This is known to veteran cyclists, but as you descend very long, very steep hills, stop occassionally to cool your brakes and your rims. The steepest hill I ever descended was in VA on the TransAm Trail as I headed towards Vesuvius, VA. I stopped to cool things off about three times, and didn't have a problem.
Later that day I spoke with the owner of a local bike shop, and we discussed that very hill. He told me of many cyclists who failed to take proper precautions on the hill, and sometimes paid dearly for it. They lost control of the bike or the hot rim blew the tire out, and ended up with severe injuries, such as a fractured skull or severe lacerations.