Re: Re:
I don't have first-hand experience with Arenberg, but most of the pros say it's the sector with the worst cobbles. They're big, uneven and badly cut. There's also the fact that the sector is incredibly narrow with barriers to prevent riders from riding off the pavé and that the forest blocks out the sun and keeps the moisture trapped underneath the trees, which makes the cobbles damp and mossy, and thus slippery.
However, the most important reason for Arenberg being feared as it is is one that can't be replicated merely by riding it alone during a training ride. Arenberg comes relatively early in the race, when the selection process has hardly even begun and riders are reasonably fresh. That means that you have 150-200 nervous riders going full blast into a path of uneven, slippery cobbles that's only three meters wide, knowing full well that they have to ride hard and fight at the front to have any hope of winning the most important classic of the year. There's very little room for error in that situation, but the tension is as high as it will ever be. That's just a recipe for disaster. At Mons-en-Pévèle and Carrefour the riders are tired and usually split into small groups, which means the speed is lower and the frantic fighting for position is less of a factor.