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Being in the tire business, I can concur that "most" nails and objects are in rear tires. I also agree with Dan, that a mud flap would keep the debris to the road more, but don't see the logic in how it would stop the rear tire from still running it over.
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IMO the fender extension only keeps the bike a little less dirty. I've got at least 500k under my belt on 2 wheels and I've only had 1 rear flat fortunately and it was due to a faulty valve stem. Now getting a flat due to a drunk driver is a different story.