Two other considerations are the dropout "drop" dimension and the upper pulley tooth count.
The "drop" dimension is the distance from rder pivot/mounting bolt center to rear axle center. I only have SunTour data handy. In '78 SunTour offered 8 different road rear dropouts with 4 different drops, ranging from 27.5mm (racing) to 33mm (touring). The longer drop's extra 5.5mm clearance between the upper pulley and largest freewheel cog can make the difference between 26t/28t or 28t/30t.
By '83 the drop dimension range narrowed, with the Superbe drop at 27mm and every other dropout at 29mm. I don't know for certain why, but it could be related to improved/expanded derailleur/cage designs, or a tightening of dimensions driven by the burgeoning (OT) contemporary index-shifting components.
Standard der pulleys are 11t. SunTour offered aftermarket 9t pulleys to help sneak an extra couple of teeth in the rear (with the smaller pulley in the upper/guide position), as well as 13t pulleys to help take up a bit more chain slack. You could put the 9t on top and 13t below to shift a bigger cog while maintaining the same chain tension/wrap.
BTW, rders with angle adjustment can be coaxed to shift larger cogs by turning in the angle adjustment screw, which pivots the rder body angle back a few degrees. 10-15deg can make that shift to a slightly-too-big cog a little less balky and a little quieter.
Happy tweaking!
Paul Brodek Hillsdale, NJ
On Sat, 13 Nov 2004 14:26:04 -0500, "Stephen Barner" <steve@sburl.com>
wrote:
[snipped]
>
>This is a recurring theme, but I've seen NR derailleurs pushed to 42/52 x
>14/30. Some will claim this is not possible (I believe there was slack in
>the typically unused 42 x 14 combination), and others will claim that they
>have seen NRs pushed to even greater limits.
>
>Even so, I have set up many a bike with 28t cogs in the back.
>The trick is in getting the chainlength just right and the wheel in the
>right position fore and aft in the dropout.
Paul C. Brodek
Hillsdale, N.J. U.S.A.
E-mail: pcb@skyweb.net