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From sliding tectonic plates to the honey in beehives, the conversion of kinetic energy into heat energy through friction is a universal process that’s been going on for millennia. It wasn’t until we figured it could be used to our advantage in the ignition of fire, though, that it started to become useful to us as a species.
Friction features extensively in the automotive world, where it is both useful and not so useful. By far and away the largest generator of friction in an automobile originates in the action of the braking system.
Up until the last few decades, motorsport braking systems solely used cast iron discs as the frictional medium. It’s cheap, has good longevity and good thermal characteristics, making it perfect for most applications.
Traditionally, brake discs for road cars and motorsport alike were manufactured from grey cast iron, differentiated from standard cast iron in that it contains around three per cent carbon by weight, and has higher sulphur and manganese content. This gives it some useful properties.
The microstructure of grey cast iron endows it with a number of useful properties. It has relatively high thermal conductivity, resists thermal fatigue well and has better corrosion performance than many other ferrous alloys. It also has higher hardness, which makes it more wear resistant.
Iron in all forms has a relatively high thermal mass (or heat capacity in chemistry terms), which means it takes a lot of energy to increase its temperature, reducing the rate of temperature increase during each braking application. Once hot, it has a high thermal conductivity, which means it will lose heat to the air through convection relatively easily.
Surface temperature
Bringing an 800kg F1 car down from 200mph to 60mph dissipates approximately 2900kJ of kinetic energy as heat into the braking system in just a few seconds. To give you some perspective, that’s roughly equal to the heat generated
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