I’m no expert on asymmetrical chainrings but I couldn’t let this pass:
The idea behind the chain rings is that the varying diameter alters the effective gear ratio during a pedal revolution, on the downstroke it producing a higher gear than on the upstroke, in effect eliminating that dreaded dead spot.
On a crankset with cranks set opposite each other (i.e. every crankset that I’m aware of) by definition the downstroke on one leg occurs at the same time as the upstroke of the other.
The Osymetric chainring clearly has 180º symmetry. From the eighth photo it seems pretty clear that Froome is pushing his biggest effective gear when his pedals are at approximately 3 and 9 o’clock, and the smallest effective gear when the pedals are at approximately 6 and 12 o’clock. Therefore the explanation should read something along the lines of:
…on the downstroke it producing a higher gear than on the downstroke midstroke, in effect eliminating that dreaded dead spot.
I tried the Osymetric chainrings for a few months and wrote this about them: http://cyclingtips.com.au/2012/09/osymetric-gimmick-or-miracle/
In short, I thought they were awesome for things like TT’s or long sustained efforts. Not for rides that required sprinting or sudden accelerations. The front shifting was horrible…
It’s possible that the purpose of the custom derailleur hanger is to position the derailleur lower than it would be on a stock hanger. It would allow the rear derailleur to clear bigger cogs for upcoming stages. If Osymetric chainrings aren’t available in compact then he’d have to use bigger cassettes to get lower gear inches. There’s only so much you can do with the B screw.
I have no doubt that there have been millions of dollars invested in research for the Asymetric chainrings, but I do have to wonder why they look like something I came up with in my shed one night…
Froome’s Pinarello Dogma with wavy stays and the Osymetric chainring… a bike Salvador Dali would happily ride.