dmusicant

Yes, of course, it's largely semantic, but you are absolutely correct. Of course, no tube is thorn proof, nor can be short of being made of steel, or perhaps kevlar, I don't know. Anyway, what I'm after is enough "resistance" to justify the additional weight and cost. I figure the additional weight is 22 ounces if I use two of these tubes over "normal" tubes. Also to consider is the fact that accelerating the bike with tubes that add 22 ounces requires more leg power than accelerating a bike with 22 ounces heavier frame. That's because you have to increase the angular momentum of the wheel, so there's an additional amount of force needed to get the bike to achieve a given additional speed. Anyway, I won't worry about that if I can reduce my flats considerably, and I figure this should. It's not something I can quantify. One thing I do know, those polyurethane inserts I use drastically reduced my flats frequency. I'd say by at least a factor of three. I'm using inserts that were designed for wider tires. They work anyway, and by using them I'm sure I have avoided some flats I would have gotten otherwise by virtue of objects penetrating somewhat up the sidewalls of the tires.

This is slightly off topic, however I'm wondering why bike tires need such frequent reinflation. What causes air to leak from the tubes? I use those Schrader valve caps, but I was told those are not used in order to keep air in, but to keep dirt and debris out.