“Stop the car!” I barked. My startled husband yanked our rental car to the side of the road. Lucky for me, he has a good sense of humor and was able to understand my urgency when I explained that I thought I had just seen a bicycle fly through the air. A quick reverse up the block and we found ourselves pulling into Valmont Bike Park, where BMX bikers were, indeed, leaping off jumps and soaring through the air. We walked into the park and settled on a stand of metal bleachers. From that vantage point, we could see a pack of helmeted BMX riders cued up at the top of a nearby hill, ready to test their will and technique on dirt hills made for leaping and jumping. I held my breath with each launch as riders careened down hills, jumped into the air, and landed on unimaginably steep …