How to cure: Chain Jumping from front crank (old Bottecchia/Nervar)
 

I wonder if someone can offer advice/help:

An old Bottecchia I'm using as a backup road bike has started to jump chain off the front small chain ring (in toward the BB)
this happens only when the chain is on the two largest gears in back.
It only happens when there is added torque on the drive train (when I am starting up, up off the saddle, or up a slight incline)

There is a new chain on this old steed, BUT... the chain was changed earlier this summer, and I two rides back to back days, 30+ miles one day,55+ miles the next day, without a problem.

the only adjustment since then is I shortened the cables so the gear levers line up with the downtube better.

can anyone offer any ideas? The gear seems to be okay, no broken teeth. Alignment... okay
Just baffled because I road this bike over a few good rides without a flaw... and now it will jump off at the slightest stress...

I can ride it in the middle gears at moderate speeds with out probs

I'm having the exact problem on a new yeti mountain bike. Chain is new and nto stretched. Teeth are fine. Biek probably has only 300 miles on it. Whats up? I really hate nine speed the stuff feels so brittle. Any suggestions?

well maybe someone can offer some opines

The Bott only has a 6-speed freewheel
It's almost impossible to recreate the situation on the workstand as it appears to be an extreme chain tension issue

Exactly, I can't get it to jump unless I stomp on the pedals I have even check to see if my crank was flexing?

are the chainrings true?

Sounds like a slightly out of true chain ring under tension, not on bike stand but under full force, the crank is flexing a little bit causing the chain to literally be shifted off to the bb side.Wow!
I would start by looking at the length of the bottom bracket spindal, is it too long?, then i would check the tightness of the crank arm bolts, and rotate the crank to check for true. Adjust your inner limit screw on the front derailler a little on the tight side as a stop or preventative measure, to prevent chain from coming off, and lastly, maybe the chain is too stretched or worn out , replacement might be necessary. Good luck!

King

Much obliged for the comprehensive assessment -- given the fact that you can't see the patient. Just last night I did note a very slight wobble on the chain ringas I was putting it away. The FD adjust is a nice temp trick that just might get me thru for a while. As noted in the OP, the chain is basically new, having a 100+ miles of over the road miles... The bike is a recent project that I acquired, and I did some maint, but there may yet be some BB attention needed. It's a 72 Bottecchia so I rekon the BB may be original.

it sound like a beautiful bike!! good luck!
you have some great old gems!

Here is a link to an article by Lennard Zinn on a gadget that is supposed to keep the chain from coming off. Does anyone have any experience with these. http://velonews.com/tech/report/articles/8254.0.html
When I talked to the mechanic at my LBS, he said, "a properly adjusted derailleur will not threw a chain". I quoted him the article, to which he responded that "this Zinn guy is in the minority". When I told him my bike threw a chain the day after he tuned it up, he implied it was operator error.
I don't have any plans on going back to this shop, but I am curious what your thoughts are on this gadget.

The problem does not seem to be a derailleur related issue. If I spin the crank, the chain passes thru the FD with room to spare...
it is possible that the crank twists leans under duress (torque) when I really step down on it, and the lean hits the FD and knocks it off... The chain does not jump when I am shifting... it just voluntarily jumps

I have not had chance to really work on it a few days, but I'm thinking it might be an out of line cog, or the crank is loose. Diagnosing is hard because it is hard to duplicate the real-time scenario on a repair stand