Let the air out of your tires, then pinch the sidewalls until they touch. That will open the cuts and cracks so that you can flick out the glass shards and rock chips with an ice pick or the tip of a razor knife or a small screwdriver. Worthwhile to do it every 1000 miles or so. Takes awhile to work your way around the tire, so probably best done watching soccer or hockey or the like on TV. Vacuum or sweep the floor when done.
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Longbikes Slipstream