I just had another one of those unrepairable torn valve stem flats, about 10 miles from home. Luckily, I was about 0.5 miles from a payphone. Anyway, I've owned two mountain bikes in my life, an old school Mongoose hardtail, and now recently I upgraded to a Schwinn FS Delta_88. It is definitely the case that V-brakes can rip a tire and tube right off a rim. My old cantilever brakes had no such stopping power. The other factor I have noticed, primarily because my Mongoose is so old, is that a tire slip on a wider single walled rim is easier to forgive than a tire slip on a narrower double walled rim. In the past I would notice that the valve stem was crooked, deflate the tire, slip it back, and reinflate. No flat. On the double walled rims, the valve stem remains upright, while the tire and tube slip laterally, causing a rupture at the base of the schaeder valve stem. I have been scolded by those who say I am running with too low a tire pressure, but even having the tires hard as a rock, it still happens. Riding with high pressure, riding with talc powder on the tube, riding with roughed up rim walls, these are all bandaid solutions to the real problem, which is a rim architecture that is too unforgiving. I have opted for what I call the "relaxed fit" shraeder installation. For those of you who routinely drop from height, you may not want to try this on your nice rims. I took a 3/8th's inch drill bit, and reamed out the shraeder hole. I followed that with a 1/2 inch bit on the inside (tube side) wall only. I finished the mod by removing the burrs with a rattail file. The next time the tube slips, I should get a warning "lean" from the valve stem, which may buy me enough time to unslip the tube before it ruptures again. I run with thornproof tubes and vinyl strips, and do not see the utility of being forced to buy tubes by the dozen because of an unforgiving rim.