Review on Wheels Manufacturing Presta Valve Saver: Protecting Your Bike's Valves for Longevity by Matt Jenkins

These little things are needed when replacing Schrader valve tubes with Presta valves. I'm not sure why Trek still put these types of discs/cams on their mid-range bikes, but it's annoying. I keep them in my son's seat pocket for when I need to replace the tube which all the ones I buy already come with. They're easy to fit and I would also suggest placing them under the rim tape to ensure they stay in place and don't cause the presta valve to pull out of the tube. They sell and feel high quality. They are terribly expensive for what they are which is the reason for the 4 star rating. They're easily at 1/2 the price.