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Or a RD that's been damaged by contact in the Porte crash.
It's possible - he gets a glancing blow from DM's feet on the drive side. That said the problem he has is quite a few KM's down the road, and as the NS mechanic fiddles with it it limply slides down in to the smallest cog. What luck - damage to the mech that only kicks in six km down the road, and then the mech is still aligned, but just 'dead'.
Really think mechanics are forgetting to charge Di2 in the biggest race in the world?
I'm not suggesting that. Could be a duff battery caused by some production issue. Froome had a similar issue it seemed, he had time for a bike change.
Two speed, stuck in smallest cog is classic battery flat symptom