As above, there were lots of attempts at index shifting, some dating back to the turn of the century. Shimano’s SIS combined several enhancements that made it successful:
- The Uniglide sprockets and chain that sped up shifts and helped prevent hanging on tops of cogs.
- The “Centeron” upper derailleur pulley that allowed slight misalignment between derailleur and sprocket.
- Special cables and cable housing that for more consistent coordination between the shift lever and derailleur.
- Shimano’s tighter specs for dropout and derailleur hanger alignment.
It wasn’t one big thing… just a bunch of little things that Shimano insisted on that created their steamroller in the ‘80’s.