I've seen that as well. Kinda cool to watch, though it cooled off (or the water trapped in the rim tape all evaporated) pretty quickly. I'd just come off a twisty ridge averaging 12-15% grade for just under a mile. You can well believe I made a point of not touching either rim for another half hour!


Oh, and no leaks from the tube.


Better to BRAKE and coast, lather, rinse, and repeat. Dragging your brakes down a long slope can heat the rim up pretty quickly. OTOH, no braking, no heat in the rim. I've come down an eight mile road coming off a ridge with up to 6% grade, and had no temperature buildup. Curves were such I barely had to brake.