The gently curving county highways of Palomar Mountain are ever rising, ever descending, and almost never flat. Bicycle riders who come here fill their lungs again and again with cool draughts of fresh air on the ascents, and get a nice break for their efforts on the exhilarating descents, which range from carefree coasting to cautious braking. For a relatively easy ride, try covering the 7-mile, one-way stretch between Palomar Mountain State Park and Palomar Observatory -- or double the distance by riding the round trip. The real Palomar challenge, though, is a 27-mile loop route involving a 3000-foot ascent of South Grade Road, followed by a descent on East Grade Road, with Highway 76 as the connection between the two. This direction on the loop takes advantage of a short, steep ascent on the first part, and a much longer descent with little need for brake work after nearly all the work is done. Be sure to ride as early in the morning as possible to take advantage of both the freshest air and the lessened auto traffic.