Auto Academy: Disc vs. Drum Brakes
Most of you know that there’s a difference between drum-style brakes and disc-style brakes. Our Micah Wright covered drum brakes in an earlier article, so the majority of this article will be focused on disc brakes, namely how they function and why they are preferred in performance/aesthetic applications. (Who doesn’t think a Brembo Big Brake Kit looks impressive?!)
I was recently writing a piece on maintenance tips, and I mentioned that brake fluid is hygroscopic (absorbs moisture from the atmosphere) and that water-laden brake fluid can diminish brake performance. But it occurred to me that it might make sense to give a bit of a primer on how brake systems work overall. (Editor’s Note: Owen’s maintenance piece will be published later this week.)
In the late 1800s, the very first cars had hand-activated tire brakes. A lever was used to apply pressure to the wheel, which helped to slow the vehicle. This system functioned reasonably well when the top speed of the car wasn’t much more than a walking pace, but it quickly became clear that it wouldn’t be acceptable in the long run. The next development to become popular was the use of external drum brakes. This involved wrapping a cable or band around the outside of a drum that was connected to the axle. Unreliable, weak, and quick to wear out, this technology eventually yielded as well. From there, it became a battle between internal drum brakes and disc brakes.
Drum brakes involve a drum (duh!), which is connected to the axle, being stopped by two shoes, which are mounted to the backing plate, pressing against the inner surface of the drum. The shoes are lined with a heat-resistant friction material and are pushed outward against the drum by the wheel cylinder. As you can see above, they are fairly complex systems with a lot of parts. For most of the 20th century, the shoes, as was the case with disc brake pads, were lined with asbestos or a blend of asbestos and other materials. This practice was stopped in the 1980s due to health concerns.
Shoes are mounted to the backing plate at one end. A “leading shoe” has its mounting at the “end” of the shoe when compared to the (forward) rotation of the drum while a “trailing shoe” has its mounting point at the “beginning” of the shoe. A leading shoe will be wedged between its mounting point and the drum, which increases the braking force in what is known as a self-servo action. The trailing shoe is continually pushed away from the drum by the rotation, which makes the braking force it exerts less powerful.
Brake drums come in a variety of combinations of leading and trailing arrangements. The most common one for passenger vehicles combines one leading shoe and one trailing shoe that are actuated by the same wheel cylinder. For more specialized applications, it’s possible to have two leading shoes in what’s known as a twin-leading shoe arrangement (through the use of two wheel cylinders) or to have a setup where the primary leading shoe pushes on a secondary leading shoe called a duo-servo setup.
The three major components of disc brakes are the rotor, the pads, and the caliper. The caliper covers a portion of the rotor and squeezes the pads against both sides like a vice. This has an advantage over drum brakes because the surface area that the pads contact, which is known as the swept area, is significantly larger. This means that it will remain cooler because the friction is being applied to a larger footprint. Additionally, the rotor is open to the air so it can be cooled by exposure to the atmosphere and the conductive cooling of air passing over it as the car moves.
Disc brake systems come in two styles: floating (or sliding) and fixed. Floating brake calipers have a piston on one side of the rotor and a bracket that reaches across to the pad on the opposite side. When the piston is powered, it pushes the pad it’s touching against the face of the rotor. Once that pad is in contact, the piston pushes the caliper back within the caliper bracket. This movement pulls the bracket on the opposite side of the rotor into the pad and, eventually, into contact with the opposite rotor face.
Fixed brake calipers are solidly mounted and do not move within their brackets. They will have a piston on both sides of the rotor, which makes them more expensive and complex. Some systems, such as those in high-end performance cars, will have multiple pistons on each side. To ensure even braking, the same number of pistons will be present on each side of the rotor so there will be an even number (4, 6, and 8. I’m not aware of anything larger than 8). These are either called 6-piston or 6-pot calipers.
Having multiple pistons allows for a more even distribution of force across the brake pad and for an increased total force applied to the pad. This can mean a greater normal force and increased friction. Increased friction will do a better job preventing rotation of the tire, but it’ll also increase wear on the rotor and pad. Multiple-piston calipers are larger so they typically accompany larger rotors. Larger rotors present a larger surface, which further increases the swept area, and helps to keep the material cool during intense braking.
Most rotors are made from carbon steel, which can heat up under periods of prolonged braking (even glowing if they get hot enough, as seen above). If enough heat is generated, the material of the brake pad can begin to sublimate (transition directly from solid to gas), which creates a lubricating layer between the pad and the rotor. This effectively renders the brake system ineffective. Carbon ceramic brakes can help to prevent this condition due to the fact that they have a specific heat of nearly twice that of carbon steel. This means they heat up less under the same conditions and help to keep the brakes functional under extended heavy use, like a track day.
Like classics? It’s always Throwback Thursday somewhere.