Bicycle Mechanics - 8 Speed Chain Coming Off

My friend just bought a new bike from Performance Bike. It's a Zum GT, whatever that is. It's a campus type commuter hybrid type thingy. I dunno, lol. It has an 8 speed cassette in the rear and a single chainring in the front. I've adjusted the rear derailer for him and it shifts around just fine.

The problem is that when in the big cog in the back, the chain will try (and sometimes succeed) to jump off of the chainring to the inside and land on the BB housing. It looks to me like it's simply a matter of the chain not being flexible enough. But since there's nothing I can adjust in the front, and I can't seem to stop it from jumping off, I'm at a loss for what to do.

I've taken a few pictures to help (sorry for the resolution, it's just my old phone).

http://i422.photobucket.com/albums/pp305/kchinger/bike%20stuff/0901082031.jpg

http://i422.photobucket.com/albums/pp305/kchinger/bike%20stuff/0901082032.jpg

http://i422.photobucket.com/albums/pp305/kchinger/bike%20stuff/0901082032a.jpg

http://i422.photobucket.com/albums/pp305/kchinger/bike%20stuff/0901082033.jpg

http://i422.photobucket.com/albums/pp305/kchinger/bike%20stuff/0901082033a.jpg

http://i422.photobucket.com/albums/pp305/kchinger/bike%20stuff/0901082033b.jpg

Any ideas from the wise mechanics?

If the chainring is ramped (to facilitate shifting) you could drop a couple bucks on a single-speed style chainring with nice tall teeth to keep it in place.

Alternatively, use a chain minder http://biketoolsetc.com/index.cgi?id=850284316154&c=Repair-Parts&sc=Chain&tc=Chain-Catcher

None of those pics are particularly useful. Ones that would be is middle cog/front chainring. There's nothing you can really do about the problem anyways, except put a chain keeper on.

That's what I was thinking. I couldn't think of any adjustments that would help. Would a more flexible chain help? Or should he just get a chain keeper?

I don't know if it matters or not, but your rear derailleur cage is stretched pretty far forward in that big cog (low gear) in the back. Normally you would want your derailleur cage to be a bit more relaxed in that gear. I think adding 1 or 2 links to the chain (if you have some that match), or trying a new chain wouldn't hurt.

He doesn't have any links for it, but I was kinda thinking the same thing; that's why I put that picture in there. I was also thinking that a more flexible chain might help him. I'll be at the LBS today for my bike, maybe I'll pick him up a new chain and a chain keeper (just in case) while I'm there. What type of chain does an 8 speed with no front derailer use (I think the one that's on there is original equipment and it has some stiff links)?

Well, I had to take my bike to the LBS today, so I let me roommate tag along and bring his bike. We swung by Performance (where he bought it) where they looked at his bike before I took mine to another LBS.

The mechanic there said that the BB had been recalled on those bikes because it was too wide, which caused the chain to not be able to reach the big cog without slipping. He had fixed all of his bikes before selling them, but the other Performance Bike that my roommate got his from apparently hadn't. He has an older Shimano BB laying around that is the right size (110), so he's gonna put that on for free and have it back to us in a couple days.

Thanks for your help.

Good to know :)

wow.... a chainkeeper would work,,,, i would think..... thats really interesting that they would outright use the wrong bb-----

Even with the right BB, you still end up with bending the chain sideways when sweeping across the rear. Effectively, the inner and outer cogs act as a derailleur and pulls the chain sideways off the chainring. The solution really to use a chain-guide of some sort, like a front-derailleur. There are also chainring-plates that prevent the chain from coming off sideways. A STIFF chain actually lessens this effect.