Brake Ducts and Cooling

Brake Ducts and Cooling

rake ducts lower disc temperatures by forcing more air through the internal disc vanes. That forced air is coupled with the centrifugal force generated by the spinning directional cooling vanes, drawing additional heat from the disc. The picture below shows an AP Racing J Hook Competition Disc with a high number of directional internal vanes. As a disc spins, those vanes act like a pump or fan, pushing hot air out and drawing cool air in. This is one of the primary reasons why a proper directional racing disc is far more effective and efficient than an OEM-style disc, which in many cases does not even have directional vanes.

The final way heat escapes a brake disc is via conduction as it travels through the connected metal components of the disc hat, hubs, studs, brake pads, and bearings. On a good 2-piece floating disc, conduction has a much smaller impact on disc cooling, and there isn’t really much we can do to change the amount of conduction cooling.

In a properly cooled brake system, both halves of a brake disc operate at approximately the same temperature. If one half of a brake disc is run at a substantially higher temperature, several problems can occur. One of the biggest is distortion and coning of the disc. Iron brake discs grow when heated. At track temperatures, a disc expands radially, increasing in diameter and circumference. It expands in thickness as well, but the amount is much smaller.

A common approach for directing airflow is to mount a sheet metal or carbon fiber plate on the spindle which points a 3-4” hose at the disc. Others leave the OEM disc splash shield in place, and simply cut a hole in it. In many cases that plate covers some portion, if not all, of the inner brake disc face, turning that plate into a heat barrier and reflector. Rather than allowing heat to escape into the wheel well, the duct mounting plate traps and reflects the huge amount of heat radiating from the inner disc face back into the disc causing the inner disc half to run significantly hotter than the outer half. Just as you don’t want to run an inner disc face cooler than the outer face by dumping air on it, it’s equally undesirable to run the inner disc face hotter by limiting its ability to radiate heat. Doing so will lead to the disc distortion and coning problems described above.

If you are quickly and frequently cracking your brake discs on the track, there may not be a problem with your discs. It could be your driving! At any given HPDE we see drivers at full tilt before they reach the end of the blend line on their out-lap, then standing on their brakes into Turn 1. When they do so, they’re introducing a huge amount of thermal shock to their discs. The discs go from ambient temperature to many hundred degrees in the blink of an eye. That rapid temperature spike places incredible stress on the disc, or what we call thermal shock, and it can crack even the toughest iron disc. Unless you’re trying to win a race, you should treat your brakes like you do your tires on the initial laps of any track session. You should gradually bring your brakes up to temperature over the course of the first lap or two, progressively increasing your aggressiveness on your brakes. We can’t stress enough how important this is! Treating them in this manner can dramatically increase disc longevity. At the end of all sessions before pulling into the pits, make sure you complete your last lap off the brakes as much as possible. That will allow for a gradual cooling of your discs, which will lead to a substantial reduction in overall brake temperatures. If you exit the track and park in the middle of a hot lap, your discs will be searing hot. While those discs sit in stagnant air, their heat will transmit into your pads, calipers, and fluid, and the odds of boiling your fluid becomes markedly higher. Discs cool in an even (low stress) manner when rotating. They cool unevenly when parked with the section under the caliper remaining hot. Often cracks are created or extended when the car is parked with hot brakes

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