Originally Posted by
cyccommute
Just because you see steam doesn’t mean that the rim is at the boiling point of water. Water vapor can condense at various temperatures depending on the dew point at the time the water vapor evaporates from the wheel. Here in dry Colorado, you would likely never see “steam” in a situation like you describe. The amount of water in the air is too low for the dew point to be low. In the more humid areas of the world, seeing “steam” in that situation would be more likely at a lower temperature. Remember that you can see very dense “steam” at cold temperatures and the water vapor is only at 98°F.
Not the "steam" you're describing, but what ChemEs call "live steam" -- water vapor that is pushing droplets out by the force of the steam pressure trapped inside the wheel.
I have years of steep downhills of much longer distance than 3 miles under my belt. Rabbit Ears Pass is just one…7 miles of 7% grade (but no turns). Colorado National Monumen is another with 6 miles on the front entrance and 7 miles on the back entrance, both of which have tons of switchbacks with 15% grades. Done both on downhill (two separate occasions). I’ve been down mountain bike trails and jeep roads with cantilevers. Saxon Mountain outside of Georgetown is 6 miles long with an average of 8% grade but with some switchbacks as high as 15%. It’s a damned steep road. The one thing that all those rides…and the thousands of other descents I’ve done…is that I have never once overheated a rim brake. I’ve never even gotten them hot to the touch. If you use your brake properly, you can descend anything without heating the wheels.
On a couple other occasions, I've wasted some water out of my bottle to wet down a finger, then flicked it against the rim like grandmother used to do with her cast iron pan to see if it was hot. Like the cast iron pan, the water hissed and fizzled before quickly evaporating. I've also seen the remains of a wheel after a tire blew on the Vesuvius grade. Not me! I stopped to let my rims cool for 10 minutes halfway down.