Shimano Biopace-SG chainset question.

Folks,

After 50 years of riding nothing but 3-speed Sturmey-Archers,
I have just acquired my first MTB, a Schwinn Sierra MOS with a
Shimano Biopace-SG chainset (ca.1990 I'm told).

I tried a search for Biopace-SG but can't come up with anything.

I want to know what the `SG' designation means. Can anyone say?

Bill

www.nyx.net/~wboas

"Do what you can, with what you have, where you are." T. Roosevelt