Originally Posted by
pdlamb
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.
Not sure what or who ChemES is but I find your description hard to believe. Even if the rim were at the boiling point, you would have to get water into the rim which would be difficult considering that the water is cooler than the rim which means the water would vaporized before it got into the rim…think water drops on a hot skillet. If the water
did happen to get into the rim, it would quickly cool the rim below the boiling point so the whole system couldn’t be “live” any more.
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.
If you are talking about Mt. Vesuvius, I’ve got similar (or worse) within 100 miles of my house. Saxon Mountain that I referred to above has a similar drop (2450 feet) over a slightly shorter distance (5.7 miles) and a higher % grade (8.5%) with far more switchbacks that are far steeper (some up to 15% grade). I’ve done it with cantilever brakes and never over heated the rims.
Oh, and by the way, Saxon Mountain is dirt with a mostly ungraded surface.
Overheating wheels on a downhill is a user error. I don’t check my rims after (or during) a downhill because I know that I haven’t overheated them. Braking on a bicycle should be down in short, hard pulses rather than a long sustained pressure.