March 8, 2006

Discs vs. Drums
By Jim Kerr

Why do some vehicles have four-wheel disc brakes and others have front disc brakes combined with rear drum brakes? Which system is better? What are ventilated disc brakes? And why do I see some aftermarket disc brakes with holes in the rotors? I am often asked these and many more questions about brakes. Let's find some answers.

Four-wheel drum brakes were the standard for decades. In a drum brake, hydraulic pressure is applied to a piston, which in turn forces a curved brake shoe outward. Friction material bonded or riveted to the shoe presses against the inside of the brake drum, slowing the rotation of the drum and the axle. Then these older cars stopped - hopefully!

Actually, drum brakes worked well at the time, but anyone who has tried to stop a car from high speed using only drum brakes quickly learns their limitations: they fade. As the brake drums heat up from friction, they expand. The brake shoes have to move out further to contact the drum, which means the brake pedal has to be pressed further. Gases formed from the hot friction material are also trapped between the shoes and the drum interior, reducing braking capability. You might get one good stop from high speed, but try it twice and you were pushing your luck.

Manufacturers added fins or bonded aluminum to brake drums to help cool them, along with metallic brake linings, but this wasn't the answer for high-performance brakes. Along came discs.

Disc brakes had been used in aircraft and industrial applications for some time. The friction material (the brake pad) clamps onto the spinning disc by hydraulic force applied to the brake caliper. Discs don't tend to "grab" like drum brakes can, so they provide much better directional stability when stopping. They also are out in the open, unlike closed drums, which has advantages and disadvantages.

Disc brakes cool better, because air can circulate across the friction surface easily. Ventilated rotors have two friction surfaces separated by a series of fins. These allow air inside the rotor between the friction surfaces for even better cooling. Most front disc brakes are now "vented" because they do the majority of the stopping, but most rear disc brakes are non-vented and have a "solid" rotor, since rear brakes simply don't generate as much heat.

Another advantage of discs is that contaminants and gases are thrown off the spinning disc, unlike a drum that traps the contaminants. Water, oil or gases from the friction material are quickly dissipated, providing better braking. Those discs with holes or slots in them are partially for show, but they do have a purpose: the holes allow water or gases sitting between the brake pads and the friction surface of the rotor to be forced into the holes so the brakes apply immediately, instead of having the rotors turn one revolution to wipe them clean. This may be important in racing situations, but is not critical on the street. The holes reduce the area of the friction surface and can even catch small stones, so they can require more maintenance.

The biggest disadvantage of disc brakes is that they are exposed to dirt. Dirt and dust between the rotor and pads can wear the disc quickly. If the rotor is too thin, it cannot dissipate the heat and in extreme situations it can crack; rotors that are worn too much must be replaced.

Front disc brakes sit in relatively clean air, but rear discs are exposed to all the dust and debris thrown up by the front tires. This is why rear disc brakes often wear faster than front disc brakes, even though they are only doing a small percentage of the braking.

Anti-lock brakes work best with disc brakes because they release quickly and smoothly. Changes to shoe design and placement have helped drum brakes release quicker, but discs are still better. For this reason, and for maximum stopping power, you find four-wheel disc brakes on many vehicles.

I personally prefer front discs and rear drums. I still get all the advantages of discs on the front and can stop very well, but rear drum brakes are not exposed to the dust so they wear much slower under many conditions. Less rear brake maintenance leaves more money in my pocket. Just don't ask me to go back to four wheel drum brakes - they were scary!

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

More Auto Tech...