Hey Doc your the man,
i was just about to post the same thing. I run 4 strips of inner tube under across the rim ( or should i say thread it across when the tyre is on before i bead it. As the dirt bikes use Tubes your not so worried about the seal, but stopping the rim spining and taking out the valve.
Used my spare wheel with soft tyre on TM last year, Learned that lesson to do it to them all, race one in Tassie Nats, ripped the valve right out but it stayed in place and went down by the last lap. Only Rim without the tapes and only one to spin out a valve in over 10 years.
I use 4 inch strips now but cut them off so they dont flap about and have never had a rim spin, as the rim moves it tries to roll up the rubber stip in the bead and locks in, i have heard of some of the speedway guys go one better and use spray adhesive.
Another one i have tried and works is to assemble with hair spray, when it dry's out it goes tacky and sticks forming a adhesive seal between the rim and the bead, seen that work on a 500 single on a flat track runing 6psi, so i know it works for sure.
