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Motorcycle Wobble And Weave—Making Sense Of “Death Wobble” Talk

What are the root causes of instability on two wheels?

Marco Melandri battled instability during the first part of the 2018 World Superbike season. “Everybody said it was aerodynamics or weight distribution because I am small,” he said that June at Laguna Seca.
Marco Melandri battled instability during the first part of the 2018 World Superbike season. “Everybody said it was aerodynamics or weight distribution because I am small,” he said that June at Laguna Seca. “In the end, it was none of this. Every time I changed tracks, I was so worried because it was a very strange problem.”Ducati

Lately, the internet has lit up with talk of “motorcycle death wobble” so it’s time to set forth the basics of how this happens and what may be involved. What follows here is intended to give riders some understanding of what motorcycle instabilities are and what influences them. Vehicles on land, sea, or in the air are capable of instabilities.

Geoffrey de Havilland Jr. was killed in 1946 when the DH108 aircraft he was testing developed an uncontrolled pitch oscillation. V-hull powerboats can be destroyed in the twinkling of an eye at high speed by chine walk, and decelerating hydroplanes can porpoise extravagantly. The front wheels of autos can wobble violently if their pivots and steering joints become loose.

Motorcycles are capable of three basic instabilities:

  1. Overturn mode. Just staying upright is our big problem while learning to ride a bicycle. We all eventually solve it by learning to steer to keep the tires centered under the bike's center of mass.

The other two instabilities result from what a motorcycle or bicycle is, a pair of casters joined at a common pivot. A caster is a wheel that is made self-centering—it aligns with the direction of motion—by having its footprint trail behind a pivot. The front wheels of a supermarket shopping cart are a common example of a caster in daily life.

A motorcycle’s front caster is its front wheel and steered assembly, pivoted on the steering-head bearings. The front wheel follows behind the steering axis by a distance known as “trail.” The rear caster is the rest of the machine—frame, engine, swingarm, and rear wheel—also pivoting around the steering-head axis.

Each caster can under certain circumstances develop an oscillation. We have all seen this in the case of shopping carts.

Vehicle dynamics people have given the motorcycle’s two caster oscillations different names:

  1. Wobble is the rapid oscillation at 8–10 cycles per second of the front caster. Wobble typically occurs in the speed range of 35–40 mph, often in step with front wheel rotation and can be excited by front-wheel imbalance or out of round. Riders of some bikes experience a bit of wobble if they take both hands off the bars in this speed range to adjust gloves or face shield. In most cases, the damping effect of putting their hands back on the bars causes this to die away. Wobble damping increases with speed.

  2. Weave is the slower side-to-side swing at 2–3 cycles per second of the rear caster. All motorcycles have a weave threshold, but care is taken in design to place this above the machine's top speed. Despite this, the addition of extra equipment, changes in loading, wear of tires, or loosening of pivots can lower this threshold, making weave possible. Weave is especially dangerous because its lower frequency tempts some riders to think they can "steer" out of it, which often has a reverse effect.

How Oscillations Develop

The usual model given for oscillation is that some disturbance deflects the castered wheel from its centered position, and the restoring force that results gives it angular momentum that carries it past the centered position, whereupon a restoring force in the other direction drives it back. If there is not enough damping (friction) in this system to promptly remove energy from this oscillation, it can persist at some level.

Levels Of Instability

Stability is a relative thing. Professional test riders evaluate the stability of motorcycles in much the same way that aircraft stability was investigated. As an example, a professional test pilot may apply a “stick pulse,” basically a firm thump on the control stick in either the pitch or roll direction. The airplane’s possible responses define four categories:

  1. High stability: The disturbance caused by the stick pulse promptly dies away.

  2. Moderate stability: An oscillation is produced, which dies away after X cycles. The smaller X, the higher the stability.

  3. Steady instability: The stick pulse results in a steady oscillation that neither dies away nor increases.

  4. Divergence: The stick pulse produces an oscillation whose amplitude increases without limit. If a divergent oscillation is unchecked, control will be lost.

Chuck Yeager's Stability Research

Famed test pilot Chuck Yeager was chosen to evaluate the stability of the experimental Bell X-1 rocket plane as it entered the transonic region. He performed sequences of tests of the previously mentioned kind at increments of 0.05 Mach, roughly 34 mph, showing that stability was gradually being lost as speed increased. His accurate stability studies resulted in a redesign that enabled the X-1 to controllably exceed the speed of sound on October 14, 1947.

Kenny Roberts experienced at 150 mph a weave oscillation so violent that it pushed the tire off its rim, causing it to go flat.

Example Of Levels Of Motorcycle Stability

On a new square-tube aluminum racing chassis, Kenny Roberts experienced at 150 mph a weave oscillation so violent that it pushed the tire off its rim, causing it to go flat.

When quick-steering 16-inch front wheels appeared in 500cc Grand Prix racing in 1980–’82, spectators could often see them wobble for several cycles when disturbed. Stability researcher R.S. Sharp, author of the 1971 paper, “The Stability and Control of Motorcycles,” had predicted this would happen as wheel diameter was reduced.

In the 1979 Goodyear pre-Daytona tire test, I could hear Mike Baldwin’s Yamaha TZ750 engine make a woo-woo-woo sound as its rear tire spun, accelerating between turns 1 and 2. I realized that this was the weave frequency, being excited by grip loss.

Although there are mathematical models that describe motorcycle oscillations, tire behavior is extremely difficult to model accurately. A large literature of motorcycle stability research provides the reader with basic advice as to what conditions or changes tend to increase or decrease wobble or weave stability.

These oscillations consist of masses oscillating against a restoring force, restrained by whatever damping forces exist in the system. The more mass there is in motion and the farther it is located from the pivot, the harder it can be for existing damping forces—mainly from the tires—to prevent those masses from oscillating.

In the case of wobble, we are usually told that adding mass to the motorcycle’s steered assembly may make wobble more likely. The reverse of this, Bimota’s forkless Tesi, is often cited for its high stability, for only its front wheel and brake are steered; there are no fork legs to move with the wheel.

In the same fashion, we are told it is best for weave stability to load a motorcycle forward as many Europeans do, with a tank bag, rather than to add such extra weight behind the rider and passenger. Another point often made is that if weight must for some reason be carried to the rear, such as heavy old-time police radio gear, it must be firmly rather than flexibly mounted. A 1960s article in the Japan Society of Auto Engineers (JSAE) publication showed that greater weave stability was associated with positioning engines closer to the front in chassis.

Various researchers have found that stiffer chassis attenuate weave more rapidly than do more flexible ones. The Honda NSR250 ridden by Max Biaggi in 1997 was for research purposes made quite flexible and later had to be reinforced to allow it to reach its top speed without weave.

Another piece of general advice is that while a steering damper can control wobble, it has less effect on weave.

The Effects Of Tires And Tire Wear On Stability

Motorcycle tires are designed and developed to be stable as manufactured, but tire wear can alter their profile in ways that affect stability. Some motorcycle models are placarded to be operated only on the make and model of tire with which they were delivered new. Such a placard is to be strictly obeyed; use of non-placarded tires, even of the same size and general appearance, can affect stability. There may also be placarded warnings regarding use of optional equipment such as fairings, windscreens, or luggage.

Influence Of Tire Pressure On Stability

Tire pressure is another important variable in stability, for it controls the size of the tire’s footprint. That footprint provides most of the damping (friction in the system) that is responsible for maintaining stability. Therefore maintain the tire inflation pressures called out in the owner’s manual or tire placard. Higher tire pressures reduce the amount of rubber in contact with the pavement—the tire “footprint”—thereby reducing damping and possibly affecting stability. Always respect the tire maker’s advice regarding tire pressures, as they are an integral part of a tire’s design.

I have seen weave develop in racing motorcycles when fast maneuvering momentarily unweights the tires, reducing the damping from their footprints.

In similar fashion, I have seen weave develop in racing motorcycles when fast maneuvering momentarily unweights the tires, reducing the damping from their footprints. I once saw Anthony Gobert hesitate for a fraction of a second each time he hit the top of the right-left direction change at the bottom of the Laguna Seca Corkscrew. When I asked him why, he said, “If I don’t do that, the bike goes crazy.” That is, it would give a violent shiver of weave.

Structural Looseness Or Flexibility

Other possible causes of instability are loose pivots—steering head, swingarm, and wheel bearings, wire wheel spokes, or even cracked frame members. When we drove an old Ford Thames van full of racebikes down the New Jersey Turnpike its worn-out front-end joints allowed both front wheels to wobble at about 40 mph. Other motorists were frightened!

When I did AMA roadrace tech, I discovered all kinds of crazy stuff. Two different bikes I inspected had aftermarket swingarms on them that were installed without bearings! Another bike’s steering-head bearings were loose; I wondered why its rider didn’t feel a “clunk” as he applied the front brake.

For the non-specialist, motorcycle wobble and weave are problems that must be referred to the dealer. Common sense dictates that motorcycles must be well-maintained, shod with tires in good condition, correctly inflated, and loaded in accordance with manufacturer callouts in the owner’s manual.