Fly,

There are more than just those two companies making them - like
www.taabs-int.com, www.imiproducts.com and some reference to
http://www.tirelife.com/
An article that discusses that is at
http://www.landlinemag.com/Archives/...tires_life.htm

There are several independent reviews of the devices and all indicate
they work. There is an interesting demo machine that Centramatic.com
distributies to ilustrate the balancing effect on a demo wheel. A demo
video is available at http://www.centramatic.com/Demo/video2.mpg

As far as I see it does not violate conservation of angular momentum.
A free body in space will always be in angular balance about its CM
whereas that is not the same case for rigid bodies rotating about a
fixed axis - maybe that is the key: there extra bodies free to move.

As to why they are not used more... probably the extra cost is
prohibitive, to say an automobile where a $20 tire balance will do. But
for big trucks it can pay off.

I am not promoting the devices, but just have an open mind. Its so easy
to put down others and not be creative oneself.