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Comment by jessaustin | Hacker News Reader

...leading to a 6.13 W/kg. [Sky] also claim that their chain rings hamper the measurement of power by 6%, and give 5.78 W/kg to account for this.

I can't think of any set of facts that would make this adjustment make sense. Can any mechanical engineers help?

I doubt Shimano will be happy if Jean-Louis Talo starts claiming that it was his weird elliptical chain ring that turned a 5.78 W/kg loser into a 6.13 W/kg champion.

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Presumably the chainring losses are roughly similar for every rider which is why no-one complains about them. Around 6% seems plausible when you remember that for cars crank horsepower is usually 15% higher than wheel horsepower. Transmission losses are an unavoidable cost.

TFA with its 6.06 W/kg estimate is talking about power at the wheel, because it's using Strava rather than e.g. PowerTap data. (Although Strava claim to account for "rolling resistance".) By making the adjustment Sky seem to be claiming that Froome's equipment wastes less power than everyone else's does. Again, if they spell that out, Talo will be happier than Shimano, their nominal sponsor.

Crank and pedal based power meters usually dont give accurate power readings when used with oval chainrings.

Quick googling suggests Team Sky still use Stages as a power meter, and Stages say on their website: "Through our own testing, and using a hub-based meter as a control, we conclude that there will be a 4-5% increase on the readings from a Stages Power meter when used with a non-round chain rings." [1]

[1] http://support.stagescycling.com/support/solutions/articles/...

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