I've seen it many times on the trail. Some guy gets a flat but can't figure out why, that is until he goes to air the tire back up only to find his valve stem missing. To add insult to injury a torn valve stem isn't an easy trail fix. (unless your tire has come off the bead, then it's pretty simple) Fortunately this is easily preventable by just checking your valve stems regularly for signs of cracking. If yours looks like the one in our picture it's time to get it to a tire shop pronto!