Egg on my face! I've had it in my head that radials for 20" tube type rims were a thing of the past. Not so. Like you say, Yokohama still shows them in their on-line catalog.
http://www.yokohamatire.com/tires_commercial/truckBusRefGuide.pdf. Probably the others do too.
This raises the issue about whether radials should really be run on old bias rims. While new rims of all kinds are rated for radials, old rims obviously were not because radials didn't exist yet. This arguement generally focuses on car and light truck wheels that are more prone to flex than are big truck rims. You hear stories of rims fracturing or having hub cabs come off under the loads radials place on rims. Probably not a problem on big trucks, but it might not hurt to ask your tire guy next time you go in. Stu
