Disc vs Drum Brakes

Disc brakes have lately become a status symbol. Along with the tall rims and low profile tires come calipers painted red.

The new see-through wheels expose brakes as never before. And nothing looks dumber than a set of tall blingy wheels showing off little bitty drum brake cans.

So we are abandoning the drum brake because it doesn't look cool, not because it isn't safe.

While I am mystified by the appeal of many automotive fads, including the current big wheel mania, I applaud the demise of the ancient drum brake.

Brakes are a key safety feature, but are often overlooked. They convert the energy of motion to waste heat which must be exhausted from the brake.

While the disc vs. drum brake debate has been around for a long time, there is a new angle. With economy cars staging yet another comeback, manufacturers have retained the old fashioned drum brakes on many small cars to stay competitive on price.

While more expensive cars use an auxiliary hand brake and four wheel disks, the price-sensitive entry-level segment has many entrants with rear drum brakes.

Experts and many users say rear drums are fine, providing adequate performance for these value centered cars. They point to significant improvements in braking technology since the 1970's, noting today's front discs, which assume 60 to 90% of braking, have excellent stopping power.

Price conscious buyers, many buying their first new car, don't think twice about the absent safety features, like side curtain air bags and anti-lock four wheel disc brakes. They feel that as long as the car stops in an acceptable distance, and has an iPod jack, they are safe.

Front wheel drive cars, they say, have nothing to gain from rear discs. Manufacturers know the application is not race car rocket science, it's just a daily commute and grocery run. The typical consumer cannot discern a difference in normal driving.

There was at least one economy car, Mazda Protege, that reverted back to a disc/drum system after having all disc standard for a couple years.

My favorite:

"under normal street driving rear drum brakes are more than sufficient 99.9 percent of the time"

Yeah, but what about the other .1%? The emergency stop, snow slick blacktop, or loaded-for-vacation mountain descent.

So the conventional wisdom says only high performance and luxury cars benefit from four wheel disc braking.

Wolf Creek Pass

Who am I to disagree?

Just somebody who has spent his whole life nursing hot (expletive string deleted) drum brakes down the side of one mountain or another.

Truck drivers used to get out, lie under the rig, and manually adjust the slack on all 10 drums at the top of a mountain. The problem was that any one drum set too tight would do more than its share of the braking. If the hill was long, pretty soon smoke would be pouring out the back and the brake pedal would be on the floor without much effect.

First mountain descent rivals first trip into Manhattan for highest number of student truck drivers calling it quits at the next Greyhound Station. Others ask the instructor for permission to turn around and ride the roller coaster again.

Hot drum brakes fade. They loose effectiveness when used repeatedly over a short interval.

Automatic slack adjusters on commercial trucks have changed the scenario described above and in the C. W. Mcall hit single "Wolf Creek Pass," but new NTHSA stopping distance proposals have reignited the disc verses drum debate in trucking.

So what does this have to do with a trend toward retaining drums on economy cars?

Plenty.

I took a rented mini-van up Pike's Peak Highway a few years back, did the tourist thing on top and started down. Of course I knew how to come down a mountain, but many four wheeled drivers ignored the signs about using lower gears.

About two thirds of the way down was a cop holding a high tech pistol. He wasn't clocking speed. This cop was measuring the heat coming off the front disc brakes and showing each driver their reading. Those with real hot fronts were encouraged to pull over and cool off.

My minivan showed 440 degrees Fahrenheit, but the officer waved me on. I pulled off to watch anyway.

Cars over 500 degrees, hot enough to boil brake fluid, got more attention. No tourist was ticketed. California Highway Patrol has sometimes waited at the bottom of the hill, ticketing truckers with hot drums.

Every car with hot brakes on Pikes Peak was either older, or a loaded minivan or economy car.

No car with all wheel discs was stopped.

Disadvantages of Drum Brakes

Drums are generally found on the rear of older and cheaper cars. They are widely recognized as old fashioned and inferior to disc brakes.

Disc brakes provide better performance and do not fade under heavy, prolonged braking

OK Professor, if drum brakes suck so much how come we still have cars with drum brakes?

Most vehicles now have disk brakes on the front, but drum brakes are cheaper to install on the rear wheels. The main reason is the cost of the legally required parking brake system. Disc brakes are hydraulic, not mechanical, which makes it difficult for engineers to incorporate a park brake. As the discs cool, they contract, requiring more pressure.

Drum parking brakes are typically held tight by a simple cable. As the drum cools, it contracts, getting tighter.

Many all wheel disc installations have a costly additional drum for the park brake. This, and other safety features, would be price breakers on entry level cars.

Self-Adjustment

Here is the punch line:

Rear drum brakes provide acceptable performance when they're in good condition and properly adjusted.

With a few exceptions where owners are fastidious about maintenance, this means only on "brand new" cars.

Remember, most cars being tested for 60-0 mph deceleration are well prepared, brand new "press cars".

The problem with drum brakes is that they degrade with age, and are very difficult to maintain properly.

Primitive braking technology used a round drum that rotated with the wheel. Inside, a pair of "shoes" applied pressure to the inside of the drum.

Early drum brakes were terrible; they were constantly out of adjustment, faded easily when hot, didn't work when it rained, and pulled to one side or the other.

In the 1950's a self adjusting brake mechanism was added, giving new life to the old technology.

The automatic drum brake adjustment feature has come far since the fifties, but it still smacks of the 19th century. This adjustment requires the car to be repeatedly reversed at high speed and then aggressively braked, an unusual behavior. Many owners do not know this and avoid fast backing. Their brakes are often out of adjustment.

Inside the drum there are lots of old fashioned dust caked parts and springs. Among them, the star wheel spreader assembly, which incorporates the self adjustment feature, is perhaps the most antiquated.

For readers unfamiliar with brake technology, a picture is worth thousands of words.

See this quick animation from HowStuffWorks.com, which shows a drum brake adjustment pawl moving against a star wheel. (Opens a new window.)

Drum Brake Self-adjustment Animation

The main problem is that the little pawl must push against, and slip off the tight star wheel, when the brakes are fully adjusted. Pawls wear and become rounded or bent, slipping off the star wheel.

Over time, a percentage of these pawls will fail to adjust the brakes. Little rear brake shoe wear will occur, and a casual inspection by a mechanic will often result in: rears fine, but front needs expensive new rotors and pads.

There are many rear drum installations that have run 100K miles without shoe replacement. Why? Because the automatic adjustment feature has failed to keep up, the rear brakes are effectively not working. Rather than providing 70% of stopping power, the front discs are doing it all.

Since pawl failure is unpredictable, left and right rear brakes may not be adjusted alike.

Rear brakes receive lots of debris thrown up from the road, they are subject to corrosion and clogging. While drums insulate internal parts from some of this, they retain brake dust. This builds up on star wheel bolt threads, further reducing self-adjuster performance.

So as the car ages, drum brake performance is degraded. Only enthusiast maintained vehicles will avoid this fate. Manual adjustment, done awkwardly through a small aperture in the backing plate, often further damages the pawl.

Many cars on the road today have worn, warped, cracked, or machined front rotors and worn pads, with virtually no effective rear braking. They stop in multiples of the new press car test braking distances. Of course, a neglected all disc system could have problems too.

Maintenance Headache

There is no job more despised by do-it-yourself mechanics than rear drum brake maintenance. Brake dust, supposedly safer today than when it contained asbestos, builds up inside the drum, and pours out when the assembly is disturbed.

Petroleum-based sprays are supposed to reduce the dust breathing. Brake mechanic has historically been right down there with body shop and welding work for low life expectancy.

First, the workings are concealed within the drum, which must be removed even for simple inspection.

Often the brakes must be backed off manually just to remove the drum.

Removing the drums reveals a mass of filthy springs and retainers to be disassembled.

This takes lots more time and some additional skill compared to disc maintenance. This time is money at brake shops and dealerships.

A serious drum brake design defect, in my humble opinion, is the fact that the brake assembly must be built around the axle to allow for the wheel to bolt on.

As a result, the forces acting on the brake shoes are asymmetrical. The hydraulic cylinder which forces the shoes against the spinning drum is at the top, not centered. There are primary (leading), and secondary (trailing) shoes on a rear drum system.

The asymmetrical forces cause the entire assembly to flex and turn in use, causing the leading edge of shoes to wear faster.

This "servo mechanism," which transfers force to the secondary shoe, also accounts for the variable input pressure that was required for hard braking in an old 4-wheel drum system.

Disc Brakes Recommended Here

Disc brakes employ the same basic principles of physics, friction and conversion of momentum to heat, to decelerate a vehicle.

A plate-like rotor and vise-like caliper halt wheel rotation. Pads face the rotor and are easy to inspect and maintain.

Disc brakes are superior to drums due to the design simplicity, lower number of moving parts, lack of need for adjustment, resistance to fade when hot, and linearity of brake pedal force required.

Discs shed heat and water easily compared to drums.

In dirty, salty, and severe service environments, rear disc calipers can bind if not kept clean. In dusty conditions, rear disc brakes may wear faster than front disc brakes as they pick up more abrasive particles.

Common problems are warping caused by heat and incorrect torque on wheel lug nuts, scarring due to neglected pad replacement, and cracking, usually where performance modifications are attempted.

Despite the imperfections of discs, no vehicle with rear drum brakes is recommended here. Buyers are urged to get every safety feature available. Studies are beginning to show higher resale values for cars with safety options.

I have four cars, all with 180K miles and up. I personally maintain them. Future cars will have four wheel disc brakes, no exceptions.

New Cars with Rear Drum Brakes

Here is a partial list of 2007 models with drum braking;