Why are there two different rear brake cable arrangements? My Schwinn Voyageur has two cable stops at each end of the top tube for the rear brake cable. The cable housing terminates at these stops with the naked cable running between them. This is a clean, neat arrangement with no apparent disadvantages.
On the other hand, my Bianchi has three small “tunnels” that are brazed to the top tube. The rear cable complete with housing runs right through these tunnels and terminates at the caliper and at the brake lever. The problem with this tunnel method is corrosion. Sweat, water or whatever becomes trapped between the cable housing and top tube causing rust in fairly short order. The simple cable stop method on my Voyageur has no such corrosion problems.
So, why do some bikes have the seemingly inferior tunnel method of routing the rear brake cable?