See also also bike pic for other bike-related technical discussions and more about poor designs and failed parts.

The Shimano FC-E700 crank has a slightly-out-of-round chainring on what looks to be a standard 144mm bolt circle diameter (BCD). However, unbolting the chainring and rotating it shows the holes are not in a circle:

[X]   [X]   [X]  

The bolt hole radii, measured clockwise from the arm, as viewed from the bicycle side are approximately:

The irregularity is small, but is enough the ring cannot be attached in another orientation. The maker's reason for this is unclear, but it is customer-hostile in that no other chainring fits, and it is not possible to rotate or flip the chainring to distribute wear.

(Wear tends to be highest at about 90 degrees and 270 degrees from the arm, and the backsides of the teeth are not worn appreciably in use. Thus, rotating and flipping are commonly used to extend a chainring's service life. Chainrings in front derailleur use are less often rotated due to ramps and pins used as shifting aids and due to a pin between chainring and arm that prevents chain jamming when it overshifts and falls off the outside. Chainrings in front derailleur service are rarely flipped due to recessed chainring bolt holes, and due to offset teeth such that flipping the chainring slightly changes the lateral position of the teeth.)

The bolt pattern is so close to round it provides no important difference in stiffness or strength compared to a standard round pattern.

The chainring is not round, but is close enough to round it is not obvious it would be problematic to rotate or flip it.

The nonstandard bolt pattern means the customer cannot fit a different size of chainring. Doing so defeats the chain guard feature (a smaller chainring cannot be fitted and a larger ring will overhang the guard). However, it is common for riders to change chainring diameters to fit their terrain and fitness.

The nonstandard bolt pattern also means when the chainring wears, the customer must replace the whole crank, as replacement chainrings are not available. It is unclear if spares were ever available, but even if they were, they were never common. lists the crank manufacture date as 1991.

The same design was also offered in a 2-chainring model. From as of 2008/09/15:

[X]   [X]  

It is idententified as having 170mm cranks, 32/44 "Biopace" chainrings and weight 830g.

It is unclear whether the inner chainring bolt pttern is round.

Note the right crank uses two bolt circles and thus is a different arm than the single-chainring model pictured above.