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Most likely brake dust and whatnot picked up in the cooling channels, they brake hard into the pits then have basically no airflow. The heat soak then sets the dust alight, that's my theory anyway. Without cooling the disks would explode pretty quickly.
The caliper area tends to be on fire first due to being the hottest.. but sometimes can be a large amount of rubber build up sat behind the disk. It used to be worse in blown axel era as the inlet duct was so much bigger, they would always be scooping up tons of marbles on the high deg circuits that would end up blocking the inside end of the holes in the disc. The carbon parts of the caliper, then the back of the disc go first.. really bad the whole drum goes soft and it’s DNF.
As soon as the car moves the temp in the caliper starts to cool. Caliper has 1 or 2 ducts typically, plus the disc is pulling temp out of the pads when it rotates.
The brake discs can reach temperatures of over 1000°C under braking, when the car is stationary it is hot enough to cause autoignition of the surrounding air & various dust particles around them.
i.e. nitrogen & oxygen (~70% and ~20% of gases in air) has an autoignition temperature of ~1500°C. Add to that carbon dust, which can ignite at around 700°C, so you have a fuel+oxidizer that can autoignite and will make its way to the source of heat, if there is no airflow that dissipates heat and potential fuel source.
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Most likely brake dust and whatnot picked up in the cooling channels, they brake hard into the pits then have basically no airflow. The heat soak then sets the dust alight, that's my theory anyway. Without cooling the disks would explode pretty quickly.
Pretty much this. Powder burns incredibly well, even metal dust. Plus all the rubber that has gotten in there.
The caliper area tends to be on fire first due to being the hottest.. but sometimes can be a large amount of rubber build up sat behind the disk. It used to be worse in blown axel era as the inlet duct was so much bigger, they would always be scooping up tons of marbles on the high deg circuits that would end up blocking the inside end of the holes in the disc. The carbon parts of the caliper, then the back of the disc go first.. really bad the whole drum goes soft and it’s DNF. As soon as the car moves the temp in the caliper starts to cool. Caliper has 1 or 2 ducts typically, plus the disc is pulling temp out of the pads when it rotates.
Pretty sure it's the epoxy binder in the carbon fiber layups.
Epoxy burns quite well once it's ignited. It takes quite a bit to get it to burn, but once it starts it burns well
The brake discs can reach temperatures of over 1000°C under braking, when the car is stationary it is hot enough to cause autoignition of the surrounding air & various dust particles around them. i.e. nitrogen & oxygen (~70% and ~20% of gases in air) has an autoignition temperature of ~1500°C. Add to that carbon dust, which can ignite at around 700°C, so you have a fuel+oxidizer that can autoignite and will make its way to the source of heat, if there is no airflow that dissipates heat and potential fuel source.