Wobble under load

Hi There, new to this forum.



I have an XJ650 that I purchased from a man on ebay about 2 months ago. I have been preparing the bike to take on a rather long trip across the country. The front breaks were shot when i got it and i bled the system and breaks grab again. when i am breaking, though, they slip a little bit in a particular spot on the disk. (please advise)

Just recently I mounted two 20mm ammo cans to the sides of the bike and attached a luggage rack/sissy bar.

Total equipment load including rack and ammo cans is about 150lbs. i also weigh about 150lbs. I have been noticing a little bit of wobble from the front tire around 40mph. no wobble on acceleration. on center stand, i spin the front wheel and the brakes rub on the disk in a certain spot, the rest spins free. i also notice some pretty major wobble at very low speeds if i sit forward a bit, leaning back tends to make it go away. at 75mph there is no issue at all.

i drove the bike about 300 miles a month after i got it and noticed some wobble around 35/40 with a 50lb pack on the back.


is this "normal" under load? uneven weight distribution?

thanks,

Aaron

 

Have you checked the bearings in the steering? With the bike on the center stand get the front wheel off the ground (a buddy pushing down on the back end is one way to do it) and try and move the front forks from side to side or front to back to see if there is any play in the steering. If so you need to deal with that bearing.

Actually if you are planning a long trip it would be a good idea to check and repack/tighten this bearing beforehand. The swingarm bearings in the rear are another set that often get forgotten during preventative maintenance.

 

Besides the head bearings, the front tire may be out of balance. If the front caliper is grabbing, this may cause a slight shimmy, too. The head bearings are the usual culprits, though.

 

thank you for your replies!

ok, i put it up on center stand and gave the forks a tug left/right forward/back and it didnt feel like there was any play. forward and back there was a little bobbing, but it felt like that was due to the weight of the wheel and caliper, it appears to be rather snug. i am new to this so i could be wrong.

how might i check to see if the front tire is out of balance?

aaron

 

Pull the tire assembly off and take it to a bike shop to have them balance it. You can do it yourself if you have a balancing stand. I have a picture of one in my gallery if your are interested in making one yourself. They run about $200 from the bike shop. You can make your own for about $20 (bearings and a rod that will fit through your rim) and some scrap lumber. Good luck on the trip.

 

I had a similar wobble which worsened under load.

Steering head bearings showed no slack or "dead spots" when checked with the front wheel off the ground - but loosening them and tightening them to spec made 90% of the wobble disappear...

 

You also want to check for "cupping" on the tyres. I had similar wobble and a new front tyre fixed it. Also make sure the wieght you are carrying is as low as possible on the bike and tyre pressures are OK.

 

Make sure you check your rear tire as well. I just spent a long time trying to take a problem out of the front end to realize in the end that I needed a new rear tire even though it looked fine sitting on the stand but at speed there were belts seperating.

Tell me how you attached to ammo boxes to your bike. Any detailed photos? That looks freaking awsome. And at a good price. Looking around I found them for about $14. Great idea.
The Buffalo

 

Take the load off of your sissybar area and give it another go.

I remember an article YEARS ago where the motorcycle journalists tried loading up a bike with luggage in various combinations to see what the effect on handling was. The top box was the only piece of luggage that consistently degraded handling to the point of inducing wobbles or tank slappers. This would have been about 1980-83 sometime, so it would be relevant to our bikes.

 

I found this on another forum. It had been copied from another forum, which in turn had been copied. I could not find author information so I hope it is OK to repost here uncredited.

WOBBLE

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...