Today's stage up the Blockhaus will undoubtedly provide some huge fireworks and may prove to finally solidify the true contenders for the maglia rosa. Unlike most mountain top finishes, the riders will hit the slopes of the Blockhaus after only 36 mostly flat miles. Add in the fact that this stage follows the Giro's second rest day and you will have the scenario of a lot of racers who think they can win the stage.
This scenario is pretty uncommon in the grand tours. Back in 1991, the Tour de France rolled up to Alpe d'Huez with only flat miles in their legs instead of a couple of category 1 ascents such as the Galibier, Glandon or the Croix de Fer. What transpired at the bottom of the Alpe was an explosion as both the stage hunters and overall riders were swept up in a wave of aspirations. Two groups of riders with different goals, but riding side-by-side up one of the world's most storied climbs.
The GC riders don't like this type of situation. They have to ride harder and punchier(not steady) than they would like in the third week of a grand tour. The guys going for the stage win will launch attack after attack and they GC riders usually respond. This has changed a bit with power meters and riders knowing what they can and cannot do on the bike, but still there is a group mentality which pervades and unless the GC riders call a truce, they can get sucked up into the fray.
Guys like Levi Leipheimer aren't punchy riders and a stage like today could cause him more difficulty. Personally, I hope not, but Levi may have a tough decision to make whether to ride within himself or go with the moves. The podium is on the line for the Team Astana rider which makes the decision even more difficult.
The final ascent is a 17km(11-mile) 1200m(4000') climb that averages about 7%. This should take the top riders about 45-minutes. Look for racers like Menchov, Sastre and Basso to be forcing the pace with Di Luca hanging on near the front. BTW, Ivan Basso won here in 2006 on his way to his first, and only, overall Giro win.
In the late 1960's Eddy Merckx won his first ever mountain-top finish in a grand tour. A few years later, the Giro hosted a similar stage as today in the morning and then ran a 200+km flatter stage in the afternoon. These "split stages" were common in grand tours up until the 1990's. Nowadays the UCI has outlawed them.