If my rims reach 450 degrees, I'll eat my underpants.
(actually, I won't 'cos I'll have plunged into a ravine when my tyre exploded)
I've been asked to autoresurrect, dive in and explain.
I appreciate a good sceptic, after all, I'm one too.
How does a rim reach 450 degrees C or more to melt off bits of aluminium that then melts into and settles into a rubber brake pad?
Firstly, aluminium fully melts at a much higher temperature than 450C but I used this figure because at that temperature it is pretty malleable (Like putty) and can easily, under pressure ooze. Therefore, under braking conditions, a bit of alu at 450 degrees can ostensibly start to flow and transfer.
Now, as for the rim reaching 450. It clearly doesn't....only bits (and very tiny bits the size of a needle tip) of it heat to that temperature . Imagine a needle and a lit candle. You can hold the needle by the eye, dip the tip into the flame and it glows red. This is approximately 450 degrees too, since steel glows red at that temperature. However, you can still hold the needle for a while before the heat reaches your fingers. If you remove the needle from the flame and wait a bit, the temperature will even out and entire needle will peak at about 30 degrees before settling back to ambient. The peak temperature depends on two things: the ratio of the thermal mass of the hot and cold bits and, the peak temperature of the tip in the candle.
I'll explain: If you were to do the experiment with a thumb tac two different ways, you could heat the tip of the pin to red hot, remove the pin from the flame and wait. The settlement temperature of the entire tac would them reach say, 30 degrees. Alternatively, you could hold the tac by the pin and heat the dome to red hot and then let it settle. The end peak temperature of the tac would then probably be something like 60 degrees. This is because more heat can be stored in more mass. Conversely it takes more heat to raise the temperature of a large piece of steel than a small piece of steel.
To labour the point from a different angle. If you were to heat a disc of copper plate over a candle but holding it dead centre above the candle, the heat gradient can be imagined like a dart board. The bull's eye will be hottest and the temperature will drop as you go towards the edges. At the doubles ring it will be the coolest and remain so until the candle burns out. Heat needs time to travel and some of it gets lost to radiation, especially towards the larger surface area around the edges of the disk. All I'm trying to illustrate here is that hot-spots are perfectly normal.
Back to our hot rim: The heat is generated at the interface. This interface isn't a smooth flat surface as you imagine but a surface resembling two pieces of rough sandpaper moving against each other. You can imagine that only the tips touch. At these tiny touch points the flash temperatures are high. But the tips are small and cannot transmit much heat back into the bulk of the rim, hence it rises only a few degrees. These contact points are tiny - microscopically small.
I could speculate that the nucleation point for a hot-spot is a bit of grit. The explanation would be that hat gets the hot-spot going which then builds up with aluminium. However, after analysing dozens, perhaps hundreds of pads with embedded metal, I'm yet to come across such a piece of grit - always pure metal. What I have discovered is that the metal is never shaved off but always oozed off under temperature and pressure (round edges rather than sharp shavings). This indicates that even rubber can create enough heat that could melt metal asperities at the molecular level.
Lo0ng story short, therefore it is not abnormal to find localised melting/softening in a large object that is relatively cool.
There's no need to eat your underwear because you are right. Your rim does not reach 450C
I'm pretty sure back in the past I've posted pictures here of my experiments with Koolstop pads. I used to ride with one Koolstop and one Shimano/Campag pad in the same calliper and after some months remove them and compare. It was remarkable how much pick-up there was in Campag and Shimano compared to Koolstop (Salmon compound).
Advice to the OP: Get Koolstop salmon as Ajax said. Ignore the sideshow about warping. The metal
is from your rim - you can even pinpoint the exact spot if you take a magnifier to the job.
I'm not going to ask how you're supposed to get a micrometer on the rim sidewall either.
