Consider this 86BCD Stronglight chainring.

I know the extra pair of holes aren't quite ready to accept chainring bolts, but are they at correct distances to be adapted for use with larger BCDs? If so, what might those two other BCDs be?

I just recently discovered these - https://www.sheldonbrown.com/cribsheet-bcd.html

You just print the sheet at 100% scale & then drop a Chainring or crank on to get the BCD

I used one of those to make my triplizer for SL 93 cranks. The middle set of holes is at 122 mm, so all you have to do is counterbore them for the flange of the nut to nestle down flush, to avoid snagging the chain.

Oh and on the crank, you have to remove the shelves on each of the 5 spider arms where the inner went when it was a double. I just hacksaw them off then file smooth.

Some of those extra holes were used to mount chsinring guards that were available from Stronglight.

Some manufactured triplizers require the same thing, but that has always struck me as an unnecessarily brutal modification. I have delicate sensibilities, it looks like.

Yes I'm familiar with them. Unfortunately, I didn't have that chainring in-hand, so couldn't make use of it or of my calipers...

The shelves can be seen as structural, in that absent them all you have is the chainring bolts; fine if they fit and are tight...
However there are places where crank parts have surprising little maerial (circled):

I just checked my chainrings. The middle set of holes does correspond with the 122mm BCD used by Stronglight 93 and 104 cranks (but you'd want to counterbore them for the chainring bolts. The outer set of holes does not correspond with either 130mm or 144mm BCDs. I do not have a 135mm BCD ring to compare. It does correspond with the Stronglight chain guard.

I don't see that the shelf provides any support for the type of load that the chainrings exert on the bolts. It serves to facilitate alignment.

One man's brutal is another man's fun, I guess. (Don't quote me out of context!) I like turning road cranks into track cranks. Are track cranks brutalized? Oh and BTW those shelves don't do anything, they are virtually unloaded in use. They are excess metal that should always be removed, if you're a weight-weenie! Though I'll allow them to stay if historical accuracy is the goal, on a vintage bike. Just know, they are decorative, like the serifs on a typeface. I like my cranks sans-serif.

This tandem left-side crank (for timing chain on a crossover) started life as a right-side road crank. I installed a steel thread insert to make it left hand thread for the pedal, and removed the shelf, which would have hit the chainstay.



Fun Fact, a lot of the sponginess you feel when you stomp on the front (captain's) pedals on a tandem is from the bottom tube being bowed by the chain tension. The only leverage the chain has to bow the tube is the offset out of the center plane of the bike (chainline measurement), so reducing that to the minimum is pure "bolt-on stiffness" with no weight penalty (or any other penalty). So all tandems should have the timing ring just skim the chainstay like this, and should have their inner shelf brutally sawed off. Oh and use the largest chainrings that'll fit, because chain tension (and flex) goes up as the ring gets smaller. I'm a big fan of "some" flex, but a lot of tandems have too much of that spongy feeling at the cap'n pedals.

Exactly. The fact that the TA design, which dates to the 1940s I think, is still used today, is evidence that chainrings do OK with no shelves there. If TA chainring bolts break, it's because they're too small and made of crap steel. Nothing to do with the lack of shelves.

128, maybe?

--Shannon

The outer holes are at a larger radius than 130mm BCD but smaller radius than 144mm BCD.
128mm in a Nervar BCD.
135mm is a Campagnolo/Miche BCD

Inner hole-to-hole measures ~ 50; Sheldon sayeth 50.5 = BCD 86, Stronglight 80, 99, etc.
Middle ~71, Sheldon 70.7 = 122, also Stronglight as above
Outer ~ 93, so BCD = 158 (smallest ring almost certainly >44).