Cable guide

Cable guides below the bottom bracket can be cheaper, just a piece of moulded plastic, and, for some bikes with very small chainrings, eliminate interference between the rear derailleur cable and the bottom of the front derailleur cage.

They also make for a cleaner appearance and easier to clean frame in the bottom bracket area.

Poor lubrication of bottom-bracket cable guides is a common cause of auto-shifting.

Despite the advantages this routing is almost exclusively found on older bikes.

Some bicycles use a cable guide on one side of the seat cluster for a rear cantilever brake cable, rather than use a short length of housing between two housing stops.

Black plastic cable guide on the bottom of a Trek 5000 bottom bracket shell
Metal, braze-on cable guide on the top of a bottom bracket shell