after watching snipits of paris-roubaix last night, i was interested in the seemingly spontaneous crashes that took place on the cobblestone sections. i have never ridden on that type of surface myself.

are they just slippery on the surface? tires get caught in the grooves? other?

on a side note, the bikes set-up specifically for that race looked pretty sweet, i would imagine the tires used would need to utilize a compound designed to handle a whole mixture of surfaces, overall that race looks very challenging.