Brake technology, just like suspension technology and fuel-system technology, has come a long way in recent years. What began in the ’60s as a serious attempt to provide adequate braking for performance cars has ended in an industry where brakes range from supremely adequate to downright phenomenal. The introduction of disc brake components like carbon fiber, sintered metal, and lightweight steel, along with the adoption of ABS, have all contributed to reduced stopping distances and generally safer vehicles (though ABS continues to provide controversy).
One of the first steps taken to improve braking came in the early ’70s when manufacturers, on a widespread scale, switched from drum to disc brakes. Since most of a vehicle’s stopping power is contained in the front wheels, only the front brakes were upgraded to disc during much of this period. Since then, many manufacturers have adopted four-wheel disc brakes on their high-end and performance models as well as their low-line economy cars. Occasionally, however, as in the case of the 1999 Mazda Protege’s, a manufacturer will revert from a previous four-wheel disc setup to drum brakes for the rear of the car to cut both production costs and purchase price.
Why are disc brakes better than drum and how much, if any, loss of braking occurs when using rear drum brakes on a modern car? That is what we are going to find out.
Friction and Heat
Before you can appreciate the difference between drum and disc brakes, you must understand the common principles that both systems use when stopping a classic car: friction and heat. By applying resistance, or friction, to a turning wheel, a vehicle’s brakes cause the wheel to slow down and eventually stop, creating heat as a byproduct. The rate at which a wheel can be slowed depends on several factors including vehicle weight, braking force, and total braking surface area. It also depends heavily on how well a brake system converts wheel movement into heat (by way of friction) and, subsequently, how quickly this heat is removed from the brake components. This is where the difference between drum brakes and disc brakes becomes pronounced.
Drum Brakes
Early automotive & vintage car brake systems, after the era of hand levers of course, used a drum design at all four wheels. They were called drum brakes because the components were housed in a round drum that rotated along with the wheel. Inside was a set of shoes that, when the brake pedal was pressed, would force the shoes against the drum and slow the wheel. Fluid was used to transfer the movement of the brake pedal into the movement of the brake shoes, while the shoes themselves were made of a heat-resistant friction material like that used on clutch plates.
This basic design proved capable under most circumstances, but it had one major flaw. Under high braking conditions, like descending a steep hill with a heavy load or repeated high-speed slowdowns, drum brakes would often fade and lose effectiveness. Usually this fading was the result of too much heat build-up within the drum. Remember that the principle of braking involves turning kinetic energy (wheel movement) into thermal energy (heat). For this reason, drum brakes can only operate if they can absorb the heat generated by slowing a vehicle’s wheels. Once the brake components themselves become saturated with heat, they lose the ability to halt a vehicle, which can be somewhat disconcerting to the vehicle’s operator.
Disc Brakes
Though disc brakes rely on the same basic principles to slow a vehicle (friction and heat), their design is far superior to that of drum brakes. Instead of housing the major components within a metal drum, disc brakes use a slim rotor and small caliper to halt wheel movement. Within the caliper are two brake pads, one on each side of the rotor, that clamp together when the brake pedal is pressed. Once again, fluid is used to transfer the movement of the brake pedal into the movement of the brake pads.
But unlike drum brakes, which allow heat to build up inside the drum during heavy braking, the rotor used in disc brakes is fully exposed to outside air. This exposure works to constantly cool the rotor, greatly reducing its tendency to overheat or cause fading. Not surprisingly, it was under racing circumstances that the weaknesses of drum brakes and the strengths of disc brakes were first illustrated. Racers with disc brake systems could carry their speed “deeper” into a corner and apply greater braking force at the last possible second without overheating the components. Eventually, as with so many other automotive advances, this technology filtered down to the cars driven by everyday people on public roads.
Drum vs Disc Today
In today’s automotive pantheon, it is not uncommon to find four-wheel disc brakes as standard equipment on medium-priced, non-performance-oriented models. Most new vehicles, however, continue to utilize a front-disc/rear-drum brake setup. What does this say about the current state of braking systems? Are these manufacturers sacrificing vehicle safety to save a few bucks by installing disc brakes on only the front wheels?
While a “yes” answer would certainly be great for increasing Town Hall traffic, the truth is that today’s disc/drum setups are completely adequate for most new cars. Remember that both disc and drum brake design has been vastly improved in the last 20 years. In fact, the current rear drum brake systems on today’s cars would provide better stopping performance then the front disc setups of the ’70s. And today’s front disc brakes are truly exceptional in terms of stopping power. Combined with the fact that between 60 and 90 percent of a vehicle’s stopping power comes from the front wheels, a well-designed, modern drum brake is all that is required for most rear wheel brake duty.
High performance cars like the Viper, 911 and Corvette can justify a four-wheel disc brake system, especially if their owners participate in some form of sanctioned racing activity on the weekends. The rest of us get more of a benefit from the lower cost of drum brakes. Expecting every vehicle built today to come with four-wheel disc brakes would require an across-the-board increase in purchase price, and that could stop new car buyers much quicker than any brake system.
There are a lot of reasons you should consider when it comes to upgrading to disc brakes. The main reason is safety. For starters, drum brakes are easily damaged by heat and moisture. Get them wet or hot, and they will not work–period. Even when they are in good working order, they offer less-than-optimal stopping power, especially in today’s traffic conditions. Chrysler actually introduced standard disc brakes on the front of their vehicles sometime in the 1950s, but it wasn’t until the mid-1960s, when power-assisted units, which made disc brakes easier to operate, hit the market. Some modern vehicles still come with drum brakes on rear wheels. Rear drum brakes are cheaper to produce than disc brakes, and since a vehicle’s front brakes do 70 percent of the work, rear brake performance can be sacrificed for cost.
Since drum brakes do have their limiting factors, and adding front disc brakes to your car is an upgrade in safety and performance, is an upgrade to rear disc brakes worth doing on your car? This is a question that only you can answer for yourself. Some guys feel that rear drum brakes are more than sufficient for stopping their cars, and it’s true that a good working drum brake system on the rear of a car can work properly during normal driving conditions–if you have front disc brakes. If you have drum brakes on all four corners, just think back to the last time you panic-braked when some idiot pulled out in front of you and tell me that an upgrade is not worth it.
It goes without saying that you cannot install rear disc brakes on your car if the front has drums on it. Well, you could but when you hit the brakes, your rear tires will lock up before the front ones even start to slow down. But if a car equipped with front disc brakes and rear drum brakes is sufficient, why upgrade to rear disc brakes? For starters, the front discs can only do so much. Have you ever noticed that your rear drum shoes last much longer than the front shoes/pads? We have been told that by adding rear disc brakes to your car, you can effectively add as much as 30 percent more braking force to your braking system. This means you stop a lot quicker, which could come in handy when that idiot pulls out in front of you.
Finally
But you might be worried that adding a rear disc-brake kit to your car is going to be too expensive, and granted, you will have to shell out a few hundred dollars for a kit that works on your car. But how much would the bodywork be if you ran into the aforementioned idiot?
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