Cable routing and why is there no optimum solution
What are the reasons of the other cabling schemes?


Most Cyclocross bikes will route all the rear cables along the top of the top tube. In a cyclocross race, you will get off and shoulder your bike, the cables would get in the way on the underside of the top tube. Many CX bikes have a flat bottom top tube as well for easier shouldering. Routing the cables along the top keep them cleaner too, cyclocross races get muddy and if they put the cables on the bottom of the downtube, they would get very wet, muddy and allow grime into the housing.