> "They just do it......." can't really say it better, but an engineer
> will always try, won't they?
>
OK!
> Here is a shortened version of the enginerd explanation.
>
> Every time the tire goes around, the pavement slaps it. This
> rhythmic slapping creates a "standing wave" near the contact patch on
> the pavement. The radius of the "cup" might be related to the radius
> of 'bend' created as the tire tread bends from "round" to "flat" and
> returns to "round" again every time its slapped. The tire probably
> scrubs pretty hard on the pavement as it's being bent form round to
> flat.
>
Ah yes, the rolling without slipping of a semi-riged object problem. Here's
a real mind blowing concept for people: The velocity of the contact patch
is zero. This is called the instantaneous zero velocity point. If the tire
contacts the ground at the contact patch, and the tire isn't slipping or
skidding on the ground, then the relative velocity between the contact patch
and the ground is zero. Therefore since the ground has zero velocity, so
must the contact patch. This means that the contact patch must decelerate
from its curvilinear and rotational velocity it had to zero when it contacts
the ground, then accelerate again. I know, you say, impossible, but its
true. I have a photograph of a spoked wheel that is rolling without
slipping. The spokes of the wheel's region that is in contact with the
ground are in clear focus, and the spokes elsewhere are blurred. This is
what causes the "rhythmic slapping" that creates a frequency of oscillation,
or standing wave.
The way to minimize cupping is to have your tire dynamically balanced. Any
extra mass on the perimeter of the wheel/tire creates larger forces in the
tire's material due to the need for greater deceleration/acceleration
forces. Also, proper tire inflation helps a lot. More tire inflation, more
rigid tire, less cupping. However, this also means harsher ride and
squaring off the tire in the middle.
> You can try higher tire pressure or longer wheelies. Best to try to
> forget about it or the enginerds won't give you any peace.
>
Hey! I resemble that!
Jeff