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Old 07-19-04, 07:58 PM
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slvoid
2-Cyl, 1/2 HP @ 90 RPM
 
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Join Date: Oct 2003
Location: NYC
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Bikes: 04' Specialized Hardrock Sport, 03' Giant OCR2 (SOLD!), 04' Litespeed Firenze, 04' Giant OCR Touring, 07' Specialized Langster Comp

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There are 3 types that I've seen separated into 2 categories. Active and passive.
Active ones are fluid filled where the fluid pools around the puncture to seal it.
Passive ones include using kevlar strands to resist punctures or just plain using a really really thick tube. I have the latter on the rear wheel of my commuter and I can definitely say the tube is about the same thickness as my road bike's TIRE and weighs about the same amount.
I've never really believed in kevlar (which BTW is in the aramid family of synthetic materials) in resisting punctures. If you've ever stabbed your lover with a sharp ice pick while she was wearing a bullet proof 20 layer kevlar vest in the heat of passion, you'd realize that the shank either slices through the kevlar or pushes the weave aside and gets through. When a bullet hits a vest, the impact converts kinetic energy into heat and deforming the vest. The slug that you pull out of your vest looks like a glob. It works because the glob has huge surface area in which to dissipate impact energy. The term should actually be bullet-resistant, not proof, as kevlar just helps resist penetration of an object. If you were shot with a .50 in the chest, you will probably get knocked down and have a huge bruise if not broken ribs. Armor piercing bullets have a central core that mains integrity as the metal jacket deforms around it, thus allowing it to slice right through the kevlar and penetrate.
I just can't see how the weave can be dense enough in a bike tire to prevent something from puncturing it if it were sharp and hard enough to make it through the rubber thread of the tire, then again I'm no expert YMMV. Plus if it were a piece of glass working it's way into it, several hundred revolutions of compression as the piece hits the ground should be enough to eventually slice it's way through the tire and the tube.
I think first defense inc. or something like that has a really really dense kevlar vest that resists shankings.
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