All vehicles display forms of instability. On a flight to Minneapolis in the 1980s, I looked out my window to see the wingtip of the 737 describing a steady oval orbit. Evidently, the aircraft’s yaw damper was out of adjustment. As a swept-wing airplane yaws to the left, its left wing becomes effectively shorter and its right wing longer, causing a roll to the left and leading to an oscillation known as “Dutch roll.”
As a deep-vee boat hull moves faster and faster across water, its chines (right and left edges) rise out of the water. Because the water’s surface isn’t perfectly smooth, an oscillation begins, called “chine walk.” The boat begins to oscillate in roll, touching its chines to the water alternately. If allowed to build up, chine walk can abruptly dig in a chine, causing the hull to dart in the opposite direction, possibly flipping over in the process.
A major goal of fighter aircraft development has been rapid control response, but the quicker the response is made the less stability remains. To deal with this, artificial stability systems were developed that are capable of making the necessary extremely rapid corrections far exceeding human capability. The result is extremely rapid maneuver response but at the cost of carrying triplexed artificial stability systems. Should artificial stability fail at the next small disturbance to airflow, the aircraft is liable to flip end for end like an arrow shot backward.
When I first saw professional racers on 16-inch wheels in 1981, I noticed that at each disturbance the front wheel displayed short vigorous shiver. Bikes on the alternative wheel size of that time, 18 inches, were visibly more stable—no wiggles. Some riders adapted to the lower-stability 16s and others returned to 18s. The eventual result was the era of 16.5-inch wheels followed by the 17-inch compromise orthodoxy of the present day.
When the new four-stroke MotoGP class began in 2002, a novel effect was the quite large engine-braking torque of its 990cc engines. This, by dragging or hopping the rear tire during braking, caused uncommanded corner-entry slides that looked as though the rider was steering the bike with a rear-wheel thumb brake. During straight-up braking, the resulting lack of rear-wheel damping (a tire that slides doesn’t care much which direction it goes) could lead to a side-to-side oscillation that could build up faster than a rider could respond. This was the beginning of the modern concern with braking stability, a concern that led to the development of “throttle kickers” to cancel engine-braking, together with slipper clutches and seamless-shift transmissions.
Riders of large touring bikes have sometimes noted that taking their hands off the handlebars (perhaps to do up glove snaps) at around 35–40 mph can produce a very rapid head shake that thankfully disappears the instant their hands again grasp the bars. Other riders have wondered why some bikes bear a placard forbidding the use of tires other than a specified type. Others yet have noticed that riding with a large load on a rear luggage rack can be associated with a disquieting side-to-side motion at high speed.
A first step for all motorcyclists has generally been learning to ride a bicycle. I repeatedly dragged my heavy fat-tired Schwinn to the top of a grassy slope, climbed aboard, and again and again felt gravity pulling me on a course to yet another crash. Finally I “got” it. I had to continually steer the wheels to keep them centered under the mass of myself and the bicycle. This falling over is the first of the three forms of instability that can be displayed by bicycles and motorcycles and is called “the overturn mode.”
Bicycles and motorcycles are a pair of castered wheels joined at a common pivot, the steering head. The front caster is quite short; on a motorcycle the center of the front tire’s footprint trails the projection of the steering axis onto the road by roughly 4 inches. The rear caster is much longer, feet rather than inches. Casters, as we can see any time we push a supermarket cart, are capable of oscillation, the castering wheel swinging rapidly from side to side. Motorcycle engineers call oscillation of the front caster “wobble” and oscillation of the rear caster “weave.”
The short front caster of a motorcycle oscillates very rapidly, typically at eight to 10 cycles per second. A standard test for control stability in aircraft was the “stick pulse,” basically to give the control stick a thump in either roll (aileron deflection) or pitch (elevator deflection) directions. Professional motorcycle testers do something very similar. With hands off the bars they deliver a steer thump. A highly stable bike responds by quickly self-centering with almost no perceptible oscillation. A less stable bike responds with wobble that quickly dies away. A bike with marginal stability may enter a steady-state wobble, and the worst case is a wobble that increases without limit (engineers call such a sudden increase “divergence”).
Wobble is generally easily damped by hands on the bars or by a steering damper. Natural damping of the wobble mode increases after roughly 40 mph. Stability against wobble is increased by reducing the steered mass (front wheel, brake, fork, plus any added load). Bimota’s hub-steered Tesi, for instance, has a very lightly steered mass (brake and wheel only; no conventional fork) and is less subject to wobble because it has less mass in which to store oscillatory energy.
Weave occurs in the range of two to three cycles per second, just slow enough to tempt the occasional rider to feel he/she can steer out of it. Weave is dangerous to riders because its damping decreases with road speed and because it is not effectively controlled by a steering damper.
Weave has been associated with circumstances such as being loaded heavily to the rear or with flexible loads such as heavy old-time police radios on flexible racks or with loose pivots (wheel, swingarm, steering head). When a customer at the dealership where I was once a partner requested “road test at 120 mph” for his bike with extensive camping gear loaded onto a sissy bar of the 1970s, we declined! You will notice that European motorcyclists tend to load their gear on a gas-tank rack. This is because European speed limits have been high.
The major source of chassis damping on a motorcycle is its tire footprints. Inflation of tires beyond the recommended pressure range can therefore reduce stability by reducing tire footprint area.
Production motorcycles are carefully designed to operate stably, just as are boats, aircraft, and other vehicles. In a few cases, certain motorcycles were found to be most stable on particular tires; for this reason they were placarded for operation only on those tires. Tires wear in use and pivots may become loose, so it is important for stability that motorcycles be well maintained.
As an AMA roadrace tech inspector for a time, I found that I could discover loose pivots by kneeling beside a front or rear tire while another person supported the bike, then grasping the top of front or rear tire and vigorously shaking it side to side. Looseness of wheel bearings, wheel spokes, and fork or swingarm pivots was revealed as a clicking or lost motion (yes, I discovered each of these conditions in tech inspection).
Such a check is just common sense. If your motorcycle feels different or odd when you ride it, look for a cause or ask your dealer to investigate.