Blake Sessions didn't really even ride bikes much until he moved to Boston for college. The 20-year-old Los Altos resident is a budding entrepreneur and inventor, though, and saw an opportunity. Now, he's launching a business catering to a niche market of bicycle riders, selling customized sprockets for fixed-gear bikes. Sessions is a Gunn High School graduate and a junior at the Massachusetts Institute of Technology studying mechanical engineering and physics. Last spring, he wanted to make his bike look more interesting so he carved a design — symbols of seasons, like a leaf and a snowflake — into the metal sprocket, or chainring. He noticed other people seemed to like it.