Just tried to put a Biopace II chainring on a Sugino XD500 crankarm.
To my surprise, it's impossible to set up the chainring on the crankarm so that the pin that usually goes behing the crankarm is correctly placed.
It turns out the spider for the Biopace chainring is designed to have the crankarm positioned between two chainring bolts (see pic below)
The Sugino XD500 spider is designed so that the crankarm is positioned in front of one of the 5 chainring bolts (pic below of the way I mounted it). Notice that one of the 5 crankbolts is behind the crankarm, and the chain retention pin is 180º away from where it should be.
I can't get the pin behind the crankarm because the spider on the XD500 is different from the one on the Deore Biopace II crankset.
I guess that the most correct way of mounting the Biopace II chainring on this crankset is to bolt it flipped 180º from the way is was designed (pin standing 180º away from the crankarm, instead of directly behind it).
Am I right?. Will I still reap the benefits of Biopace this way? (I'm guessing that the chainring can be flipped 180º because it is symmetrical)