After not riding a bike for (many, many) years, I recently purchased a Trek 750 off of CL. It seemed to be in good condition, but it turns out one of the shifters is broken.
They say "Grip Shift" on them (always a SRAM brand?), with one additionally labled "ST-F3" and the other "ST-F2". They connect to Suntour XCE derailers. It's the rear one that wouldn't shift, and taking it apart, looks like some of the plastic inside has broken off.
Thinking they'd be easy enough to replace, I headed to amazon, but am now confused as to what I need. Looking at SRAM Grip Shift sets, it appears some work with a SRAM 1:1 ratio rear derailer, and others for Shimano 2:1 ratio. Unfortunately, I couldn't find info on what I have, the Suntour XCE. Are those at a 2:1 ratio as well, and therefore I could use any shifters designed for the Shimano setup?
I guess the real question is can I use "modern" index shifters with the derailers I have. I'm just looking for a fairly inexpensive fix, as I'd rather not have to replace the derailers at this point.
I never learned to ride a bike. It is my deepest shame.
When Suntour existed, Sram made gripshift shifters specifically to work with Suntour derailleurs. I suspect the "ST-" denotes that yours are suntour specific shifters. I do not think Gripshift still makes these.
THe options for cable pull are not just 1:1 and 2:1... theoreticaly, any amount of cable pull can be used to create any amount of derailleur movement in different derailleurs if the derailleurs and shifters are not made to work together. Shimano compatible Gripsifts might be close to working with the Suntour derailleurs, but will not be perfect.
So your options are: find a set of used or NOS suntour compatible shifters - bar-top thumb levers, below-bar thumb levers (called X-Press, I think), or ST - compatible Gripshifts. Or get a set of friction levers (cheap Falcon or Sunrace or nicer Shimano bar-top levers with a friction option). Or change your derailleur.