I've never seen 20" tires on 1-piece rims. Are you sure they're not the Firestone "widow makers" with about a 2" wide steel band near the center of the wheel just outside of the center plate? It would be virtually impossible to get a 20" tube type tire on a single piece rim.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!