Originally Posted by
FBinNY
Variable cable pull is easy to imagine in theory, but not so easy in practice. The cable winds onto a drum, so all that's necessary is to change the drum diameter, according to the desired cable travel. this is far easier than trying to vary the click spacing. OTOH, when you look at it, varying the drum diameter isn't all that simple in practice, though it could probably be designed to be so.
According to the
wikibook, to get all the basic Shimano and Campy RDs to work, having the ability to have a base barrel with nested shims that alter the diameter for each RD type would look something like this:
[TABLE]
[TR]
[TD="align: left"]Cyl. Thickness[/TD]
[TD="align: left"]Nested Dia.[/TD]
[TD="align: left"]15° Travel[/TD]
[TD="align: left"]1:1.7 Travel[/TD]
[/TR]
[TR]
[TD="align: right"]10.34[/TD]
[TD="align: right"]10.34[/TD]
[TD="align: right"]1.35[/TD]
[TD="align: right"]2.30[/TD]
[/TR]
[TR]
[TD="align: right"]0.45[/TD]
[TD="align: right"]11.24[/TD]
[TD="align: right"]1.47[/TD]
[TD="align: right"]2.50[/TD]
[/TR]
[TR]
[TD="align: right"]0.68[/TD]
[TD="align: right"]12.59[/TD]
[TD="align: right"]1.65[/TD]
[TD="align: right"]2.80[/TD]
[/TR]
[TR]
[TD="align: right"]0.23[/TD]
[TD="align: right"]13.04[/TD]
[TD="align: right"]1.71[/TD]
[TD="align: right"]2.90[/TD]
[/TR]
[TR]
[TD="align: right"]0.91[/TD]
[TD="align: right"]14.85[/TD]
[TD="align: right"]1.94[/TD]
[TD="align: right"]3.30[/TD]
[/TR]
[TR]
[TD="align: right"]0.21[/TD]
[TD="align: right"]15.26[/TD]
[TD="align: right"]2.00[/TD]
[TD="align: right"]3.40[/TD]
[/TR]
[TR]
[TD="align: right"]0.25[/TD]
[TD="align: right"]15.75[/TD]
[TD="align: right"]2.06[/TD]
[TD="align: right"]3.50[/TD]
[/TR]
[/TABLE]
Now, I only threw the core, 2nd, 3rd and last rings into the image, but the rest are similar. Since they are nested and very infrequently changed, the shims do not have to have any significant strength themselves which should make manufacture easy.
Manufacturers can easily mass produce pistons, rings and valves that work at vastly higher temperatures, pressures and duty cycles than anything on a bike. I truly fail to see why this is difficult, other than protecting profit associated with brand image and expense.