# Why drum brakes on cars? **URL:** \ **Category:** Factual Questions\ **Created:** [August 11, 2002, 11:37am UTC](https://boards.straightdope.com/t/why-drum-brakes-on-cars/123391 "2002-08-11T11:37:42Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![zwede](https://avatars.discourse-cdn.com/v4/letter/z/ac91a4/32.png) [@zwede](https://boards.straightdope.com/u/zwede)\ **Post date:** [August 11, 2002, 8:47pm UTC](https://boards.straightdope.com/t/why-drum-brakes-on-cars/123391/21 "2002-08-11T20:47:25Z")
> [@](#): > > \*Originally posted by bernse \* > \*\* > > Whaaat? A vehicle with 4 wheel disc has drum brakes in the back for a _parking brake_? Where did you get that idea from? Do you have an example of this? 😕 \*\* Just as an example: My 1971 Corvette has this setup. 4 wheel disc brakes with a tiny drum brake in the hub of the rear discs. But that’s just one example. It is actually quite common. Markus ---
**Author:** ![LouisB](https://avatars.discourse-cdn.com/v4/letter/l/dfb087/32.png) [@LouisB](https://boards.straightdope.com/u/LouisB)\ **Post date:** [August 11, 2002, 10:55pm UTC](https://boards.straightdope.com/t/why-drum-brakes-on-cars/123391/22 "2002-08-11T22:55:48Z")
> [@](#): > > I’ve heard of vehilcles having a drum brake on the driveshaft for a parking brake, but even then, none of those vehicles had disc on the rear wheels either. I remember this set-up being used on Chrysler products from the mid 1940s through the 1950s. I had a very specialized tool used only for adjusting these things. I don’t remember any other manufacturers that used this approach. ---
**Author:** ![Doc\_Nickel](https://avatars.discourse-cdn.com/v4/letter/d/5fc32e/32.png) [@Doc\_Nickel](https://boards.straightdope.com/u/Doc_Nickel)\ **Post date:** [August 12, 2002, 4:12am UTC](https://boards.straightdope.com/t/why-drum-brakes-on-cars/123391/23 "2002-08-12T04:12:54Z")
Having done a couple of 4W disc conversions, I can speak with a smidgen of authority. As mentioned, the Corvette uses a dinky, miniature drum brake on the inside of the rear rotor hat as a parking brake. Late 70’s Cadillac ElDorados with 4-wheel disc setups had rear calipers that had a fast-helix screw setup on the inside of the puck. A cable-actuated lever on the back, “screwed” the puck out to lock against the rotor. Effective but expensive; a rebuilt front caliper is $29 with pads. A rebuilt rear is $135 with pads. Drums, as said, are somewhat cheaper, as stampings are less expensive to produce than castings. Most small cars nowadays even use stamped drums, with a cast-iron liner as the wear surface. Second, drums are “self energizing”. Meaning that the geometry of the pivot point and the scrubbing action of the shoe against the inner face of the drum, “naturally” wants to push the shoe harder as the drum turns. It’s hard to describe, but simple to illustrate, if I could find a photo… I haven’t checked, but you might look at [howstuffworks.com](http://howstuffworks.com). This “self energization” reduces the pedal force required (the same small cars you see 'em on tend also to not have power assist, though this is by no means true across the board.) Plus, as mentioned, it’s easier to have cable-activated drums (parking or emergency brake) than it is for discs. In most disc brakes, the hydraulic pressure pushes the pucks (and thereby, the shoes) against the rotor, but the square-section “o-ring” that seals the puck to the inside of the caliper, is what _retracts_ it so the shoe doesn’t rub on the disc. It’s quite self-adjusting, in that the more the shoe wears, the more the O-ring slips to let the puck travel outward. The problem here is that if the puck is _kept_ extended- IE, with the manual parking brake- the O-ring can “relax” and allow the shoe to scrub on the rotor almost constantly. Which, of course, increases wear, costs (marginally) mileage and makes noise. The drawback to drums is the handling of heat. Not only is the heat generated inside a semi-enclosed body, with minimal air allowed to circulate around the shoes, but also the “growth” of the metal drum as it heats is _away_ from the shoe, which in turn requires you to apply more pedal to keep the same level of braking. With a disc, the growth is outward- _against_ the caliper, which increases the braking force naturally. Second, the rotor is quite exposed, and internally-ventilated types are actually pretty decent air-movers, which improves air cooling by significant margins over even good drum setups. And last, the disc is actually far better than the drum at handling mud and water contamination. The shoe on a disc scrapes the rotor clean once per pass. In a drum, the water and mud can get trapped between the shoe and drum. Some tests with water have shown the moisture can actually “boil” and form a layer of microscopic steam bubbles that actually keeps the shoe from contacting the drum. Obviously, this is not greatly conducive to good stopping power. Bottom line, cost, use and corporate inertia keep the drums in place. ---
**Author:** ![Anachronism](https://avatars.discourse-cdn.com/v4/letter/a/e56c9b/32.png) [@Anachronism](https://boards.straightdope.com/u/Anachronism)\ **Post date:** [August 12, 2002, 4:32am UTC](https://boards.straightdope.com/t/why-drum-brakes-on-cars/123391/24 "2002-08-12T04:32:39Z")
> [@](#): > > \*Originally posted by Johnny L.A. \* > \*\*[qoute]Disk brakes require power to work.[/quote] > My Yamaha has disc brakes, and there is no power assist. I’m pretty sure my Porsche didn’t have power assist. \*\* I stand corrected, at least I can code correctly 😉 And to the rest of you I call ‘piling on’, I’ll take my 15 yards. > [@](#): > > \*Originally posted by Doc Nickel \* > \*\* > Second, drums are “self energizing”. Meaning that the geometry of the pivot point and the scrubbing action of the shoe against the inner face of the drum, “naturally” wants to push the shoe harder as the drum turns. It’s hard to describe, but simple to illustrate, if I could find a photo… I haven’t checked, but you might look at [howstuffworks.com](http://howstuffworks.com). > > This “self energization” reduces the pedal force required (the same small cars you see 'em on tend also to not have power assist, though this is by no means true across the board.) > > \*\* > [@](#): > > \*Originally posted by zwede \* > \*\*With drum brakes the pivoting of the brake shoe will assist in brake force. This enables drumbrake systems to operate fairly well without a brake booster. > > Although it is true that you can have 4 wheel disc brakes with no booster, the pedal effort will be very high compared to drums. On a small, light, car you can still get away with manual discs. Hevy cars will require a booster. > \*\* > [@](#): > > \*From how things work \* > \*\*Back in the day, when most cars had drum brakes, power brakes were not really necessary – drum brakes naturally provide some of their own power assist. Since most cars today have disc brakes, at least on the front wheels, they need power brakes. Without this device, a lot of drivers would have very tired legs. > \*\* This is what I was thinking of, I misunderstood this information as meaning that disk brake _needed_ power assist and drum brakes did not, :o ---
**Author:** ![bernse](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@bernse](https://boards.straightdope.com/u/bernse)\ **Post date:** [August 12, 2002, 1:35pm UTC](https://boards.straightdope.com/t/why-drum-brakes-on-cars/123391/25 "2002-08-12T13:35:05Z")
> [@](#): > > \*Originally posted by zwede \* > \*\* > > Just as an example: My 1971 Corvette has this setup. 4 wheel disc brakes with a tiny drum brake in the hub of the rear discs. But that’s just one example. It is actually quite common. > > Markus \*\* OK. I know what you mean now. I would not go as far as calling that “common”. I believe that it was only used on the Corvette (possibly another GM model as they usually share some parts). I don’t know of any other mfgs that have done that. I’d really apperciate a couple more examples so I could see how other people did it. The 4 wheel disc setups that I have all worked on drove the rear piston out with the park brake. > [@](#): > > I remember this set-up being used on Chrysler products from the mid 1940s through the 1950s. I had a very specialized tool used only for adjusting these things. I don’t remember any other manufacturers that used this approach. Toyota for certain. Our old landcruiser FJ40 had. Usually older iron had that type of setup. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [August 12, 2002, 2:02pm UTC](https://boards.straightdope.com/t/why-drum-brakes-on-cars/123391/26 "2002-08-12T14:02:49Z")
In re 4-wheel disc systems using a drum & shoe type parking brake, **bernse** said: _…I would not go as far as calling that “common”. I believe that it was only used on the Corvette (possibly another GM model as they usually share some parts). I don’t know of any other mfgs that have done that. I’d really apperciate a couple more examples so I could see how other people did it. The 4 wheel disc setups that I have all worked on drove the rear piston out with the park brake._ At least some models of BMW, Infiniti, Jaguar, Land Rover, Lexus, Mercedes Benz, Nissan, Porsche, Subaru, and Volvo use this design–and that’s just from checking the imports. Don’t know how you’d define common, but it certainly isn’t rare. There are probably hundreds of different models of cars. Experience on 5 or 8 or 12 of them is not likely to provide a representative sample of all the various design approaches used. [Previous page](https://boards.straightdope.com/t/why-drum-brakes-on-cars/123391.md?page=1)