Last wednesday I was walking, my (GX-35 friction drive mountain bike) into my workplace building. As I was pushing my bike I heard a clicking noise, and then my bike could no longer be pushed. I first checked my engine and friction drive but they were fine. The problem was actually with my front wheel. My front u-brake "return spring wire", came out of its mounting place. And then the left side rubber brake pad, lodged under the front rim, and then froze the front wheel. If the return lever would have come off, while I was driving 25 mph with cars behind me, it would have been VERY dangerous. I believe part of the problem, was that my left rubber brake pad (was installed to low), and that allowed it to get under the rim, and then freeze the front wheel. This u-brake malfunction was (very) serious. And u-brake inspections should be included in everyones motored bike safety inspections. Make sure your "return spring rod" is tightly mounted. Also make sure your brake pad, is installed high enough along the rim (so it can't get under the rim.) And also make sure all your brake components like pads, ex.ex. are tightly installed. I have learned that its very important to make these brake inspections, Chad.