swissycle

I have an '89 Giant Sierra that's fun to ride on gravel trails and is going to be my winter commuter. Looking at those horizontal drop-outs, I can't help but think about converting it to single speed. But maybe a chain tensioner would help?

PhilFo 

It’s only an OK candidate for single speed conversion due to your rear brake. I recently built up a 1987 Raleigh Edge Mtn. Trials as a “dingle speed” with the intention of using a 16/20 2 speed freewheel and a 26/30 crankset. The 4 tooth difference allowing me a quicker 30x16 for riding to and from the local trails, and the 26x20 for slow speed technical riding. When I got it all together, the chain was rubbing on the bottom of the rollercam right on the post. I didn’t want to but I had to install a chain tensioner. If you’re still ok with having a tensioner or derailleur, then go for it. You can measure how much movement you have available with your short horizontal dtopouts and you should be able to get a good chain tension from the right gear combo as well as using a half link. Again, if you intend to use a tensioner, none of that is necessary. Not using a tensioner may cause your chain to rub on the brake unless you use a large chainring and cog combo. It’s perfectly doable though, just make sure you do some planning.

swissycle

Thanks for the info. Yes, with the short dropouts and brake on the chain stays, I'm assuming a tensioner will make the conversion easier, regardless of what ring and cog size I go with. I'd like to use an existing ring, either 48 or 38. 48 would pair with a 20T cog; 38 with a 16T to give the gear ratio I'm after. One benefit to the 38 chain ring is that it's already in the middle position. Can the outer and inner rings simply be removed? And/or is it possible to move the outer ring to the middle position?

PhilFo 

You can move the outer to the inner, with the caveat that your chainring may hit the chainstay. This is most likely to happen unless you get a bottom bracket with a longer spindle. As long as you keep using a 3/32” chain for a multiple speed bike (same type of chain you have now) you’ll be fine if the chainline is off a few millimeters here and there. If you keep the 38, you can get a “pants guard” or “bash guard” which can replace the outer ring, just make surebyou get one for 110 BCD and get the proper size according to a few more teeth than the ring you are choosing to run.

EnzoRWD

assuming that's a HG cassette and not a multispeed freewheel, you need an Endless fibonacci spacer set and a DMR tensioner and you're good to go

JohnDThompson 

You can move the outer to the middle, since they share the same bolt-circle diameter. The inner ring uses a smaller bolt-circle diameter. You'll want to get shorter mounting bolts, though, since you're two rings sharing the same bolt-circle diameter to only one. From the middle position, the ring is unikely to come in contact with the chainstay.