We all know that the sun rises in the East and sets in the West. Even small children know that the Moon orbits the Earth. And everyone knows that disk brakes are much better than drum brakes. Common Sense, Conventional Wisdom, Accepted Practice are all names for the type of knowledge that we all collectively know. But every so often we learn that what we have always thought to be absolutely true is in fact completely wrong. At one time we thought that the Earth is flat, that traveling faster than 20 miles per hour will cause suffocation and that tomatoes are poisonous. And there are a few select instances when drum brakes are a better choice that disk brakes.
Drum and disk brakes differ in a variety of ways; generally speaking, disk brakes are lighter, dissipate heat better and resist fade better than drums. Once considered exotic technology that migrated from aircraft applications to racing cars in the 1940's. In the 1950's only exotic foreign cars were available with disk brakes although such classics as the Mercedes Gull Wing 300SL did not get factory disk brakes until the production was nearly ended in the early 1960's. Front disk brakes did not appear of US domestic cars until the mid 1960's as an extra cost option. By the early 1970's front disk brakes became the accepted norm and disk brakes on all four corners became the standard for most luxury and performance cars by the 1980's. But even to this day rear drum brakes are common on lower priced cars and trucks with four-wheel disk reserved for upscale and high performance cars.
Nearly all forms of race cars use four wheel disk brakes unless the organizing body decrees that production based race cars must use the same type of brakes that came from the factory. Amongst the car modification hobby, it is considered essential to replace rear drum brakes with much more stylish rear disk brakes. And consumers are willing to pay extra for the perceived advantage of four wheel disk brakes, and they will identify a car's performance potential by the presence of four-wheel disk brakes.
All the evidence seems to point to the undeniable fact that disk brakes are superior in every regard. Except when they are not.
Large cars need large brakes. And large brake drums are heavier and less efficient than large disk brakes so disk brakes still retain an advantage over drum brakes. But smaller cars can use smaller brakes and small drum brakes are actually lighter than small disk brakes. Strange as this may seem, the rear drum brakes and all the assemblage required to make them work are lighter than the total collection of parts needed to make rear disk brakes work on small Honda Civic-sized cars.
But the braking advantage of rear disks would negate the weight disadvantage, right? In actuality the rear brakes on the majority of street cars contribute very little to the car's stopping power. Most street cars have as much as 60% of their total weight over the front wheels and the forward momentum of a car in motion puts even more of the car's total weight over the front wheels. The rear wheels are relatively un-loaded during stopping and can not contribute much stopping power under the best of circumstances. That is why the rear brakes are the first to lock up during a panic stop. In street driving, rear drums are perfectly adequate for all driving conditions, and that is why manufacturers have no problem selling cars with rear drum brakes. It is only consumer demand that drives the sale of so many cars with rear disk brakes, the average driver will never notice the difference between rear disks or drums.
Before consumer demand began to dictate that manufacturers sell performance cars with four-wheel disk brakes it was common for sports cars to come with rear drum brakes. You might be shocked to know that the original Datsun 240Z came from the factory with rear drum brakes, albeit the drum brakes were aluminum rather than the usual cast iron. The aluminum drums were lighter and they dissipated heat almost as well as the disk brakes.
And the American Muscle Car crowd is very hesitant to adapt rear disk brakes to their drag racing cars. Not just because they seem to have a phobia of adapting current technology to their nostalgia-mobiles, but because drum brakes have no parasitic drag when released while disk brakes rub ever so slightly even when not engaged. For drag racers who only use their brakes once at the end of a long straight, the lack of total braking power that drums suffer is not a disadvantage.
The advantages of disk brakes over drum brakes are numerous. Disk brakes are the only choice for the hard working front brakes of any street car and rear disks can be an advantage in the rear for some cars. But it is not universal that rear disk brakes are always superior to rear drums.