In the automotive world, heat is bad. Take brakes for instance. They work by converting the rotational kinetic energy of the wheels into heat by friction. As more heat is generated, a point comes when the brakes rotors cannot absorb any more heat. They lose their friction and become useless – a phenomena known as brake fade. Therefore it is essential, that a brake can get rid of heat fast.
Vanes are often added on brake rotors to aid their cooling. Many vehicles come with stock brakes equipped with straight vanes. These are cheaper to make as they do not need two different kinds of castings for their manufacture as they are specific to which side of the car they can be mounted. These vanes pull in more air onto the rotors to speed up their cooling. They cut the air as the wheel rotates and direct it for a better and faster flow. Directional rotors also pull in air from the side of the wheel, and not just from the top like straight vanes. Air coming from the side of the car is obviously cooler than that coming from the top, which could be coming straight from the hot engine. It would be anyone’s guess then, that directional rotors would perform better than straight vane rotors. Some of us however, are not so easily convinced.
Recently a YouTube video decided to test this for themselves. A Honda S2000 was installed with straight vane rotors on the left front wheel and directional rotors on the front right wheel. A data logger equipped with a magnetic thermal probe was used to measure their temperature. Two different tests were performed. In the first test, the initial temperature of the rotors was noted. Then the car was accelerated to 60 mph and stopped using half brake and half throttle. Then the car was again sped up to 60 mph and then finally stopped. Readings were taken immediately after the run and the same test was repeated two more times.
The results from these tests would seem baffling to anyone who expects the directional rotors to perform better. In all three runs, the directional rotors heated up to a greater extent than the stock ones - 116, 82 and 105 degree centigrade respectively. Stock rotors’ readings were consistently better at 96, 86 and 92 degree centigrade. The second test was done to test braking while going downhill. The car was sped up to 40 mph and then put in neutral while going downhill. The car’s speed was then maintained with just the brakes without any engine braking. Once the car had stopped, the temperatures were noted again. This test too, was conducted thrice at three different locations. In this test, the temperature changes for the stock setup were 49, 77 and 14 (from 138 to 152) degrees centigrade. The temperature changes for directional rotors were 59, 82 and 24 (from 141 to 165) degrees centigrade.
Are we caught in some made up marketing ploy to sell more expensive rotors? Let’s dig deeper. If we account for the ambient temperature, the actual temperature difference between the two setup only happens to be 2% for the first test and 1% for the second test. However a negatively biased difference does exist for the differential rotors. The next thing to consider was the mass difference between the two rotors. The stock rotors are in fact a pound heavier than the directional ones. Considering the energy to be the same for both rotors, the same energy is being stored in a denser package in directional rotors.
The most critical point however seems to be the temperature itself. The temperature ranges that this test involved (40 – 190 degrees) were not exactly high. On the track, brake rotors get up to temperature ranges 10 times higher. A better test would then have been, to test the two different setups on the track throughout the day. Under that kind of serious performance, the cooling efficiency really comes into play, more than the mass of the brakes. It’s really a give or take between heat absorption (aided by mass) and cooling efficiency (presence of vanes). This was further proven by a test conducted by Science of Speed, where another Honda S2000 did 56 laps with the two different brake setups. With consistent lap times, directional rotors ran 32.4 degree centigrade cooler on average. The temperature ranges involved here were also as much as 500-1000 degrees centigrade. The conclusion then may be, that performance brakes only shine when they are tested hard. And that we should not let confirmation bias cloud our judgement.