The lower exit of the S tube should be on the axis of rotation of the rudder pedals so that when the rudder is used the distance between the exit and the forward attachment point stays the same. If not the cable slides slight to and fro through the S tubes giving the prospect of wear especially if there is no plastic sleeve insert extending out of the tube - worse still if the ends of the tube are sharp and not bell-ended. Schempp-Hirth gliders have this right as the exit is on the rotation axis outboard of the axle. Most other makes have the lower part of the S tube welded to the vertical of the pedal some close and some not so close to the axis. At least one design has the tube exit projecting forward from the pedal so that not only does the cable move in the S tube it is also angled slightly on full rudder deflection.