Aluminum Rotors Vs. Cast Iron Rotorsby Richard Rowe
Brake technology has come a long way since the first auto engineers wrapped a leather belt around a steel drum and tightened it to slow the car. Over a century later, we have brake pads made of sintered bronze, fluid made from liquid silicone and rotors made out of everything from cast iron to synthetic diamond. Somewhere in between are aluminum rotors, which are an improvement over iron rotors in all but one somewhat crucial area.
Disc brakes work by converting kinetic energy -- movement -- into thermal energy. They do this by squeezing a spinning metal brake rotor between a pair of stationary friction pads. But that thermal energy doesn't just go away -- it soaks into the rotor and adjacent components, and ultimately disperses into the airstream. Quick dispersion is a must, since brakes can easily soar to 1,000-plus degrees F in street applications, and twice that in racing applications.
Aluminum does have a few things going for it, but especially weight. Aluminum weighs about one-third of what cast iron does, which is particularly helpful when you're looking at unsprung weight. "Unsprung weight" refers to dead weight sitting right on the tires, as opposed to weight not controlled by the suspension. Lower unsprung weight means less strain on the springs and shock absorbers; less strain on the suspension means that you can use softer springs and shocks, which in turn means that you don't have to sacrifice ride comfort for handling prowess.
There's another side to the brake rotor's importance in terms of weight, and that part has to do with rotational inertia. A brake rotor acts like a flywheel, storing mechanical energy. The flywheel always tries to maintain speed, resisting both acceleration and deceleration. Lighter aluminum rotors reduce this flywheel effect, which helps to boost performance in terms of both acceleration and braking. A spinning rotor also acts as something of a gyroscopic stabilizer, meaning that a heavier iron one will increase steering effort while dulling steering feedback. The lighter aluminum rotor decreases effort while increasing steering feedback and precision.
Heat dissipation and electrical conductivity are very closely related concepts, enough so that they're nearly interchangeable in engineering terms. Silver is one of the best electrical and thermal conductors known to man, followed by copper, gold and aluminum. Cast iron's thermal conductivity is comparatively dismal; about 3.5 times lower than aluminum. This means that iron soaks up heat slower and hangs onto it longer, which allows heat to transfer into the brake pads and brake fluid instead of radiating out into the air where it belongs.
Aluminum and cast iron perform similarly in terms of friction coefficient, or the ability to grab and hold the brake pads. Depending upon the specific alloy used, aluminum can easily surpass steel's friction coefficient, especially when combined with other metals specifically engineered to enhance that coefficient. Cast iron may also contain alloying elements, though, so this part's pretty much a wash. But the aluminum's reduced inertia helps it to win out in terms of performance, since the net effect is a positive one in terms of performance.
Heat and Durability
You may be wondering at this point why, if aluminum is so wonderful, we use anything but recycled cans for brake rotors. It's because, quite simply, aluminum returns to a liquid state at around 660 degrees Fahrenheit -- well below the temperatures encountered during spirited driving. This has historically made the aluminum-vs-iron debate fairly academic, as even low-grade cast iron can withstand 2,100-plus degrees. Steel rotors can take even higher temperatures -- upward of 3,000 degrees, depending upon the alloy. This alone makes cheaper but less ideal cast iron rotors preferred for most performance or heavy-car applications.
Solutions and Developments
Manufacturers have been working on ways of making aluminum work by tweaking the alloys and bonding aluminum cores to steel outer discs in an effort to keep the rotors together. There have been some significant advancements in brakes, making them universally functional for non-motorcycle applications, but other materials are making inroads as well. Carbon-ceramic and carbon-carbon brake rotors offer all the weight savings and benefits of aluminum, but can withstand higher temperatures than steel. Granted, these materials also cost 10 times as much, so aluminum may yet have a serious future in automotive performance and heavy-vehicle applications.
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