Bicycling is for road bikes only.  Trail riding is strictly prohibited.   Bicycles can be rented at the Yosemite Store, and are single-gear, fat-tire-road-bike cheapos meant for the foot trails and specially built bicycle trails on the valley floor.  Although it is possible to get a good workout and a satisfying ride, bring your road bikes only.  Some people ride from the valley floor all the way to Tuolumne Meadows.  It is a VERY strenuous ride, about 40 miles long, and a 5,000 ft. gain in elevation.  Not for the out-of-shape duffer.  Also, the ride to Glacier Point is much the same, but the return trip is a point-it-down-hill-and-hang-on thrill-ride!