The chain on my SS mt bike keeps coming off the rear cog under load (like when I hit a steep hill) it will hold right after it's tightened but after a few hills it starts coming off non-stop. Talked to the LBS they so far are just telling me to tighten the tensioner, I was thinking i might need a bigger rear cog for more "chain wrap" any advice.
Most likely you do not need a larger cog. If you need more tension and you're out of drop-out space, you probably need a shorter chain. You may need to remove a link or use a half-link. Otherwise, David's suggestions are all recommended. Check your chain-line, chain & cog wear and make sure your wheel is tightened adequately.
If your front and rear cogs are inline, the most likely reason that it keeps popping off is that the chain is not tight enough. For me, simply pulling the axle as far back as it will go and tightening the nuts is not enough. I don't use a chain tensioner, I use the wheel as a lever to tension the chain.
How I Tighten My Chain
- Seat the chain.
- Looking forward push right side of the hub axle as far back as it will go such that the right side of your tire can is hitting your frame.
- Tighten the right side hub nut.
- Lay your bike down, peddle side down.
- Place your left foot gently on the the frame near the bottom bracket.
- With your left hand, grab the rear tire by the bottom bracket next to your foot.
- Pull up (perpendicular to the plane of the wheel) gently on your tire. This should have the effect of using the tire rim as a lever arm to cam your rear cog farther backward, fully tensioning the chain.
- Pull up enough so the tire is centered and no longer touching the right side of the bike.
- Using that wrench you are holding in you right hand, tighten the left side axle nut.
NOTE: Be careful you shouldn't have to pull on the rim to hard. If you need to you can always go back and adjust the right axle nut forward a little.