions for conventional bicycles are of relatively minor importance.