Bicycle Mechanics - Cold Case. Chain falls off outside of big ring!

Cold Case - Chain falls off largest ring
Hi. I don’t have this bike anymore. I don’t know how appropriate it is posting an old problem here but I virtually lost a bike over it. It is a 20+ year old mystery that I would like an answer to and I thought this forum might help me out. I had a mountain bike that the chain used to inexplicably fall off the outside of the big chain ring. I took it to three different bike shops and they could not solve my problem. I replaced the chain after the second shop of the bike shop visits and this did not solve the problem. I presume that because the problem occurred while I was peddling, not on or just after a gear change, that the de-railers were properly aligned. The bike had bio-pace chain-rings (yes, it was that long ago). Only one bike shop offered any solution, which was possible bent teeth on the chain-ring. It used to get a stiff link after it came off the cog that no amount of ‘wiggling’ the links could fix properly; either that or I did not have the ‘guts’ to wiggle the stiff link any further. Eventually, the rear de-railer broke a pin (maybe known as a pivot) that joined it together after it ‘slapped’ itself to death from the stiff links going though it. I was a bit younger then and did not know much about bikes, so I traded the bike in on a new one. In my current knowledge and personality I may have tried to change the big-ring (large chain ring that the chain kept falling off on the outside) on it to see if that helped, however, like I said, I did not know much about bikes and no-one offered that solution to me. Like a car with bad mechanics (and you’re not a mechanical person) I got rid of it because it was turning into a ‘money bucket’ of problems.
What do you think caused the chain to fall off the big ring?
As a post script: I was in the Navy at the time and personalities are such in that organisation that I would not rule out sabotage – maybe that will help with an answer.
If you’re young you probably want to know what Bio-pace chain-rings are: they were elliptical chain-rings brought out by Shimano that were designed to smooth-out your peddling stroke. For some reason they did not survive as an idea and Shimano reverted back to round chain-rings. As an addition to the above I used to ride with someone who had Bio-pace chain-rings and the chain did whip to a small degree because of them.

Hard to tell without seein' the bike. But in my experience, the problem always stems from one or more of the following causes: outer limit screw too far out, derailleur too far from chainrings, derailleur not aligned properly to chainrings, bent chain, or bent chainring.

I have had it happen to 2 bikes that were correctly set up.
I just learn to pedal slowly when going up to the big cog,

An old mystery, so less applicable to most modern derailers, but I run into a lot of mechanics that don't have all of the needed info on dialing in front derailers. Older mechanics usually know this, but the outer cage plate's foreward tip is an over-shift prevention device that should always be bent inward until it just clears the chain in the highest gear. This provides the fastest correction to the chain's path if it starts to go beyond the teeth of the big ring, while being less likely to actually rub the chain as it is shifted to the smallest rear cog (because of it's more-foreward location from the rest of the outer cage plate).

Note that many of today's derailers have no foreward-extending tab to adjust, but their design provides chain control with a different geometry.

And as for the original bike's adjustments, one must set the cage height gap at between 1-3mm for the cage to fully control the chain, something that may have been confusing to the mechanic due to the non-round chainrings.

In the case of BioPace chainrings, which I have raced on almost exclusively in virtually every racing discipline, they don't exhibit shifting problems but in theory may be more sensitive to chainline as the cage gap is not controlled during a good portion of rotation.
Not that a mountain bike or any bike with a triple would likely have too short of a spindle sufficient to cause over-throw in itself.

I blame the cage tab adjustment.

Like a car with bad mechanics (and you’re not a mechanical person) I got rid of it because it was turning into a ‘money bucket’ of problems.

What do you think caused the chain to fall off the big ring?

If you’re young you probably want to know what Bio-pace chain-rings are: they were elliptical chain-rings brought out by Shimano that were designed to smooth-out your peddling stroke. For some reason they did not survive as an idea and Shimano reverted back to round chain-rings. As an addition to the above I used to ride with someone who had Bio-pace chain-rings and the chain did whip to a small degree because of them.I really liked BioPace rings back then. I rotated them 144-degrees away from the standard location so they worked better. This balanced out the varying torque of the pedaling motion. Shimano's orientation actually exaggerated the imbalance.

I can tell you from working in a shop for 10-years that most shop-mechanics are not degree mechanical-engineers. They can't run a lathe/mill or program CNC, much less able to braze/weld worth anything. They're trained within a system without certifications and accredited programs. As such, the quality of the repairs you get will vary widely. As dddd mentioned, there are my tricks of the trade that may or may not have been passed onto the particular person working on your bike.

Anyway, take a look a this Park Tool - front derailleur adjustment (http://www.parktool.com/blog/repair-help/front-derailleur-adjustments) site. The order of operations is vitally important and when you bring a bike to a shop, very few "mechanics" will start from the beginning. The very first thing they'll try in your case is cable-tension and outer limit-screw; too little too late. The actual first step is ensuring optimum mechanical positioning of the FD:

1. height above rings - about 1-2mm works best
http://www.parktool.com/uploads/images/blog/repair_help/ft_der_2.jpg
http://www.parktool.com/uploads/images/blog/repair_help/ft_der_2.jpg

2. cage rotation - slightly outwards to match chain in tallest gear, many modern ones do this with a stepped outer-cage
http://www.parktool.com/uploads/images/blog/repair_help/ft_der_8.jpg
http://www.parktool.com/uploads/images/blog/repair_help/ft_der_8.jpg

Only after these two adjustments are made, can you then effectively use cable-tension and outer limit-screw to fine-tune the last 10%. In your case, tightening the outer limit-screw by a tiny bit to prevent the FD from moving outwards too far may have prevented the chain from flying off the outside.

In order----height of FD above large chain ring---alignment front to rear of the cage---high adj limit screw.