MOBI > Shimano Biopace Bolt Pattern

Shimano FC-E700 crank.

[ic1886.jpg]   [ic1888.jpg]   [ic1889.jpg]  

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.

Copycats — Campagnolo

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.

Campagnolo offers Ultra-Torque™ cranks where the aluminum version has a 110 mm BCD and the carbon-fiber version has a 110 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.