Okay, my memory of this is pretty vague. A guy encounters a funny little man that gives his bicycle the ability to travel to other worlds. He does this by adding an extra gear to the man's 12(?) speed. When the bike was travelling at a certain speed he would be able to shift into this extra gear and the bike would be teleported to a sort of subspace corridor. The corridor was endless and had shapes all along the walls that were gateways to other worlds. The protagonist called some of them "Bart Simpson heads", because the outline of them was similar. He occasionally encountered other beings there, riding odd vehicles.
The books (I read two, there may have been more) were about the strange adventures he had visiting some of the worlds. I seem to remember one of the books being called, "The 13th Gear", but Google had nothing for me and neither did any of the sci-fi book databases I tried.