fujifinest

So I bought a 1991 lugged Trek 750 black/white/yellow. After cleaning it up and setting the derailleurs, I had this problem in the small cog where it kept skipping. The bike had replacement Shimano brake shifters model ST-EF65-7R/L with Suntour XCE rear and Suntour front derailleur. I used the Park youtube video to refresh my memory, but in the end I still had the problem. I brought it to my local mechanic and he said the Shimano and Suntour indexing were not compatible. He made some adjustments and it seemed to work reasonable. Today I did the complete wax job in preparation of putting the bike on Craigslist (I have 2 Trek 750s, I'm keeping the 1996 Ice Blue one), then I decided to make sure the recent tune was good. Unfortunately it's exactly like when I brought it to the mechanic. Under load it skips in the small cog. Since my mechanic is off on Monday, I went ahead and took the Shimano STX front and rear derailleur off from my other Trek 750 with idea of using them to make an all Shimano compatible system. I found out the tubing on the 1996 & 1991 Trek 750s are not the same size, so the front clamp for the Shimano STX from the 1996 Trek 750 was too big. So I put the Suntour front derailleur from the 1991 back on. After several hours, I could not get it to work correctly and was wondering if the Shimano STX rear was the problem? Does anyone have some insight about what to do next? Thanks, Nelson

hokiefyd 

Your mechanic is correct in saying the Suntour cable pull is not the same as Shimano, so Shimano shift levers will probably not index well with a Suntour XCE derailleur. However, the STX rear derailleur should be 100% compatible with the Shimano levers you have. Are the shifts between each of the other sprockets clean and smooth? Is the rear derailleur's high limit screw adjusted so that the derailleur can move outboard far enough so that the chain doesn't drag on the second sprocket? It may simply be a problem of wear on the first (smallest) sprocket. If it has a relatively new chain and old cassette, then that's likely your issue.

DorkDisk

There is another factor not mentioned: cog spacing.

From what I know, cable pull is not the main issue with Suntour 7sp, but cog spacing. Suntour used a mixed 2.0/1.8mm spacing, where the small cogs used the wider spacing. Shimano is 2.0mm throughout.

One solution is to run a cheap friction thumb shifter.

FWIW, I have a bike with all Shimano (including freehub and cogs) but with indexed Suntour thumb shifters. Works great. This might be due to the floating upper pulley that only Shimano used at that time.

headwind15

DorkDisk is right about Suntour/ Shimano sprocket spacing. . Something else not mentioned is that your new Shimano EZ fire shifters are V brake compatible. / will work really poorly if paired with your original Dia Compe cantilevers. The brakes will have little bite to them. I would highly recommend that you upgrade to V brakes.