Originally Posted by
Jipe
The energy to dissipate depend of the total mass (bike+cyclist+luggage) and deceleration, its independent of the wheel size.
For a small rim size (like for a small disc size), the rim become warmer than with a bigger one. But for the rim; heat goes to the tire and inner tire that also become warmer which can be damaged in case of extreme braking like in case of long downhill.
Warmer weather will increase the risks since the heat dissipation depend of the difference between rim temęrature and ambient temperature.
Yes, when I said mass, I meant rim mass to accept the braking heat. I would assume other masses were constant.
Heat into the tires and tubes is a factor. As I mentioned, aluminum has a high coefficient of thermal expansion, so the rim will expand in circumference and diameter and yank on the spokes. But also, aluminum begins to lose strength and temperatures a lot lower than most people would guess. But yes, I think tubes and tires will be in trouble first. Found online:
"A major concern in the design of land-based and marine aluminum structures is fire safety. This concern is exacerbated for aluminum alloys due to property degradation which occurs at temperatures as low as 150°C with a 50% yield strength reduction at ~275°C (Langhelle and Amdahl
2001)." So yeah, I agree with you, I think tubes will be in trouble before the aluminum. A think the aluminum also galls and transfers bits of aluminum into pads at elevated heat, those bits are more of a frequent problem to me than road grit, I have to pry out those shiny bits with a knife tip when cleaning the pads.
But in general...
(Mass)(Specific Heat)(Delta-T) = Heat Transfered
Like riding a bike, some things you never forget.
Oh, by the way, aluminum has about twice the specific heat of iron, so can store a good amount of heat. This has great implications in comparing aluminum and iron cookware for searing, iron having the advantage (for storing heat) in 3X the mass for a given thickness, but aluminum having the advantage of 2X the specific heat, means:
- for the same thickness, the aluminum pan has 2/3 the heat storage capacity of the iron one
- if the aluminum pan is 1.5 times as thick as the cast iron, equal heat storage capacity (but aluminum having far better conductivity)
- if the aluminum pan is 3X the thickness of the cast iron, equal mass, you really have a great aluminum pan, as that has 2X the heat storage of the cast iron one, and immensely better conductivity. It can also be seasoned, just like cast iron.
The above matters little over a restaurant industrial-strength burner, where even a thin steel wok works fine. But at home, with much weaker burners, key to getting a good sear is a pan that can be preheated to store a lot of heat, so as the food renders water and cools the pan by evaporation, a heavy pan can keep up much better than a light one, evaporating the water and searing the food, rather than cooling and boiling the food.