OK, OK, OK. I think I've got this figured out. It has nothing to do with
the inertia. It has nothing to do with braking or accelerating. It has
everything to do with the taper fit at the knockoff and the wheel hub,
as Brian pointed out. Here's what's happening.
Let's start out assuming the knockoff is absolutely tight. This means
everything is moving at the same RPM, nothing is getting looser or
tighter, all is well. Now let's assume the knockoff is a little loose.
Since the fit between the inner piece (wheel hub) and the outer piece
(knockoff) is a taper, when it's loose, the taper joint will only be in
contact at one point. In this case, the wheel rides up and the contact
is at the top. At this point, at the top of the wheel hub, you have a
smaller diameter circle in contact with a larger diameter circle. In
order for the two circles to be turning at the same RPM, the outer one
would have to be traveling at a higher LINEAR surface speed. Since they
are in contact at one point, the outer piece is only traveling at the
same linear surface speed as the inner piece. Therefore, the outer piece
isn't turning at full RPM, and is therefore "precessing" backwards on
the end of the spindle, which tightens it up as you drive! At some point,
the taper fit becomes tight enough, the circles become concentric, and
effectively the same diameter, and no more tightening occurs.
If you want to see the "Mr. Science" demonstration of this, roll a tennis
ball around in a large bowl. The ball is traveling around the bowl one
way, but the ball is actually turning the opposite direction. Better yet,
tip the bowl towards you and rotate the bowl. The ball rolls over one
whole turn long before the bowl has. The outer piece travels backwards
with respect to the inner piece because the inner piece turns faster.
If the taper went the other way, as in having a nut that holds the wheel
on with the outer taper being the wheel, you would want it to tighten IN
the direction of rotation, because the smaller circle still turns faster
when loose.
Class dismissed.
|