I've logged 70,000 km on my 640 and never had the bead slip to cause a flat..... But I always run 26-30 psi in both tires and have gotten use to the way the bike handles in varied terrain with that pressure..... Also I leave the nut off the valve stem and instead slip a small length of rubber hose down the stem to the rim so if it does move a bit it wont rip the stem out....
If you plan on running in the low 20s psi I would recommend running rimlocks for sure.....
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