I've got an ongoing problem that I'm hoping someone could enlighten me about. I'm a X-country rider on very gently sloping and smooth DG fans in Arizona, nothing gonzo about the terrain other than the cacti. Recently I've been using Bontrager thorn-resistant Presta tubes (that's what my shop carries) and for the 4th time in as many rides the entire Presta valve assembly promptly rips itself right out of the tube on my rear wheel, resulting in an instantaneous ride-ending flat (bummer). Since it seemed that maybe both the tire and tube are 'moving' on the rim, I've marked the tire's position on the rear wheel before rides and confirmed that in fact the tire repositions itself about 3/4" forward on the wheel relative to its starting position. I'll assume this means that friction between the tube and tire is dragging the tube along with it, resulting in the Presta valve separating from the tube; seems logical anyway. What I can't understand is why this problem seems to occur only with the Bontrager tubes and not (yet) with several other brands I've used over the years. I always seat the valve stem fully vertically with the silver locknut cinched up firmly against the rim hole, and ride with ~ 45 psi (I weigh 190 lbs). This morning I tried using yet another brand-new Bontrager thorn-resistant Presta tube in a brand-new Hutchinson Python tire, but before mounting it on the rim I coated the tube and inside of the tire with talcum powder to try and reduce friction. Result: yet another ripped out Presta valve after 20 minutes of riding. I've talked with the guys at my shop and they suggested it might have been a 'bad batch' of tubes so they replaced them out with some from a newer batch, but they're still a problem for me. These tubes get expensive after awhile with this problem, so you can see I'm anxious to find a solution. Can anyone out there pass along some ideas? Thanks much.