Shimano FC-E700 crank.

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:

• 75.5 mm
• 76.0 mm
• 78.5 mm
• 74.0 mm
• 77.0 mm

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:

• Customers cannot install different chainring sizes: the only chainrings that fit are those few designed for this crank.
• When the chainring wears out, the customer is forced to buy this specific — and probably hard-to-find and expensive — chainring, or to buy a new crank.
• As a chainring wears, cyclists commonly rotate it, as most wear happens at 90° and 270° to the crank; rotating the ring occasionally can double its service life. This chainring is so close to round that it will work well in any orientation, but the non-round bolt pattern used here means this chainring works in only one orientation, thus halving its useful service life.
• If a chainring is damaged (e.g., an accident or vandalism), it is unlikely a nearby store will stock the odd chainring, so then only prompt repair is to replace the crank and chainring.

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-Übersicht: FC", 2007. From http://www.fa-technik.adfc.de/Hersteller/Shimano/FC.html as of 2009/05.