This section is the direct continuation of the discussion in the previous section, “Gyroscopic.” We continue our scientific quest to investigate the bicycle.
In the process we will start by introducing a term called “Critical Velocity.” Once we have that definition in hand we will next look at the front fork, and in particular, what forces and torques control the behavior (the turning) of the front fork. The “Critical Velocity” is defined as the velocity at which a bicycle becomes stable in and of its own properties — and without the need for human intervention to otherwise prevent falling over. In short, if a bicycle is traveling at too low of a speed, it will fall over. Once Critical Velocity is reached, the bicycle will remain upright on its own.
This section goes into experimental aspects of a discussion necessary in order to get to the mysterious concept of critical velocity of a bicycle.
Understanding the Front Fork Hierarchical Chain of Command
When a front-steered bicycle is being considered, the torques acting on the front fork have a pecking order or hierarchical chain of importance. Rider applied handlebar torques are dominant and the king. Next in line, but relatively far behind, are the castor-camber forces, these being the results of the ground contact forces which act on the front tire. The third in line, but somewhat close to castor-camber torques in magnitude, is the front wheel’s gyroscopic reaction torque. In essence, it is the castor-camber forces along with precession that cause a bicycle’s front fork to want to turn into the direction of fall or tilt. Gravity, inertial, head bearing friction, and aerodynamic torques also come into play but they are usually lower down yet in order of magnitude.
When a bike is ridden “no-hands” it happens that the king is asleep, and hence the secondary and even lesser torques are allowed to come into play. These secondary and lesser torques are relatively weak, and as such one has to be a little more delicate as well as less aggressive. That is why it is important to ride a bike “no-hands” with a degree of finesse, and that we usually tend to make our upper torso lean moves sufficiently in advance, such as riding a bike “no-hands” when a turn is anticipated.
Another reason why a rider has to be more cautious in the no-hands riding mode is that the human’s neural delay for upper torso movement is approximately 0.3 seconds, whereas the neural delay associated with hand and arm movements is faster, typically 0.1 to 0.2 sec (Weir 1972). Moreover, the use of upper torso leaning is further exacerbated as the dynamic response of the bicycle to body leans is relatively slow as compared to steering control that is handlebar actuated.
As a point of clarification regarding aerodynamic torques, if we would ever elect to use a full cover front wheel, which seldom happens in practice and for good reason, the aerodynamic forces can in certain circumstances be intensified, as well as being destabilizing.
University of Illinois undergraduate students in the 1980’s went on to conduct additional bicycle experiments. Perhaps the most notable, in addition to fun, is what we call Rear Steered Bike I.
Rear Steered Bike I
Travis Williams, a scholarship athlete at the University of Illinois gives a try on the unrideable challenge bike — under the watchful eye of Dr. Klein. Travis tells us that our prize money is quite safe.
Rear-Steered Bike I has never been ridden, although hundreds of average and even skilled riders have tried. The best attempt made so far occurred in the 1980’s when the bicycle was placed on loan for about one month to the then president of the University of Illinois Unicycle Riding Club. With practice the rider, a skilled unicyclist, was able to eventually remain upright on the bike, but in doing so the rider had to do two things:
1.Configure the chain connecting the handlebars and the rear fork in a conventional circular loop, as opposed to being in a “figure 8” or crossed which is the usual connection configuration. See photo below of the connecting chains in the crossed configuration.
2.The rider ended up riding haphazardly in an open and flat parking lot as opposed to being able to follow a prescribed path.
Being able to follow a prescribed path is one of the requirements in order to qualify for the US$5,000 prize. In addition there are additional requirements, all quite reasonable but nonetheless bike riding challenge requirements must be adhered to. The contribution of John Becker, former University of Illinois graduate student in Mechanical Engineering, is acknowledged as it was through John’s effort that this particular bike was built.
As the photos show, this UIUC bike has been through its share of trials and tribulations. Hundreds upon hundreds of overly optimistic riders have tried for the prize reward — but to no avail! Our money is very safe. We’ve hauled and crated this popular bike to seemingly countless shows and exhibitions across the nation. We never fail to attract attention as well as suckers who think that it’s easy to ride. In short we have tons of fun just laughing as this bike draws would-be riders like a pile of hundred dollar bills flying about in the wind draws all who want to scoop up what then can grab.