Converting Schrader Rims For Presta Valves - If you use a Presta-valve tube in a rim drilled for Schrader valves, you'll notice that the hole is too big. This isn't a problem unless you ride with low air pressures as some off-road cyclists like to. In that case, the Presta valve may creep as the tube shifts inside the tire. This can lead to a bent or broken valve over time. To prevent this miscue, install rim grommets, O-ring-like rubber washers that fit in the valve hole reducing its diameter to match the Presta's.