Of Wobbles and Weavels

Motorcycles are only stable when they are moving. We all know that.If you walk away from one without putting some sort of stand under

it, it'll fall over. The problem is that a moving motorcycle isn't

necessarily all that stable either. There are forces that constantly

work to make a bike crash; it is the designer's job to make sure

these upset-ting forces are completely damped. The fact that

motorcycles continue to weave, wobble and tip over after nearly a

century of continuous development is proof of the difficulties

involved.

WEAVE AND WOBBLE

Most of us have experienced a wobble or a weave or have been told

about them. Few of us have a clear understanding of what a wobble

or weave actually is. Motorcycle instabilities are usually lumped

into a catch-all description wobble. Before one can do anything about

a problem, one must first understand its nature. There are three

destabilizing forces acting on a motorcycle as it moves down the

road: weave, wobble and upset. When a bike weaves enough to notice,

the front fork assembly remains straight and the rest of the motorcycle

moves from side to side several times each second. A bike that is

seriously wobbling shakes its head while the rest of the machine remains

mostly straight and stable; the shaking (wobble) is very rapid and

difficult to control. Upset simply describes the same tendency to fall

over as the machine has when it is stopped.

When interpreting the dynamics of motorcycle stability, it is useful

to view a motorcycle as having two major dynamic assemblies: the chassis

aft of the steering head and the front fork assembly. Like the wheels

of a shopping cart, these assemblies can sometimes embarrass us.

Any motorcycle chassis constantly tries to oscillate (weave) back and forth

about the steering head at between two and five times per second. Every

bike has this instability; it is held in check by damping forces created

mainly by the rear tire's self-correcting tendencies. If the tire's

natural oscillation frequency happens to coincide with the chassis,

the bike will weave. This seldom happens,though, as tires have much higher

frequencies than the typical chassis. For the tire to do its work, it must be

properly inflated and rigidly connected to the chassis. Worn swingarm

bushings, or poorly damped suspension springs can defeat the best tire's

stabilizing influence. Weight attached rigidly to the rear of a motorcycle

can slow the weave frequency and bring it into a range close to the

tire's, and serious weave instability can result.

Wobble describes the natural oscillation of the front fork assembly in the

steering head bearings. Proper front end geometry combines with self

correcting torque from the tire to damp this wobble force. Like the rear

tire, the front tire must have an effective connection to the fork

assembly to do its job. Underinflation,dented steering bearings, loose

fork sliders or limber fork tubes can defeat the tire's corrective efforts.

Chassis upset instability is simply the motorcycle trying to fall over,

as it does when at rest. Rake and trail act to harness the self corrective

forces of the moving front tire's contact patch. The net effect is to

keep the bike headed straight and the machine's center of gravity over

the tires' contact patches.

Upset instability is never, by itself, a serious threat. It can contribute

to a crash when a weave becomes so serious that the motorcycle's lean

angle during each weave cycle becomes large.

WHAT YOU CAN DO TO PREVENT WEAVE AND WOBBLE

A few of the factors that act to damp wobble and weave are, largely,

out of your control. Most, however, are in your hands. In fact, each

time you install a different tire or change the dampers, springs or

fork oil, you affect the stability of your motorcycle. Fork rake angle,

trail and distribution of the majority of the weight are fixed by a

motorcycle's design.

The tires and their operating pressure, weight distribution and the general

condition of the hinges (swingarm bushings, steering bearings, wheel bearings

are the most important factors of stability, and you control them. All of

us buy tires; few replace the originals with more of the same. Even the

best premium tires can fail to stabilize a particular motorcycle. That's

the reason your owner's manual specifies certain tires for your bike;

the factory has tested those tires on that model for stability. Germany

and Japan require that only tires which have been tested and approved

for use on a specific motorcycle be used on that motorcycle; in those

countries, you will get a ticket if you have unauthorized tires mounted

on your bike, even if only the speed rating is wrong. But you won't

necessarily have instability caused by non-standard tires.

While instability seldom arises when any premium tire is mounted on any

motorcycle, it can and has happened. A wide and sticky rear sport tire

can make many motorcycles weave. The excellent traction, which is why

a buyer chooses such a tire, can cause many a chassis to flex enough

to become unsettled and initiate a weave. There is nothing wrong

with the tire; there is nothing wrong with the various chassis. They

simply don't work together. Before you install an unknown tire on

your bike, ask around. See if anyone has had any problems when that

tire was mounted on the same bike model as you have. See what the bike

manufacturer says. If you can't get such feedback and mount the tire

anyway, be cautious about running at high speed. Slowly approach the

maximum speeds at which you intend to ride. And check the pressures often;

that is the most common maintenance failure, even among experienced riders.

Hopefully, the fear of wobble or weave will instill in you renewed

dedication to take care of the "hinges" on your motorcycle.

Worn rear tires are the most common cause of weave instability on a bike

that, until it suddenly started to squirm, was as stable as marble. Even

minor wear in the middle of the tire can create the slight alteration of

profile that creates instability. Many stability problems are the result

of deterioration of the swingarm bushings or steering head bearings. If

you keep them adjusted and lubricated, they will last many miles; the

act of inspecting and lubricating them keeps you aware of their condition

so you can replace them, if and when they do wear out. Most of us ignore

our bike's wheel bearings don't. Periodically check them for roughness

and play. Replace them if there is any of either. Lubricate them according

to the maintenance table in your shop manual.

Where you place weight on your bike has a tremendous effect on its high-speed

stability. Weight should be as far forward and as low as possible. Put your

tools in the tank bag rather than in the rear of the trunk. The worst thing

you can do is bolt weight to a rear rack. The late Kawasaki police motorcycles

have their heavy (47 pounds) radios mounted on hydraulically damped pivoting

mounts. They are also mounted where a passenger would normally sit. Speaking

of passengers, they actually help stability. The damping effect of a person

assists a tire in its efforts to keep you pointed in the right direction.

Aerodynamic effects can initiate weave or wobble. Saddlebags tend to spill air

around one side of the bike and then the other, just like a flag pole does. If

your motorcycle is fitted with saddlebags, you may have noticed that when you

place your feet on the passenger pegs the bike begins to slowly weave. If your

bike does that, either install a fairing of some sort, remove the bags or

don't go so fast. A fairing added to a bike with bags usually eases the weave

problem. It seems a fairing routes air around the saddlebags and eliminates

them as a source of aerodynamic upset. A poorly designed fairing or a poor

bike fairing combination can create wobble. If your bike does this, at

least you know the cause... you may decide to take the fairing off and get

one with a successful history on bikes like yours. Fairings can unload the

front wheel at high speed which encourages an undamped wobble.

WHAT TO DO IF YOU GET INTO ONE

The faster a motorcycle goes, the less stable it becomes. Think about that the

next time you casually decide to "see what she'll do."

DECELERATION SHIMMY

The deceleration shimmy is a wobble many riders experience between 50 and 35 mph

when they take their hands off the bars during deceleration is unimportant and can

sometimes be tuned out by adjusting the fork for smooth operation and, if necessary,

changing the front tire.

A real wobble, one that can force the bars out of your hands, bang the fork

against its travel stops six to ten times per second and cause the front

tire to leave crescent-shaped marks on the roadway gives you very little time

to do anything about it. Most experts recommend slowing down and easing your

grip on the handlebar. When you grip the handlebar tightly and set the muscles

in your arms and back (fear does that), you alter the frequency of the fork's

natural oscillation. Unfortunately, that alteration makes the tendency to

wobble worse. You can, under the right conditions, initiate a wobble by simply

gripping the handlebar and setting the muscles in your upper torso. If

you get into one, slow down and relax your grip on the handlebar. I realize this

action will take the resolution of a saint, but it is probably the best solution.

Weave instability seldom results in a fall. While the onset of a wobble can be

very sudden and energetic, a weave tends to gradually build in intensity with

additional speed and can be controlled by merely slowing down. If a weave is

caused by a flopping flat rear tire or if it happens in the middle of

a high speed corner, things are a little more dicey. Still, you can get out of

it if you keep your head. Should you suddenly find your bike weaving under

you, do two things: lie down (bend forward) and move your weight as far forward

as you can. That's it. Unless there is some mechanical condition sustaining it,

the weave will stop within three seconds and usually less than two. This action

reduces the polar moment of the chassis assembly, increases its oscillation

frequency and gives the tire a better shot at damping the weave.

It's not advised to power through a weave or wobble. Although a bike will

sometimes regain marginal stability with an increase in speed, don't take the

chance. Remember that if you power out of a weave or wobble, you still have to go

through the same speed range on the way to a stop.

I hope this primer gives you a basic understanding of the problems involved in

making a motorcycle stable. You now know the difference between weave and

wobble. You also know what some of the forces are that cause them, and you

know what major components most affect your bike's stability. I hope you think

about these things when you inspect your bike for wear and tear, when you buy

tires buy both and before you decide to go real fast. A motorcycle in good condition and

fitted with compatible tires is much safer at any speed. A rider who knows what to

do if things go wrong, will, in the final analysis, have more fun, which is why we

ride these things in the first place...

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