1985 Schwinn 12 speed Shimano 105 rear derailleur?
 

I recently bought a 1985 Schwinn Traveler. Its a 12 speed, 6 rear cogs, 14T-40T. I would like to replace the existing Suntour rear derailleur with a Shimano 105. can I do it? Will it fit? What should I know? I would like to do this myself rather than LBS but feel bad asking them for their knowledge and then buying on ebay.

Yes you can, if the 105 is a long cage model. You will need to use it in friction shifting. The Sun Tour is actually a better derailuer unless it is broken. I doubt the rear is 14 to 40. More likely 14 to 34. Roger

+1

If the freewheel is truly 14-40T a 105 RD isn't going to shift it. Only a few RDs will probably be able to.

The Shimano road derailleurs (like 105) have a published limit on the largest sprocket- 28 teeth. In reality, you can probably go to 30 teeth without problems. 34 would be a stretch- the guide pulley will hit the sprocket teeth, and it may not even shift to a cog that big. I'd get a mountain derailleur for 32-34 tooth cogs. And I wouldn't necessarily spend the cash to go to the Deore level or above unless I were planning a 9-speed upgrade in its future. Alivio or Acera would be fine. 40 would be impossible even for mountain derailleurs.

How wide is the chain on that bike? Thinking about clearance in the RD cage here.

Thanks. The largest cog is 40T, my mistake.

Paul

Damn. 30T.

OK, a 30T cog MAY work with a 105 rear derailleur. Also, does your frame have a built in deraileur hangar or is it a separate "claw mount"?

You can probably use either a long cage or a short cage rear derailleur with your double crankset and 14 - 30 cog set.