Trakhak

That's a clever idea. In 1974, if I remember correctly, entry-level Gitane 10-speed bikes shipped with a black plastic ring with a sloped outer surface that was installed between the chainrings. It was clearly intended to eliminate the possibility of the chain dropping into that space between the rings. Unfortunately, it instead ensured that that would happen, so we shop mechanics had to remove the black plastic ring and replace it with thin spacers.

Bill Kapaun

Your "solution" is far more trouble than just getting the correctly spaced crank.
IF you are going to "machine the spider" etc and then add spacers, why not just machine it to proper 11 speed width and have it function correctly?
You don't need the spacers. Sandwiching more "plates" together just gives it a better opportunity to loosen.

Trakhak

You might try drilling the large chainring and tapping the drilled holes for button-head bolts. I'd drill the holes where chain guides are installed on modern rings. You could experiment with different thicknesses of spacers under the button heads. Some combination of button head thickness and spacer thickness might allow smooth shifts without chain drop. Or you might not need spacers at all.

If it doesn't work out, no harm done. The drilled chainring would still be usable.

noglider 

I like this. Thanks.

HillRider

After perusing all of these suggestions, it still seems the real solution is buy a proper crank.

Bill Kapaun

Some people want to make a forest out of a tree.

sch

Wish I had thought of that button head bolt idea back in the day, a lot easier than thinning the crank spider or making and
mounting small ramps!

The comment about the Gitane shipping with a plastic attachment in the cranks in 1974 reminds me of current
compact/subcompact Shimano cranks with their black 'fillins' on the inside of the large chainwheel which may
facilitate downshifting, never thought about why they were there.

noglider 

Sure, but it's only a good idea if it works. No one has said here they've tried it with success. I'm still considering it. It would be less destructive than thinning the crank, so maybe I should try it first.

gesta

I experienced similar thing when I switched from 8spd to 10spd and insisted using existing crankset/chainrings.

I'm using Campagnolo 10spd front mech and ultegra chain with Stronglight 105 crankset. In theory, the measured spacing between chainrings should be narrow enough for chain not to fit in between, and front shifting does work when there's no real load on drivetrain from pedals. In reality, apparently chainrings (48-38) have enough flex to allow chain to be forced to the gap between rings on real-life conditions, like when actually riding the bike.

Unwilling to change or modify crankset, I decided modifying the large chainring was the best approach.

What I did with JB weld(ish substance), I added thickness to large ring and tried to imitate the sloping inner profile of these new 11&12spd chainrings. When the extra layer was hardened, I trimmed the chainring down to even thickness with 180mm grinding wheel attached to drill press.

It has been working like a charm for about 10 months for now. Only hiccup has been when first time around I did not properly clean the chainring (after light sanding to have better adhesion) and my JB welding chipped off from about 1/6 length of chainring circumference. This happened during very hard front shift on 200km brevet, but surprisingly it did not cause any noticeable problems on shifting performance.

If I recall correctly, tip-to-tip teeth distance on new 11/12spd chainrings is quite similar to 5/6/7/8spd era chainrings (because chains were wider back then) and larger than in 9 or 10spd, because it helps the drivetrain to manage more extreme chain angles these modern drivetrains have. Chain internal width has been the same since forever.

noglider 

Thanks for all of the suggestions. Changing the crank did the trick. I got a 10-speed Campagnolo on ebay. I changed it today and took it for a quick test ride. I think the problem is solved.

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