Under the right conditions they can get hot enough that the oil will separate from the thickening agent. It's been said that the oil can run out of the hub and you lose your lube. The Klunker crew that invented mountain biking used to run a downhill race in some CA mountains that was long enough and steep enough that they were on the brakes long and hard enough to cause that problem, forcing them to repack their hubs for each race. Their slang term for the mountain and the race was Repack.

I like to do long coaster wheelies. There are some roads here that have super long, steep, wide, downhill runs that are perfect for that. Get up to speed, Yank the front end up to vertical, and then modulate the brake at the teeter-totter point of flipping over backwards. The descent gives you the speed without the need to pedal, and the brake is the balance trigger. I've made some runs close to three miles and never had an overheating problem, but I am very lightly feathering the brake, so that could be part of it.

I usually don't ride if the weather is scorching hot. I'm OK with mid 90s, but above that I normally don't have much fun. This year however, I've had this weird high tolerance for the heat. Over 100 was not bothering me if I took periodic shade breaks. Well, a couple weeks ago I heard a howl coming from my rear hub during braking. It was way over 100 degrees that day and I had been on the bike for a long time and braking frequently and pretty hard. I could make the sound repeat at different tones by applying different braking pressure. Not cool. And it definitely had a very unpleasant dry sound and feel transmitting to my feet. I thought, "Crap, what the hell? My first ever rear hub problem! Ten Miles from home dammit!" I dismounted, lifted the rear wheel and spun it. No weird noises. Checked for bearing play and all was kosher. Visual inspection yielded nothing abnormal. I happened to be right by a Pita Sandwich joint, I was hungry, and they were adverting a 1/2-off-any-sandwich-Today-only-sale, so I parked in the shade and fueled up. Thirty minutes later I pedaled off and the sound was gone. Brakes were buttery smooth, dead silent, and modulated perfectly. So my guess is that the 100+ degree weather was just enough to push the grease past it happy place, separating the oil from the thickener. And evidently, that particular grease has good/decent reversion qualities. Since then the temps her have dropped a lot and I have not been able to reproduce the howl again no matter how fast-long-hard I brake.

The grease I have been using is designed especially to fight salt water penetration, but for the life of me, I can't find any temp ratings for it. Hence my search for a better batter.