Jammed Chain - Scratched Bike -- WHY?
 

Hi guys,

I was riding my new to me 2004 Giant Boulder. I was happily pedaling up a hill and shifted from the big chain ring to the middle chain ring. I pedaled some more and all of a sudden my pedals locked up. I looked down and the chain was wedged behind my front gears and the frame (chainstay?) I got off and saw that the rear derailer tensioner (I think that's what it's called) was also pulled almost horizontal.

I loosened all of this and saw my frame was scratched in numerous spots down to the bare steel! :(

Anyway, I rode a few more pedal strokes and it happened again, scratching it more! I got off fixed it and turned toward home. It did not happen again, I shifted a ton just trying to make it happen and nothing.

First, what happened? And second what can I do to make my frame not rust in those spots? Do I need to worry about that? This will be my commuter bike and will see a lot of use.

A picture is attached illustrating this.

Looks like your downshift dropped the chain off the granny and got it jammed between the BB and said chainring. Either your derailleur adjustment is off or it just happened. Some bikes are more prone to this than others because of the number of gears/frame geometry/chainline.

http://www.parktool.com/repair/readhowto.asp?id=75

F.D Adjustment.

You can also install a jump stop which prevents the chain from dropping off into that area again.

http://www.gvtc.com/~ngear/graphics/mount2.jpg

This part is about $10.

What kind of paint would you use to touch it up so that it doesn't rust? Or, do I not have to worry about it?

Acrylic? Enamel? The kind you use on plastic models?

ooh a jump stop ... me likey

Yep, that would do fine. Or pretty much any other brushable option that is a close color match.

If you're riding off road a lot you may want to cover the chain stay on that side by wrapping it with something. A couple or three layers of electrical tape is one option or there's my own favourite which is to slit an old inner tube lengthways into a strip about an inch wide and wrap the stay overlapping so there's two layers other than the at the edges.

I just had this happen on a bike I was selling. The buyer took it down the street for a quick test ride and came back pushing it. I knew I'd adjusted it properly, but I noticed the chain was on the smallest cog in the back.
I asked if he'd shifted it up that far in the small ring. (had to explain "ring") Sure enough....Just enough cross-chaining with a shiny new chain to derail the thing.
A slightly bent tooth on the small ring will do the same.....

I use an old cut up bike tire. It is 100% noiseless that way.

aren't the Giant Boulders aluminum? if so you probably don't even need anything to prevent rust.

A "jump stop" won't help your problem at all. It keeps the chain from falling off of the top of the small ring.

What you had happen is called chain suck and happens when the chain does not release from the bottom of the chainrings. Then it is pulled up between the rings and the chainstay. Take a look at this link to get a better understanding of the possible causes.

Agreed. I would get it fixed ASAP as it could lead to accidents and injuries at higher speeds.

Make sure your dérailleur is tight, particularly if it is a braze on. Then set your limit screws.
I like nailpolish. Lots of colors for matches, just put the brush back in the bottle, nothing to clean!
You can use model enamel if that is what you have, it works fine.
Technically, aluminium frames won't "rust" per se, but do oxidize. Not a big deal, but much prettier if you cover the scratches!

I've used fingernail polish before, comes in enough colors to get a real close color match and seems to hold up well. Holiday seasons brings out even more color options, and Holloween brings out the orange and black colors and shades.

Works for fishing lures too. ;)

Nope. Classic example of chain suck. The chain sticks to a tooth on the cog and gets sucked up into the chainstay and is jammed between the stay and the wheel. Overshifting onto the granny - or beyond - would result in spinning the cranks without engaging the chain. This would not stop the rear wheel from turning.

Chain suck will stop the wheel dead in its tracks. Derailer adjustment won't fix this problem. There is probably a gear tooth on the chain wheel that has a burr on it or is bent. Inspect them and see if there are any that look scratched or abraded. Some of the teeth may look shorter than others but that is normal for modern chainwheels. You are looking for a tooth that looks like it's been chewed up or that is twisted out of line with the others.

for small touch up jobs, i go into the automotive section at walmart or something and go look at the automotive touch up paint. they have a wide variety and you should find a rather close match. and on the one i got, it was a 2 in 1, where i could use the tiny brush it came with or a ball point end for very detailed touch up. this will work in especially tight clearance places. as for wjat is wrong with your bike, i think everyone else has nailed it 4 u, so no need to post my version of the same. just try to fix the problem b4 you touch up the paint or you'll likly end up having it to do again. besides, if you ever get a scratch in the future, you'll have the small bottle of touch up paint to get rid of that eye sore.