531 SL, like other 531 sets, probably came with a few different choices of chainstay shapes - I don't remember. But I saw several SL bikes that broke, which all had a particular chainstay shape that I associated with 531 SL in my mind. These stays had a sort of flattened area for the tire clearance, making the cross-section close to a "D" there, instead of the more normal indent like Columbus used at the time, or the perfectly acceptable Reynolds round-oval-round or rapid taper styles. The ones I saw that broke numbered two or three, not a valid sample size but it made me suspicious, and they all broke where the flattened area transitioned to the round - the shape changed too abruptly and the crease was too sharp, in my opinion. Also being the weaker, non-heat treated steel made it harder to get away with less than perfect shaping of the tubes -- 753 in that shape might not have broken.
Did any one else here notice this chainstay shape, and did it live a long, fruitful life or did it die young?
To the poster who worried (back in the day) that 753 might be too stiff - it was in fact a bit less stiff than 531 SL. They did make some variety of wall thicknesses in 753, but even the heaviest of the commonly available 753 sets was lighter and less stiff than 531 SL. (Not counting tandem and off-road 753)
Tubing specs for 531 SL and 753 as of 1978 can bee seen in these catalog
scans:
http://bulgier.net/
or http://tinyurl.com/
Note the thicknesses are given as gauge, where a larger number means a thinner wall. Sorry the text is so hard to read; I should have put them up at a higher resolution. In case you can't read it: All three choices shown for 753 are thinner than 531 SL in places, some tubes being the same thickness in both, and no tubes being thicker in 753.
Mark Bulgier
Seattle WA USA