Wheel balance

[Frank Clarici <spritenut@comcast.net>]

Why or how do wheels go out of balance?
Today I took a few hour tour in my Sprite and the wheels need to be 
balanced. This was not the case last Fall when I took an 8 hour tour.
I have driven a bit over the winter and at speed most of the time, once 
or twice I thought it was just flat spots from sitting for a week or 2 
at a time.
Today the wheel shake seemed to come and go and it was anywhere from 55 
mph to 75 mph but not constant.
Something else going bad maybe?
-- 
Frank Clarici
Toms River, NJ