Reason for the switch:It would explain his rather strange bike switch almost immediately after the organisers announced an "unannounced" bike test after the stage.
http://velonews.competitor.com/2011...aving-white-flag-contador’s-bike-switch_175062
Alberto Contador once again stopped to switch bikes ahead of a decisive summit finish. With about 50km to go, Contador slowed down, rolled behind the Saxo Bank lead car and quickly swapped frames. Saxo Bank mechanic Faustino Muñoz told VeloNews the switch was for a “slightly lighter” frame for the final climb up Zoncolan. Contador rode a 34×32 up the Zoncolan, which allowed him to keep a high cadence on the ramps as steep as 22 percent.
Contador also switched bikes in a similar situation on the Mount Etna. When asked by VeloNews why Contador switched bikes, Riis said it was for “safety reasons,” saying that Contador rode wider tires with slightly less pressure to make for a smoother, more secure ride over bumpy cobblestones through the Sicilian villages and roads made slick by volcanic ash.
All tested bikes weighted what they should:
http://velonews.competitor.com/2013...d-for-bike-weight-checks-at-alpe-dhuez_295850
Contador made two bikes swaps during the stage, one prior to the first ascent up l’Alpe d’Huez, and one on the flat road before the second accent. The bike checks were announced between the two swaps, leading to speculation that Contador was forced to back off a long-range attack to swap back onto a UCI-legal bike for the finish climb.
That doesn’t appear to be the case. UCI technological coordinator Mottet confimed with VeloNews that both of Contador’s bikes were checked following the stage, and both came in at exactly 6.8kg.