Originally Posted by
cxwrench
Since you believe so much in the 'table', use it and figure out how much stretch happens. Don't forget to accurately measure how much force it takes to move a derailleur.
Okay. Using Loos & Co table of constants and the formula, a 1.2mm cable has an elastic stretch of 0.17mm with a 2 pound pull and 0.8mm with a 10 pound pull. I don’t believe that derailer pull is as high as 10 lb but I think 2 lb is a bit low. Elastic stretch of almost a millimeter would probably be too much for the precision required for index shifting.
Loos & Co also says in that link
Structural Stretch is the lengthening of the lay in the construction of cable and wire rope as the individual wires adjust under load. Structural Stretch in Loos & Co., Inc. products is less than 1% of the total cable length. This form of stretch can be completely removed by applying a cable or wire rope prestretching operation prior to shipment.
1% of, let’s say, a meter of 1.2mm cable would be 10 mm. I’m not saying that cables on bicycles stretch 10mm and most modern bicycle cables will claim that they are “prestretched”.
Bethlehem Elevator Rope actually gives some information on construction stretch for elevator cables. Unstretched cables have construction stretch from 4” to 6” per 100 feet for a 6 strand rope (close to the same construction of a bicycle cable but much larger). Prestretched cables have construction stretch of 1.5 to 3” per 100 feet. A quick calculation shows that is a high of 0.5% for the latter and 0.25% for the latter.
Applying that to a meter of 1.2mm derailer cable give 2.5mm for a prestretched cable and 5mm for unstretched cable. Taking up 2 to 2.5 mm of slack out of a new cable isn’t out of the ordinary.
The TL;DR is that both elastic and construction stretch are well known, well documented, and, in the case of elastic stretch, easily calculated.