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I've watched the chain derail from the front sprocket, and there is considerable flex that is causing it. I'm a big guy with an athletic build (6'7"/280lbs), and it's a big-box bike, so I'm not expecting a miracle. Are there sprockets made for people my size that won't flex under load, and if so are they reasonably priced?
UPDATE
The flex that I have seen is actually in the chainring (front sprocket)
I don't like to be the bearer of bad news, but I've been down this road once or twice. As a hobby, I sometimes pull bike frames out of dumpsters and rebuild them to sell for my cost on Craigslist. I have learned over the years that bicycles, when new, really cost at least $300-400. "Bicycle-Shaped Objects" (BSO's) are sold at stores like Wal-Dart and Cost-Mo for much, much less. These BSO's are, though, just that. They are a loss for everyone except the manufacturer and the store. And I doubt the stores are making much on them.
If I find a box-store BSO and seek to rebuild it, I will usually remove and discard that whole front assembly--chainrings, cranks, bottom bracket. They're usually toast anyway. WHen I get to this point, I am about to put into the bike probably near what it was worth new at Cost-Mo. The bottom bracket (bearings between the cranks) will probably run $30 (UN54 baby!). The cranks I can often get decent ones used for $20-30. Chainrings another $5-20 a piece, depending on what I am trying to build.
None of these things is really expensive, but it adds up.
WHY AM I TELLING YOU all of this?
Why don't I just say, "get some used quality parts for $5-20?" Well. If you're putting that much torque on this thing to bend the chain rings, I wouldn't be surprised if the bottom bracket were screwed up, too.
Sometimes you can get decent used parts at a pretty low price, but I think it is unlikely that your crank-arms will accept replacement chainrings (sprokets). Usually on box-store bikes, these are riveted-on. They are disposable. That means replacing the cranks, too. So. You're going the whole monte.
So figure, cheapest-possible solution, we're looking at about $100. What did the bike originally cost? Maybe $150? If that's worth it to you, go do it. It's not all that hard to do yourself.
In my experience, though, when you find one thing wrong... then you find another... and another... pretty soon you have $250 invested in a bike that's worth about $150.
If the bike is "Your Bike" and you have sentimental attachment to it, I understand (I own that bike and continue pouring parts into it). Otherwise, I'd say look for a quality used bike. A mid-90's Specialized Rockhopper will run you about $150 and will come with good quality parts that can be more easily worked on.
