I went for a little ride at lunch and noticed something that I've never noticed before.
I was going about 80km/h on a very slight turn and there was a car that was stopped beside the curb so while leaning in to the turn, I eased off the throttle and applied some rear brake to slow me down so I could switch lanes. When I did this, the bike started to wobble, really really slowly, as if I was turning on those cracks that are filled with tar; though there wasn't any.
I didn't think anything of it and I went on my way until it happened again, under different circumstances.
I came to a stop sign on a curve and this time, I accelerated through the turn and felt that slow wobble again! My tire pressure is good, my tires only have about 7,000km on them, I believe my chain slack is good too.
I've never felt this slow type of wobble before, could it be the road surface? This particular road surface is known to get super slippery when it's wet, but it's been super sunny and hot today.
Are your tyres cupping? Are the rear lugs tight? What kind of tyres do you have?
Doh! I think I know what it is.
I checked my tire pressure a couple days ago and for whatever reason, 34 psi in the front and 36 psi in the rear stuck out. I check the manual and it should be 36 psi in the front and 42 psi in the rear.. I'm sure it's a combination of the road surface, cold tires and low psi!
Phew, that's a relief!
You might have experienced a deceleration shimmy and it's normal...
some bikes may shimmy decelerating through the 45 mph range... keeping
your hands on the bars should arrest most of the front end shimmy...
some bikes shimmy more than others and it's no big deal with your
hands on the bars in the critical speed range... your bike should be
immune at speeds above 45 mph...
Deceleration shimmy is chiefly the product of non OEM or a worn
tires... it ain't the product of tire cupping... but low pressure or
loose steering head bearings defeat the tire's corrective efforts...
because every bike has this instability... it is held in check by
damping forces created mainly by the tire's self-correcting
tendencies...
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.
In short...
Deceleration Shimmy = front tire problems... annoying...
Wobble = front end problems... (could grow into the dreaded tank slapper)
Weave = rear end problems... annoying...
[image: View Post]Monk, on 14 June 2012 - 12:54 PM, said:
Are your tyres cupping? Are the rear lugs tight? What kind of tyres do you have?
The bike's only got 15,000km on it, and the tires are Continental Road Attack 2's with about 7,000km on them!
Thanks for knowledge guys, I think it was a combination of what BusyLittleShop described and low pressure for sure. I'm 8psi low! Yikes!
[image: View Post]Monk, on 14 June 2012 - 12:54 PM, said:
Are your tyres cupping?
Cupping is a natural wear pattern on motorcycle tires and it will
always follow the tread pattern. It is not a sign that you have bad
suspension parts or out of balance condition. It merely shows that
your tire is indeed gripping the road when you make turns (thank you
for that Mr. Tire!). This cupping develops within the side wear bands
of a leaned motorcycle. The leading edge of the tread does not flex
much as it grips the road and the rubber is scuffed off the tire in
that area causing a depression. As the tire rotates, the pressure
moves to the trailing edge of the tread pattern where the tread flexes
more causing less scuffing so less material is ground off the tire.
The more complex the tread pattern, the more complex the cupping
pattern will be. The softer the compound of the tire, the sooner this
cupping will develop.