MOBI > Shimano Biopace Bolt Pattern

Shimano FC-E700 crank.

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Probably from 1991 [AC07].

At first glance, the five chainring bolts appear to be in a 144mm circle. Closer inspection shows each bolt is at a different radius, roughly:

Presumably this has an engineering justification. The best one I made up is "it provides better support for non-round chainrings." It's a justification. It is not a very good one. I doubt there is a good one.

Any engineering benefit is so tiny as to be irrelevant, compared to how customer-hostile it is:

A 2-chainring model was also offered, using a second smaller bolt circle holding a 32-tooth chainring. It is unclear if the second pattern was round.

Campagnolo "compact" 110mm BCD cranks are offered in two varieties. As of 2009 the models with aluminum arms use standard chainrings, but the models with carbon fiber arms use a 110mm BCD for four of the bolt holes, but the fifth uses a 113mm BCD. Users report that many non-Capagnolo chainrings can have one hole filed to fit. This weakens the joint, but gives a wider range of chainring choices and a filed standard chainring seems to work well for most riders.

Copycats — Campagnolo

Campagnolo offers Ultra-Torque™ cranks where the aluminum version has a 110&nbdp;mm BCD and the carbon-fiber version has a 110&nbdp;mm BCD except one ring has a diameter roughly 1 mm larger radius.

Some people report good results filing the 5th bolt hole to an oval.

Related: Uneven Circles

Related is putting the bolts on a circle, but still at uneven locations.

References

[AC07] Allgemeiner Deutscher Fahrrad-Club, Fachausschuss Technik. "Shimano Digest-√úbersicht: FC", 2007. From http://www.fa-technik.adfc.de/Hersteller/Shimano/FC.html as of 2009/05.