Join Date: Jun 2008
Location: Spring Hill, TN
Rep Power: 1
Hey Lou... glad it worked ! I dug around in my archives a bit and looky what I found! An email from our own Rob Tharalson! He responded after I was absolutely convinced a fork change was the cause and only threw in the tail bag addition as an afterthought.
I'll cut and paste his explanation.
"Ancient advice: if it hurts when you do that, don't do that!
Headshake, tankslapper, a "massive moment", an excursion into
puckerville: whatever you want to call it, it is simply the bike
trying to find a balance point of all the forces fed into it when, in
this case, decelerating. A huge number of variables are created by
what you did, not the least of which is a slight unbalanced force
applied to the bars when letting go, initiating a lean whether you
meant to or not. Add to this the natural tendency of the bike to
fall over. Regardless of the suspension setup, the front tire will
try to straighten itself from this force and invariably overshoot due
to its mass and inertia. As soon as the wheel has turned itself too
far in the other direction, the exact opposite happens and an
oscillation starts acting on the CG. If the motorcycle is pivoted on
the CG with the tires off the ground and spun up, this oscillation
would center and the bike would wobble in roll (horizontal axis fore
& aft) and yaw (vertical axis)about the CG, but since the tires are
more or less fixed to the ground, the forces create a second center
coincident with the ground which transfers motion to the mass above
the CG. Normally, this will damp out unless the forces are too high
in which case the amplitude of the front wheel motion will increase
with each cycle until something limits it such as a steadying hand,
the fork stops, or falling over. The greater the mass above and the
further away from the CG, the more effect they have and the more
likely the oscillation will increase instead of damping out. I think
by now you will have figured out where all this is going -- the mass
of the trunk being mounted very high and aft adds a considerable
force to the picture, increasing the likelihood of a tankslapper. I
know there will be a bunch of replies stating "I never had anything
like this happen even when carrying the family anvil collection in
the trunk." Well, good on you! The point is all that mass way back
there will have some effect on whether or not the oscillation damps
out or deposits you in the rhubarb.
Take the trunk off and see what happens: I'm willing to bet it won't
misbehave. It's up to you where to go from there."
Good advice always has a way of coming back around...