cable housing observations
 

I've noticed most on here post pictures with the housings swooping high over the top bar for brakes, but when it comes to the shifter housings the rear derailleur is WAY too short, if you want your bike to shift well with little interference you may want to get a little more loop. Proper sizing would be to insert your end into the ferrule on the chainstay and loop it around to the derailleur without it wanting to pull on the side of the ferrule, should sit straight in derailleur without kinking to the side. Just thought I would mention for any that don't know.

I think that's true for indexed shifting, but shorter runs seem to be fine for older derailleurs with a fixed angle and no indexing.

I thought it was the other way 'round -- that the loop should be tighter for modern indexed, because more loop equals more possibility for flexing of the housing, and you don't want flexing for indexing. On the other hand, "back in the day", when housing was often un-lined, and shifters on "good" bikes were usually on the downtube, a bigger loop made for smoother shifting, and since that loop was the only housing between the shifter and the derailleur, a couple more inches were no big deal.

That's the way I remember it, anyway. Seems like I remember reading it on Sheldon. I dunno.

friction is friction, think about it if you have a cable in a housing in a straight line it moves in and out easily, now put a tight bend in it and see what happens when you move it. A bigger loop better for both, though not as important on friction, though as cables get nearer time to replacement you could have some binding when moving the friction lever with a small loop.

I've thought about that, and I just go by sight.

If it looks too small and binding, I cut a longer housing.
If it looks too big and loopy, I trim it.

I'm a simple person, and rely on instinct.
Sometimes, I'm right.

Seems like pulling cable: not that big an issue, but releasing tension: would want as little interference as possible.

In homage to your OP, I just lengthened the housing on my Kestrel.
It may not be better, but I feel better about it.

Thanks.

The problem is not friction, the housing is too short for any appreciable friction. Modern housing is constant length and compression-less, so the length is supposed to stay constant even as it bends. In reality, it does change a bit as the derailleur pivots, meaning that the derailleur movement may be slightly different for the same amount of cable pull. Lengthening this housing minimizes the change in length as the derailleur pivots. Now, for vintage derailleurs this is not a big deal, because they are non-indexing, and older ones have a fixed angle anyway. In fact they typically use stainless steel coil or brake housing. Here is a typical NR setup:

http://www.speedbicycles.ch/bikes/237/titel.jpg