Chain jumps off large chainring under high torque

[cameronpack]

Just finished building up a road bike on a dawes giro 500 frame. My first time doing anything like this. Ok, nothing special but I did it. My question is this....
Why is my chain jumping off when in the large to small combination? It happened 3 times on my 22 mile ride on Sunday. All three times were on the level or going downhill (only time I could get onto that combination!) and I was giving it big strokes. A lot of torque. There is a little "jump" or "clunk" every four strokes or so which kinda points to a stiff link. But I had a quick look last night and couldn't see one. The right crank arm runs very close to the chain in this gear conbination. I was wondering if there was that much torque that the crank arm / frame was distorting slightly and the crank arm was then catching the chain on the way up, popping it off? Next chance I get I will try watching this but as I am going very fast I don't want to take my eyes off the road too long.
It's a 57cm Dawes Giro 500 frame with campag mirage 53/39 crankset, 9 speed cassette and a 113mm bb (increased to this to try and reduce the small to small chainrub problem against the large chainring).
Any ideas?
Cameron

 

[cameronpack]

Sorry, just thinking out loud regarding the chainring rub problem. Would a triple crankset reduce / resolve this?

Still wondering about the chain jumping off though.

Cheers for any help you guys can offer.

Cam

 

[MacB]

How does the chainline look? is the middle of the cassette about the middle of the two chainrings? Which way is the chain jumping, ie overshift and off or coming back down to the inner? Have you checked the chainring bolts are tight?

 

[amaferanga]

Bent chainring or front mech not set up right would be my guess.

 

[cameronpack]

Cheers gents,
Chain jumps off to outside. It shifts fine and there is no rubbing on the cage so I doubt it's a front mech problem. It only happens in large front to small rear and even then, only under severe pressure. The chainring is not bent either. The teeth are not worn. I'll double check the chainring bolts all round tonight when I have a thourough look at the chain for stiff links. The chainline looks ok. I have read a lot about the rubbing problem occuring. To be honest, if I can't fix this I am not bothered, as most literature and people more experienced than me say "never run in that combination anyway as it puts more stress on the chain". I can obtain the same sort of drive using the large chainring and a larger sprocket.
Thanks again.
Cam

 

[amaferanga]

Cheers gents,
Chain jumps off to outside. It shifts fine and there is no rubbing on the cage so I doubt it's a front mech problem. It only happens in large front to small rear and even then, only under severe pressure. The chainring is not bent either. The teeth are not worn. I'll double check the chainring bolts all round tonight when I have a thourough look at the chain for stiff links. The chainline looks ok. I have read a lot about the rubbing problem occuring. To be honest, if I can't fix this I am not bothered, as most literature and people more experienced than me say "never run in that combination anyway as it puts more stress on the chain". I can obtain the same sort of drive using the large chainring and a larger sprocket.
Thanks again.
Cam

Can you clarify what chainring/sprocket combination you're having problems with cos I'm confused now? I thought you said it was big chainring - smallest sprocket in which case that's you're biggest gear and no other combination will replicate it.


Is the chain jumping off the cassette or the chainset?


Its big chainring - big sprocket that should be avoided, not big-small.

 

[cameronpack]

Chain jumping problem - The chain jumps off to the outside off the large chainring when under stress (on the small sprocket).

Chainrub problem - to be honest not fussed about as I can replicate the small chainring to small sprocket with the large chainring and a larger sprocket. Just need to remember and avoid small to small (and big to big) when riding.

Cheers

Cameron

 

[HLaB]

When I changed my cassette to a 11-25 from a 12-25, my bad shifting (too quick) would sometimes derail my chain from the large chainring when standing up. I've micro adjusted (tightened) my H limit screw and have tried to think more about my shifting and the problem has never reoccurred. Your problem could be entirely different however.

 

[4F]

Sounds like you need a deda dog fand, see the review here from ColinJ

http://www.slimmerandfitter.com/cycling/deda-dog-fang-chain-keeper-review/

 

[cameronpack]

Thanks for your replies,
I took the whole assembly apart and had a close look last night. I then re-assembled and went on turbo trainer at full resistance. Chainring doesn't look warped and teeth don't look worn at all. The chainline didn't look brilliant though. Under high tension the chain looked like it wanted to jump off to the outside because of the angle it was trying to make to get to the small rear sprocket. The further away from the BB I can get the big chainring the better this situation will get.
Also, the right crank arm does touch the chain ever so slightly and the outside of the front derailleur. I checked an offset from the left crank to a point on the chainstay tube and then the same on the right hand side. The right crank is closer to the tube than the left one. This would suggest to me that the right crank arm is not sitting at the correct angle and needs replaced?
I think I may just go down the line of replacing the cranks and chainrings to see if that helps, unless anyone has any other ideas? I wondered about spacing out the BB from the BB housing 3 or 4mm to see if that would help. Has anyone done this?
Thanks again.
Cameron

 

[Herzog]

Assuming it's square taper, get a bottom bracket with a longer spindle?

 

[cameronpack]

Cheers Herzog, I already did that from 110mm to 113mm and it didn't seem to help that much at all. How long would you go?
Regards
Cameron

 

[Herzog]

You can get a 127mm BB (Shimano UN54, pretty cheap and reliable)

http://www.chainreactioncycles.com/Models.aspx?ModelID=17473

 

[MacB]

If it is as I'm envisaging them it needs to be a longer BB, and a fair bit longer by the sound of it. Try running a straight edge from the chainrings to the cassette and tell us which cog each one is most closely aligned to.

The only other thing I could think of is if the cassette is somehow on too far, lacking a spacer on the inside maybe.

 

[cameronpack]

Ah MacB, you may have something there. I'll check that tonight when I get home. Thank you thank you.
Kind regards
Cameron